Physical Activity Guidelines Advisory Committee Report
Part G. Section 5: Musculoskeletal Health
List of Figures
List of Tables
The Musculoskeletal Health Subcommittee reviewed the evidence for the
role of physical activity (PA) in bone, joint, and muscle health. With respect
to bone health, the review focused on osteoporosis because it is the most
prevalent bone disease and because physical activity is thought to play a role
in the etiology of osteoporosis. In 2002, it was estimated that 7.8 million
women and 2.3 million men in the United States aged 50 years and older had
osteoporosis, and another 21.8 million women and 11.8 million men were at risk
of the disease because of low bone mass. By 2010, it is expected that the
number of women and men with osteoporosis will increase to 9.1 and 2.8 million,
respectively, and the number of women and men with low bone mass will increase
to 26.0 and 14.4 million, respectively (statistics from
21 January 2008
). Performing regular weight-bearing and muscle-strengthening
exercises is one of the universal recommendations for the general population to
reduce the risk of falls and fractures (1-3). However, more
specific information on the type or volume of exercise that should be performed
With respect to joint health, the review focused primarily on
osteoarthritis (OA), particularly of the lower extremity, because of its high
prevalence. It is estimated that 27 million women and men in the U.S. aged 25
years and older have OA. The incidence rates are higher in women than in men,
particularly for knee OA (4). Physical activity and OA have
a potentially complex association, in that a certain level of mechanical joint
stress is essential for good joint health but excessive joint stress may
promote the development of OA.
In contrast to bone and joint health, muscle health in not linked with
a specific chronic disease. Despite this, muscle mass and function are widely
recognized as important determinants of risk for such chronic diseases as
osteoporosis and type 2 diabetes (5). Muscle mass and
function are also recognized as important determinants of physical fitness. The
review focused on physical activity as a mediator of both muscle quantity
(i.e., muscle mass) and quality (i.e., muscle function).
Review of the Science
Overview of Questions Addressed
This chapter addresses 5 questions about the role of physical activity
in bone, joint, and muscle health:
- Does physical activity reduce the incidence of osteoporotic
- Does physical activity reduce risk of osteoporosis by increasing,
or slowing the decline in, bone mineral density or bone mineral content?
- Does physical activity reduce or increase the incidence of
- Is physical activity harmful or beneficial for adults with
osteoarthritis or other rheumatic conditions?
- Does physical activity increase or preserve muscle mass throughout
the lifespan? Does physical activity improve skeletal muscle quality, defined
as changes in intrinsic and extrinsic measures of force-generating capacity,
such as strength or power?
For each question, the Musculoskeletal Health subcommittee considered
whether such factors as sex, age, or specific characteristics of the physical
activity are important determinants of the health-mediating effects. Effects of
race and ethnicity could not be examined because the majority of studies
reviewed either did not include volunteers from underrepresented minorities or
did not conduct subgroup analyses by race/ethnicity.
Data Sources and Process Used to Answer
Scientific articles related to physical activity and musculoskeletal
outcomes were primarily identified by using a systematic search process that
relied on the Physical Activity Guidelines for Americans Scientific
Database (see Part F: Scientific
Literature Search Methodology for a detailed description of
the Database). The systematic review and subsequent article abstraction process
was supplemented with previously published review or meta-analytic papers, and
key or important studies identified by the Musculoskeletal Health subcommittee
and consultants. Several systematic review or meta-analytic articles and
faculty-identified studies were used to document the scientific evidence
pertaining to physical activity and bone mineral density (BMD) and/or bone
mineral content (BMC) outcomes. The systematic review and abstraction process
also was used to identify articles related to physical activity and bone
outcomes in men. Along with the systematic review and abstraction process,
review articles, and faculty-identified key studies were used to identify
papers and findings related to physical activity and joint outcomes, primarily
focusing on OA. Longitudinal cohort studies and case-control studies were
located that evaluated some measure of physical activity as the exposure and
incidence of OA as the outcome. Randomized controlled trials (RCTs) were
identified to determine the risks and benefits of physical activity among
persons with OA or other rheumatic conditions, such as rheumatoid arthritis,
fibromyalgia, lupus, and ankylosing spondylitis. Exercise interventions that
were primarily clinical (i.e., therapeutic physical or occupational therapy)
were excluded. Review articles and/or meta-analytic studies and a
faculty-generated search for relevant studies were used to evaluate the
evidence for physical activity and muscle fitness.
Question 1. Does Physical Activity Reduce
the Incidence of Osteoporotic Fractures?
Physical activity is inversely associated with fracture risk (i.e.,
increased PA, decreased fracture risk), particularly for fractures of the
proximal femur. It also has a dose-response relation with fracture risk, such
that a greater volume of physical activity (i.e., frequency, duration, and/or
intensity) confers greater risk reduction. It is not currently possible to
identify more precisely the characteristics of the type or dose of physical
activity likely to optimize fracture prevention. Based on epidemiologic studies
that evaluated dose-response associations in various quantifiable manners, the
minimal levels of physical activity that were significantly associated with
reduced fracture risk were at least 9 to 14.9 metabolic
equivalent (MET)-hours per week of physical activity, more
than 4 hours per week of walking, at least 1,290
kilocalories per week of physical activity, and more than 1
hour per week of physical activity.
No large RCTs have been conducted to determine whether the incidence
of fractures is decreased in response to physical activity. Therefore,
definitive evidence for its efficacy in fracture prevention is lacking.
However, prospective cohort (6-16), retrospective cohort
(17), case-control (18-23), a small
RCT (24), and cross-sectional (25;26) studies provide moderate evidence for an inverse
association of physical activity with fracture risk (i.e., high levels of
activity, low fracture risk). These studies also provide evidence for a
dose-dependent association with fracture risk, with higher levels of activity
related to lower fracture risk. Data that can be used to develop quantifiable
recommendations for the type, frequency, duration, and intensity of physical
activity most likely to reduce fracture risk are limited.
The likelihood that a RCT of PA with osteoporotic fracture as a
primary outcome will ever be conducted is remote because of the large sample
size and long duration of intervention that would be required. In this context,
the consistency of findings, from both the population studies considered in
this section and the biomarker (i.e., BMD) studies considered for Question 2,
provides a solid evidence base for a role of physical activity in preserving
bone health. The optimal type and dose of activity necessary to maintain bone
health is less clear.
The evidence will be discussed with respect to whether the
associations between physical activity and fracture risk are consistent across
the types of studies that have been conducted, and whether findings are
influenced by such factors as sex, fracture site, or type of activity.
Type of Study
Prospective cohort studies (6-16), a retrospective
cohort study (17), case-control studies (18-23), a small RCT (24), and
cross-sectional (25;26) studies
provide moderate evidence for an inverse association of physical activity with
fracture risk (i.e., high levels of activity, low fracture risk). Overall, and
without respect to the specific factors that will be considered below (i.e.,
type of study, fracture site, sex specificity, dose-response association), all
types of observational and experimental approaches provided evidence for a role
of physical activity in preventing fractures. Of the 21 studies considered,
only 3 reported no associations (12;16;17), and 2 reported an association of
physical activity with increased fracture risk under some
Prospective and Retrospective Cohort Studies
Of the 12 prospective and retrospective cohort studies, 9 found
beneficial associations of physical activity with fracture risk (6-11;13-15); the others found no
significant associations. Of note, 2 of the latter studies focused only on
vertebral fracture risk (12;16); and
the third focused on all osteoporotic fractures (i.e., hip, leg, wrist, pelvis,
spine, rib, humerus, clavicle, radius, and ulna) (17).
Because the effects of mechanical loading on bone metabolism are specific to
the region undergoing loading, physical activity would not be expected to have
uniform effects in all skeletal regions. Also, the less consistent evidence for
an association of physical activity with vertebral fractures may be related to
difficulties associated with diagnosis.
Most of the case-control studies were focused on hip fracture cases
(18;20-23); only 1 evaluated the role
of physical activity levels as a determinant of vertebral deformity (19). Although all reported favorable odds ratios for a
physical activity-related reduction in fracture risk under some conditions, 2
studies noted a direct association (i.e., increased fracture risk with
increased activity) in certain cases (19;20). Silman and colleagues (19) found
that heavy levels of physical activity in early and middle adult life were
associated with increased risk for vertebral deformity in men
(odds ratio [OR] 1.5 to 1.7; all P<0.01), but not women. The same
study found that current walking and/or cycling more than 30 minutes per day
was associated with a reduced risk of vertebral deformity in women (OR 0.8; 95%
confidence interval [CI] 0.7-1.0), but not men (OR 0.9; 95% CI 0.8-1.2).
Stevens and colleagues (20) found that vigorous activity
was associated with a reduced risk for hip fracture in older
women and men who had no limitations in activities of daily living (ADLs) (OR
0.6; 95% CI 0.4-0.8), but an increased risk (OR 3.2; 95% CI
1.1-9.8) in those who had 1 or more limitations in ADLs.
Randomized Controlled Trials
One small RCT reported on the incidence of vertebral fractures (24). Women who had been randomized to participate in a 2-year
back strengthening exercise program or a non-exercise control group were
evaluated 8 years after the completion of the intervention trial. The incidence
of vertebral fractures was significantly lower in exercisers (1.6%) than in
Cross-Sectional Comparison Studies
Nordstrom and colleagues (26) compared the
incidence of fractures in former elite male athletes (soccer and ice hockey
players, aged 60 years and older) and age-matched male controls. The incidence
of fractures before the age of 35 years was higher in the athletes than in
controls (17.5% versus 12.9%, P<0.05), but athletes had fewer
fractures than controls after the age of 50 years (8.5% versus 12.9%,
P<0.05). Ringsberg and colleagues (25)
evaluated fracture risk in older (aged 65 to 75 years) and elderly (aged 76 to
89 years) women who reported regular participation in exercise classes (at
least 1 hour per week) for at least 20 years. They were compared with randomly
selected age-matched women from either urban or rural communities. The relative
risk for any fracture was reduced in both older (RR 0.50; 95% CI 0.33-0.79) and
elderly (RR 0.28; 95% CI 0.13-0.56) regular exercisers when compared with urban
controls, but not when compared with rural controls (older: RR 1.10; 95% CI
0.63-2.00; elderly: RR 0.63; 95% CI 0.24-1.43). Similar associations were found
when only fragility fractures were considered.
Cohort, case-control, and cross-sectional comparison studies all
provide evidence for a beneficial association of physical activity with
fracture risk. A limitation of these types of studies is that they do not
isolate the role of physical activity as being causal in fracture reduction.
However, the general consistency of favorable findings across multiple studies
generates confidence that it plays a central role, if not a causal role, in the
prevention of fractures.
Type of Fracture
Findings show consistently favorable associations of physical activity
with reduced hip fracture risk (6-9;13;18;21-23;26). Many of these studies
categorized participants by levels of activity (e.g., tertile or quartile,
hours per week) (6-9;13;18;21-23), and the relative risk for hip
fracture was significantly reduced in the most active group when the least
active group was used as the reference group (Figure
Hip fracture risk was also increased in the least active group, when
the most active group was used as the reference group: Hazards Ratio=2.56 (95%
CI 1.55-4.24) (13); reciprocals of the hazards ratio and
confidence intervals were calculated for inclusion in Figure G5.1. It should be noted that prospective
cohort studies query for physical activity level and then monitor for fracture
outcomes, whereas case-control studies query for physical activity after
identifying fracture cases and controls.
Figure G5.1. Point Estimates of Relative
Risk (± 95% Confidence Intervals) of Hip Fracture From Studies That
Examined Multiple Levels of Physical Activity (Most Active Group Versus Least
Note: Solid confidence intervals indicate studies of
women; dashed confidence intervals indicate studies of men.
Michaelsson 2007 (13); Kujala 2000 (9); Hoidrup 2001 (8); Gregg 1998 (7); Feskanich 2002 (6); Kanis 1999 (23); Jaglal 1995 (22); Farahmand 2000
(18); Boonyaratavej 2001(21)
Figure G5.1. Data Points
|Prospective Cohort: Michaelsson 2007 (13)
|Prospective Cohort: Kujala 2000 (9)
|Prospective Cohort: Hoidrup 2001 (8)
|Prospective Cohort: Hoidrup 2001 (8)
|Prospective Cohort: Gregg 1998 (7)
|Prospective Cohort: Feskanich 2002 (6)
|Case-control: Kanis 1999 (23)
|Case-control: Jaglal 1995 (22)
|Case-control: Farahmand 2000 (18)
|Case-control: Boonyaratavej 2001 (21)
Vertebral Fractures or Deformity
Although both vertebral and hip fractures are of high clinical
significance because of the associated morbidity and mortality, the former are
more difficult to diagnose because they can occur without symptoms. Consensus
also is lacking on what extent of vertebral deformity constitutes a fracture.
The few studies that have evaluated the association of physical activity with
risk of vertebral fracture (or deformity) have had discordant findings (7;12;16;19;24). Heavy levels of activity in early and middle adult life
were associated with increased risk for vertebral deformity in
men (ORs 1.5 to 1.7; all P<0.01), but not women (19). In that study, current walking and/or cycling more than
30 minutes per day was associated with a reduced risk of
vertebral deformity in women (OR 0.8; 95% CI 0.7-1.0), but not men (OR 0.9; 95%
CI 0.8-1.2). Two other studies that assessed historical and recent occupational
and leisure-time physical activity found no associations with
vertebral fractures in women and men (12;16). However, in women aged 65 years or older, participation
in moderate- or vigorous-intensity sport or recreational activity was
associated with reduced risk for vertebral fracture (RR 0.67;
95% CI 0.49-0.94) when compared with women who reported no participation in
such activities (7). In a small prospective study of women
who had participated in an exercise program focused on strengthening back
extensor muscles, the prevalence of vertebral fractures 8 years later was
significantly lower in the exercisers than in the controls (1.6% vs. 4.3%;
Although the wrist is a common site of osteoporotic fracture, it is of
lesser clinical significance than the spine and hip because the associated
morbidity and mortality is very low. In the Study of Osteoporotic Fractures (7;26), physical activity was not
associated with risk of wrist fracture, whereas favorable associations with
risk of hip and vertebral fractures did exist. Physical activity was associated
with reduced wrist fracture risk over 25.2 years of follow-up in the Adventist
Health Study (high versus none/low physical activity, relative risk [RR] 0.61;
95% CI 0.41-0.87) (10), and former elite athletes were
found to have a lower prevalence of wrist fractures after the age of 50 years
than were age-matched controls (0.75% vs. 3.5%, P<0.05) (26).
All Fractures, Fragility Fractures, or Nonvertebral Fractures
Several studies have evaluated the association of physical activity
with risk of any fracture (11;13;20;25), fractures in weight-bearing
versus non-weight-bearing regions (15), and
low‑trauma, osteoporotic, or fragility fractures (14;17;25;26). The majority of these studies found an association with
reduced fracture risk (11;13-15;20;25;26), but
there were exceptions. Participation in vigorous levels of activity was
associated with a reduced risk of fractures in women and men with no
limitations in ADLs, but an increased risk in elderly with any ADL dependency
(20). Joakimsen and colleagues (15)
found that women and men in the highest category of physical activity, compared
with those in the lowest, had a reduced risk for fractures in weight-bearing
regions (RR 0.6; 95% CI 0.4-0.9) but not in non-weight-bearing regions (RR 1.0;
95% CI 0.7-1.2). Among women and men in the Rancho Bernardo study, physical
activity was not significantly associated with osteoporotic fractures (17).
The evidence supports favorable associations of physical activity with
reduced risk of fractures. The evidence is most consistent for a reduction in
hip fracture risk. Because the proximal femur undergoes loading during walking
and all activities that involve ambulation, it is logical that an effect to
reduce fracture risk would be most apparent at this site. The less consistent
findings for an association with reduced vertebral or other osteoporotic
fractures should not be interpreted as evidence that physical activity is not
important for preventing such fractures. It is likely that the instruments
commonly used to assess total physical activity do not adequately capture or
characterize the potential site-specific skeletal benefits of certain types of
All of the studies that included women only reported favorable
associations of physical activity with reduction in fracture risk (Table G5.A1 [PDF - 211 KB], which
summarizes these studies) (6;7;10;18;21;22;24;25).
Similarly, all of the studies that included men only reported favorable
associations with reduction in fracture risk (Table G5.A1 [PDF - 211 KB]) (9;13;14;23;26).
In contrast, the studies that included both sexes had discordant
findings. Three of these studies found no significant associations of physical
activity with fracture risk when analyses were performed by sex (12;16;17) or in
women and men combined (17). However, none of these
studies was focused on hip fractures. Two studies reported an association with
reduced risk for any fracture in both women and men (11;20). Other studies found beneficial associations of in women
but not men (8;19), or in men but not
women (15). Another noted an adverse association in men,
but not women (19).
Studies that included both women and men are characterized by greater
discordance in the results than those that included only women or only men.
This causes a general concern regarding the assessment of physical activity in
studies that include both sexes. It is typically categorized by participation
in activities of varying intensity (e.g., mild = normal walking, moderate =
fast walking, strenuous = jogging (17)) and, in some
cases, quantified by the absolute intensity of the activity in metabolic
equivalents (METs). However, these approaches do not account for sex-related
differences in the relative intensity. In age‑matched
women and men, walking at a given speed or performing an activity of a certain
MET level represents a greater relative cardiovascular stress for women than
men, because women have a lower maximal aerobic power (27). Similarly, such activities may also represent a greater
skeletal stress in women, because bone size and mineral content are less in
women than in men. The failure to account for such sex-related differences in
relative intensity may result in miscategorization of level of activity. For
example, fast walking may, indeed, be a moderate-intensity activity for older
men, but is likely to be a strenuous activity for older women. Although studies
typically adjust for effects of sex (and age) in statistical analyses, it is
not clear whether such approaches adequately control for these issues. The use
of very broad categorizations (e.g., mild versus moderate versus strenous) may
obscure true associations of physical activity with fracture risk, and this
would be expected to be of greater concern in studies that included both women
Physical Activity Dose-Response Pattern
Studies of laboratory animals indicate that the adaptation of bone to
mechanical loading is dose-dependent, with the intensity of
the loading force being the key determinant of the magnitude of the adaptive
response (28). If these findings have relevance to human
physiology, it would be expected that associations of physical activity with
fracture risk would reflect a dose dependency.
Quantified Dose Response
Several studies evaluated physical activity in a manner that enabled
the evaluation of a quantifiable (in terms of frequency, duration, and/or
intensity) dose-response association with fracture risk. Some studies (6;7;18;23), but not others (8;9;12), found evidence of a linear trend
for increased volume of physical activity and reduced fracture risk. The manner
in which the dose was quantified varied among studies, including MET-hours per
week, kilocalories per week, and hours per week; none of these approaches
facilitated the isolation of intensity as a mediator of fracture risk. Among
the studies that reported a significant dose-response association, the minimal
levels found to be significantly associated with reduced fracture risk were: at
least 9 to 14.9 MET-hours per week of physical activity (6), 4 or more hours per week of walking (6), 1,290 kilocalories or more per week of physical activity
(7), and 1 or more hours per week of physical activity (18;23). These levels were associated
with relative reductions in fracture risk of 33% to 41%. With increasing
levels, the relative reduction in fracture risk was 36% to 68%. Another study
(6) found a dose-response association of hours spent
standing per day with reduction in fracture risk. Standing 40 or more hours per
week was associated with a 34% reduction in fracture risk. One study (7) also found a significant dose-response association of
physical inactivity, quantified as hours per day spent sitting, and increased
fracture risk. Sitting more than 8 hours a day was associated with a 37%
increase in risk of fracture.
Categorical Dose Response
A few studies that used categorical methods (e.g., tertiles of
activity, inactive versus active versus very active) to evaluate dose-response
associations of physical activity with fracture risk found significant trends
others did not (9;13;15-17;19;21;22). However, even in the absence of
significant linear trends, several of the latter studies found that the highest
categories of activity were associated with reduced fracture risk (9;13;15;21;22); the relative reduction in risk
ranged from 20% to 70%. Most of the methods used to categorize level of
physical activity were based on combinations of frequency, duration, and/or
intensity. Of the 3 studies that categorized physical activity by intensity
(e.g., low versus moderate versus vigorous walking pace) (6;7;17), two found
that higher‑intensity activity was associated with reduced fracture risk
Change in Physical Activity
In the Nurses' Health Study (6), the change in
hours per week of leisure-time physical activity was evaluated over the 6-year
interval before the accrual of hip fracture data. A non‑significant trend
(P=0.07) was apparent for women who were the least active (less than 1
hour per week) at the baseline assessment to have a decreased fracture risk if
they reported becoming more active. Conversely, a significant trend
(P=0.004) was seen for the most active women (4 or more hours per
week) at the baseline assessment to have an increased fracture risk if they
reporting a decrease in activity level. Women who decreased their activity
level from 4 or more to less than 1 hour per week had more than a 2-fold
increase in hip fracture risk (RR 2.08; 95% CI 1.20-3.61). Among older women
and men who were performing heavy outdoor work, those who reported a decrease
over a 2.5-year interval had more than a 2.5-fold increase in fracture risk
relative to those who maintained their level of activity (RR 2.7; 95% CI
1.14-6.62) (11). A limitation of the study was that it
could not rule out the decline in physical activity as a consequence, rather
than an antecedent, of the fracture. Among women and men who participated in 3
Danish longitudinal population studies (8), the change in
physical activity over 2 assessment visits was evaluated as a predictor of
future fracture. Participants who had been moderately active and became
sedentary had a significant increase in relative fracture risk (RR 1.53; 95% CI
1.12-2.08). However, those who moved from the sedentary to the most active
group also had a significant increase in fracture risk (RR 1.73; 95% CI
1.10-2.70). A case-control study evaluated change in physical activity from the
recalls of historic (ages 18 to 30 years) and recent levels (18). This approach revealed no significant associations of
either increases or decreases in physical activity with fracture risk.
Studies that have used either quantitative or categorical methods of
discriminating physical activity dose generally support an inverse association
between the level of activity and fracture risk. However, such findings are not
uniform across all studies. There may be sex‑and/or site-specific
benefits that are not adequately captured in the instruments used to assess
physical activity. Limited evidence indicates that decreases in physical
activity result in increased fracture risk over only a few years in older
adults. Evidence that increasing physical activity leads to a reduction in
fracture risk in older adults is lacking.
An advantage of studies conducted in laboratory animals is that the
effects of mechanical loading (i.e., physical activity) to enhance resistance
to fracture (i.e., bone strength) can be assessed in a direct and quantifiable
manner. Such experiments have demonstrated that small increases in BMD and BMC
(e.g., 5% to 7%) translate into very large improvements in resistance to
fracture (e.g., 64% to 94%) (28). In contrast, the larger
improvements in BMD and BMC in response to bisphosphonate (e.g., 14% to 15%)
(29) or parathyroid hormone therapy (e.g., 9% to 13%) (30) result in only proportional improvements in resistance to
fracture (e.g., 7% to 21% and 12% to 17%, respectively). If such findings in
laboratory animals are relevant to human physiology, it suggests that physical
activity plays a critical role in fracture prevention.
Consistency of Findings With Other Recommendations
Observational studies suggest that the minimal levels likely to reduce
fracture risk are 9 or more MET-hours per week of physical activity, 4 or more
hours per week of walking, and 1,290 or more kilocalories per week of physical
activity. These levels are consistent with the current recommendations of the
American College of Sports Medicine (ACSM) and the American Heart Association
(AHA) (3;31) and in the US Dietary
Guidelines (32). However, 2 studies found that relative
risk of fracture was significantly reduced with more than1 hour per week of
activity (18;23), suggesting that
even lower amounts have benefit on bone health. As reviewed in the ACSM
Position Stand on Physical Activity and Bone Health (33), fracture risk may be reduced both by the effects of
physical activity on bone metabolism (weight-bearing endurance and resistance
activities), and by its effects to reduce the risk of falling (resistance,
balance, and flexibility activities). Currently, no evidence is available in
humans that the benefits of physical activity on fracture reduction can be
achieved through multiple short bouts versus a single longer daily bout.
However, studies of animals suggests that multiple short bouts should be more
effective in enhancing bone strength than a single bout (28).
Question 2. Does Physical Activity Reduce
Risk of Osteoporosis by Increasing, or Slowing the Decline in, Bone Mineral
Density or Bone Mineral Content?
Exercise training can increase, or minimize the decrease, in BMD in
clinically relevant spine and hip regions. The magnitude of the effect, when
compared with changes in non-exercise control groups, is approximately 1% to 2
% per year for studies up to 1 year in duration. Studies involving longer
periods of exercise training (i.e., more than 1 year) are sparse, but suggest
that the annual rate of BMD accrual does not persist. Importantly, studies of
animals indicate that small improvements in BMD in response to mechanical
loading (i.e., exercise) translate into very large increases in resistance to
fracture. In contrast, increases in BMD in response to pharmacological therapy
(i.e., bisphosphonates, parathyroid hormone) translate into proportional
improvements in resistance to fracture.
Benefits on BMD have been found to occur in premenopausal women,
postmenopausal women, and adult men; the effects of physical activity on BMD of
children are addressed elsewhere in the report (See Part G. Section
9: Youth). Both weight-bearing endurance and resistance types of
exercise programs have been found to be effective in increasing BMD. A key
determinant of effectiveness is likely whether the exercise program
appropriately targets the skeletal region of interest.
Bone mineral density is the strongest predictor of fracture risk.
Accordingly, many RCTs and non-randomized clinical trials (CTs) have been
conducted to evaluate changes in this biomarker of fracture risk in response to
exercise training, and even more cross-sectional comparisons of BMD in
sedentary versus physically active people and athletes in a variety of sports
and non-athletes have been published.
Because several meta-analyses of these studies have been conducted,
the primary evidence base used to address Question 2 was the meta-analytic
findings (Table G5.A2 [PDF - 178 KB],
which summarizes these studies). It should be noted that 3 of the
meta-analyses included individual subject data (34-36).
The evidence for an effect of exercise training on BMD will be
summarized with respect to whether findings are specific to skeletal region
(lumbar spine [LS], femoral neck [FN], other hip regions), population (i.e.,
premenopausal women, postmenopausal women, men), type of exercise program
(i.e., endurance or impact exercise, resistance or low-impact exercise), type
of study design (i.e., RCT, CT), and dose-response association.
Meta-analyses have most commonly assessed BMD of the LS and FN. Other
sites include the total hip, regions of the hip other than the femoral neck,
the radius, and the os calcis. Because it is fractures of the hip and spine
that are of greatest clinical significance, the discussion will focus on BMD of
these regions. The methods of reporting the overall treatment effect varied
among studies, and included absolute (g/cm2) and relative (%) change
in BMD, annualized relative (% per year) change in BMD, and effect size.
Results will be discussed regarding whether changes in the reported parameters
were statistically significant and, when available, the general relative
magnitude of the effect will be provided.
Lumbar Spine Bone Mineral Density
Of the 15 meta-analyses, 13 evaluated whether an exercise intervention
had a significant effect on LS BMD (34;36-47) (Table G5.A2 [PDF - 178 KB]).
Without regard to the population or type of exercise studied, all but 3 of the
meta-analyses found that exercise intervention resulted in a significant
benefit on LS BMD (36-41;43;45-47). The relative magnitude of the benefit was generally 1
to 2% per year (i.e., difference between exercise and control groups). One
meta-analysis reported a much larger benefit of exercise to increase LS BMD
(10.7%) (45); this will be discussed further in the
population section (adult men) below.
Femoral Neck Bone Mineral Density
The second most commonly assessed skeletal region was the FN (34;35;37-40;42;47). Only
2 of these meta-analyses reported significant effects of exercise training (39;40). The relative benefits of
exercise on FN BMD ranged from 0.5% per year to 1.4% per year.
Total Hip or Femur Bone Mineral Density
Regions of the proximal femur other than the femoral neck that have
been studied were the total hip or what was generically described as the femur
(any subregion) (38;41;42;45;46;48). Significant effects of exercise training on BMD were
reported in 3 meta-analyses, with benefits of 0.4%, 2.4%, and 5.9% (45;46;48).
Meta-analytic studies generally agree that exercise training has
beneficial effects on LS BMD. Although a benefit of 1 to 2 % per year may seem
small, this is roughly equivalent to preventing the decrease in BMD that would
typically occur over 1 to 4 years in postmenopausal women and elderly men. Less
evidence exists for beneficial effects of exercise training on hip BMD. Because
compliance to exercise training studies wanes as the duration of the
intervention increases, the majority of studies have been 12 or fewer months in
duration. The rates of increase in BMD observed in studies of less than 1 year
in duration do not appear to be sustained with longer-duration exercise
training (49). Studies of laboratory animals indicate that
increases in bone mass continue only if the loading stimulus is progressively
increased, but it is unlikely that an exercise program with a continuously
increasing stimulus to bone could be carried out long-term in humans. However,
in adult men and women, an important goal of physical activity is to minimize
age-related declines in bone mass and strength. The extent to which decreases
in BMD with aging can be attenuated through long-term exercise training is not
clear. Recent evidence indicates that increases in BMD in response to a 1-year
exercise training program can be maintained for up to 4 years by regular
Premenopausal Adult Women
Several meta-analyses have either focused exclusively on premenopausal
women or conducted subgroup analyses of premenopausal women (34;37;39-41). Only
one of these studies reported no significant benefits of exercise training on
BMD (34). That meta‑analysis was of individual
subject data, and included only 3 published studies. The other meta-analyses
were generally consistent with the findings summarized above for skeletal
regions of interest.
Because the highest prevalence of osteoporosis is in postmenopausal
women, it is not surprising that the majority of meta-analyses have focused on
this population, either exclusively or in subgroup analyses (35;36;38-44;46-48). Only 3 of these
meta-analyses found no significant benefits of exercise training on BMD (35;42;44). Of
these, one excluded studies that involved any intervention other than exercise,
including calcium supplementation (42), one focused only
on tai chi interventions (44), and one evaluated
individual subject data (35). The remaining meta-analyses
involving postmenopausal women were consistent with the findings summarized
above for skeletal regions.
Fewer RCTs and CTs of the effects of exercise training on BMD have
been conducted in men than in women. The only meta-analysis of studies of men
included 2 RCTs and 6 CTs; the studies evaluated BMD at any skeletal region (45). The overall effect size (ES) of 0.028 was not
significant, but was equivalent to a difference in BMD of 2% between exercisers
(1.6%) and controls (-0.4%). Thus, the magnitude of the overall effect was
similar to what has been observed in women. Subgroup analysis for age revealed
a significant ES (0.605) for men older than aged 31 years (4.2% in exercisers
versus -2.5% in controls), but not for men aged 31 years or younger (ES 0.066).
Subgroup analysis by skeletal region revealed significant ESs for the LS (5.8%
in exercisers vs. -4.9% in controls) and the femur (4.0% in exercisers vs.
-1.9% in controls).
Because only one meta-analysis of studies of men has been published,
the Musculoskeletal Health subcommittee also considered RCTs of the effects of
exercise training on BMD in men published after the meta-analysis (50-54). Only one of these studies reported significant
exercise-induced increases in BMD (51). In that study, 24
weeks of progressive high‑intensity resistance training resulted in
greater gains in LS and whole-body BMD than did moderate-intensity resistance
training. The ineffectiveness of exercise training to increase BMD in 3 of the
other studies was likely because they were conducted at only low to moderate
exercise intensities (50;53;54) and because intensity was not progressively increased (50;54). The study by McCartney and
colleagues (52) involved a progressive
high‑intensity resistance training program, but did not result in
significant increases in BMD. However, in that study, half of the 6 resistance
exercises that were performed involved relatively small muscle groups (i.e.,
ankle dorsi- and plantarflexion, arm curls) that would not be expected to have
a major influence on clinically important regions of the skeleton. Thus, the
volume of exercise performed that would be predicted to have favorable skeletal
effects was low.
Meta-analytic findings indicate that adult women and men can increase
BMD at clinically important skeletal regions through exercise training. Two
analyses that included both pre‑and post-menopausal women found similar
relative effects of exercise training on LS and FN BMD in both populations (39;40). The other analysis that included
both pre- and postmenopausal women found similar relative effects of exercise
training on LS BMD, but effects on FN BMD in postmenopausal women only (41).
Although some subgroup analyses have suggested relatively greater effects of
exercise on BMD in men than in women, this must be interpreted cautiously. One
of the RCTs was a study of the effectiveness of resistance training (RT) to
increase BMD in men following heart transplantation, and both the decreases in
BMD of controls and the increases in BMD of exercisers were of relatively
greater magnitude than is typically observed in healthy cohorts.
Type of Exercise Program
Some of the meta-analyses evaluated effects of the type of exercise
training, either by restricting inclusion to certain types of exercise programs
(34;37;38;41;43;44;47;48) or by conducting subgroup
analyses (40;46). The types of
exercise programs have generally been categorized as either endurance (i.e.,
aerobic) training (ET), with an emphasis on weight‑bearing activities, or
RT (i.e., weight lifting). One meta-analysis focused specifically on impact
versus low-impact exercise training (40); the exercise
programs were aligned with the ET (i.e., impact) and RT (i.e., low-impact)
categories referred to below. In general, exercise programs can be categorized
as to whether they introduce stress to the skeleton primary through
joint-reaction forces (i.e., low-impact, strengthening exercises) or
ground‑reaction forces (i.e., impact).
The meta-analyses that restricted inclusion to studies of ET have
found beneficial effects only on LS (43) and hip (48) BMD. One meta-analysis included only studies of walking
and found a significant effect on LS BMD, but not FN BMD (47).
Four meta-analyses restricted inclusion to studies of RT (34;37;38;41). Three found a significant effect of RT on LS BMD (37;38;41); the one
that did not was a meta-analysis of individual subject data (34). None of the analyses found significant effects of RT on
BMD of the FN (34;37;38) or other hip regions (41).
Endurance Training versus Resistance Training
Two meta-analyses included studies that involved either ET or RT
exercise programs and conducted subgroup analyses by exercise type (40;46). When considering any regional
BMD measurement (LS, radius, femur regions), Kelley found a significant overall
effect of RT (0.7%) but not ET (46). In contrast, Wallace
and Cumming found significant effects of both ET and RT on LS and FN BMD in
postmenopausal women and on LS BMD in premenopausal women (40). They found no effect of ET on FN BMD in premenopausal
women and the available data were not adequate to evaluate the effect of RT on
Evidence indicates that both ET and RT types of exercise programs can
increase BMD at both the LS and hip in adults, but this is not a consistent
finding across all meta-analyses. In particular, study findings differ as to
whether RT has beneficial effects on BMD of hip regions. This would be expected
if RT programs did not include exercises that specifically involved the
musculature in the hip region, particularly because many of the exercises that
target other major muscle groups are commonly performed in the seated position
(i.e., very little load on the FN and other regions of the proximal femur).
Type of Study Design
The majority of meta-analyses included studies in which the assignment
to exercise and non‑exercise control groups was either randomized (RCTs)
or non-randomized (CTs) (35-37;39;41-45;47;48). Three
included only RCTs (38;40;46) and 1 meta-analysis of individual data was generated from
only CTs (34).
Randomized Controlled Trials Only
All of the meta-analyses that restricted inclusion only to RCTs found
beneficial effects of exercise training on LS BMD (38;40;46); 2 also found significant effects
on BMD of hip regions (39;46).
Randomized Controlled Trials versus Non-Randomized Clinical Trials
Studies that evaluated whether outcomes differed by study design had
discordant findings. Wolff and colleagues (39) reported
that increases in LS and FN BMD were 1.5- to 2-fold greater in CTs (1.85 % per
year, 1.39 % per year) than in RCTs (0.84 % per year, 0.89 % per year). Kelley
(48) found significant increases in hip BMD in CTs, but
not RCTs, but in another report (43), type of study design
was not a significant determinant of the increase in LS BMD. Although the
meta-analysis of studies of men found that increases in BMD were larger in
RCTs, this finding appeared to be influenced strongly by the study of heart
transplant patients (see discussion above). Finally, Kelley and colleagues (41) reported that study quality was a determinant of the
increase in hip, but not LS, BMD, with higher quality studies demonstrating a
benefit. Randomization is one characteristic that contributes to high quality,
but other factors include blinding and attrition.
It is not clear whether non-random assignment to exercise and
non-exercise groups results in an over-inflation of the effects of exercise
training on BMD. Importantly, meta-analyses that restricted inclusion to RCTs
reported favorable effects.
The meta-analyses provided no evidence for dose-response effects of
exercise training on BMD. In some cases, when a study included two exercise
groups that were distinguished by exercise intensity, the meta-analyses
included only the more intensive group (40;47). Several of the meta-analyses by Kelley and colleagues
evaluated characteristics of the exercise programs (e.g., duration, intensity,
compliance) using regression or correlation analyses, but none of these yielded
significant results (36;41;43;45;46;48). However, one of the larger RCTs (n=140) of the effects
of resistance exercise training on BMD of postmenopausal women found a positive
association between volume of weight lifted and the change in BMD (55).
Consistency of Findings With Other Recommendations
The findings from meta-analyses of the effects of exercise
intervention on BMD and BMC did not reveal dose-response effects. However, many
of the intervention trials included in the systematic reviews involved a volume
of exercise that is consistent with the current recommendations of the ACSM and
the AHA (3;31) and in the US Dietary
Guidelines (32). The ACSM Position Stand on Physical
Activity and Bone Health (33), which was based on
narrative review and consensus opinion, suggested that adults should
participate in weight-bearing endurance activities 3 to 5 days per week and
resistance activities 2 to 3 days per week at a moderate to high intensity (in
terms of bone-loading forces) to increase, or prevent excessive loss of, bone
mass. The current review did not reveal any evidence to suggest that the
recommendation is inappropriate or should be modified.
Question 3. Does Physical Activity Reduce
or Increase the Incidence of Osteoarthritis?
In the absence of major joint injury, no evidence exists to indicate
that regular moderate to vigorous physical activity in amounts that are
commonly recommended for general health benefits increases the risk of
developing OA. In addition, limited, weak evidence is available from
observational and animal studies to suggest that low-to-moderate levels of
recreational physical activity, particularly walking, may provide protection
against the development of hip and knee OA.
Osteoarthritis is a relatively common degenerative condition of the
hyaline cartilage lining the joints and affects nearly 27 million US adults,
manifested most commonly in the knee and hip (4).
Characterized clinically by joint pain, swelling, stiffness, and weakness, OA
often results in increased disability and significant negative personal effects
on physical function, mental health, and quality of life. Known major risk
factors for OA include genetic predisposition, older age, female sex, history
of joint injury, occupational load, and excess body mass (56-60). Historically, the "wear and tear" theory of joint
degeneration suggests that excess force on the joint cartilage, such as
accumulates from vigorous sports and occupational and daily living activities
may initiate the pathophysiological process that results in clinical OA (61). However, some level of physical activity is essential
for joint health. Thus, the physical activity guidelines for Americans should
include a level of movement or activity to ensure good joint health, while
minimizing potential deleterious forces.
The Musculoskeletal Health subcommittee examined the scientific
evidence from observational epidemiologic studies that have assessed some
measure of physical activity exposure before a determination of the OA status.
In selecting studies from the Scientific Database, the subcommittee used the
following criteria, which were thought to be most helpful in informing the
development of physical activity guidelines for Americans: 1) included
case-control or longitudinal cohort study design, 2) included participants
typical of the general community (not specialized subpopulations of elite
athletes), and 3) assessed and/or classified exposure in relation to the usual
types and amounts recommended for general health benefits (3;31). A total of 12 studies (8
longitudinal cohort, 4 case-control) were used to address the research
Also examined were studies of elite, high-level athletes in specific
sports activities to qualitatively assess those activities that may be
associated with an excess risk of incident OA. Although not representative of
the general population, studies of former elite and professional level athletes
provide insights that may be useful in informing physical activity guideline
development. Select sports have an increased risk of incident OA by virtue of
such factors as the inherent risk of joint injury, the extent of impact forces
delivered to specific joints, and/or the length of time and level of play while
participating in the sport. We identified 16 studies of elite athletic
populations representing a variety of sports and activities.
Data from 12 observational epidemiologic studies suggest that no clear
evidence exists that regular participation in moderate- to vigorous-intensity
PA, in amounts commonly recommended for general health, infer a significant
risk of incident lower-extremity OA (Table G5.A3 [PDF - 150 KB], which
summarizes these studies). Weak evidence indicates that walking and select
other low‑impact activities may protect against the development of OA (Table G5.1).
Five of 8 cohort studies and 3 of 4 case-control studies reported at
least 1 measure of association below 1.0. For example, in a longitudinal study,
participation in cross-country skiing, walking, or swimming was associated with
statistically significant protection against OA (62).
Theoretically, this is aligned with laboratory animal and human research
showing that exercise in moderate amounts results in beneficial changes to
hyaline cartilage (greater surface area, volume, glyccosaminoglycan content),
synovial fluid nutrition and distribution, and quality and strength of muscles
surrounding the lower extremity joints, possibly without increasing the
presence of knee cartilage defects (63-66). These changes
may improve the shock absorption ability, thereby reducing forces transmitted
to the joint cartilage.
Two longitudinal studies reported potential protective effects of
walking on joint health. One (67) reported odds ratios of
0.96 (95% CI 0.57-1.62) and 0.78 (CI 0.49-1.24) for incident radiographic,
symptomatic knee OA in adults who walked less than 6 versus more than 6 miles
per week, respectively. In the other study (68), women who
walked more than 5 miles per week had significantly less joint space narrowing
(OR 0.38, CI 0.15-0.93) than did women who walked less than 5 miles per week
(Table G5-1). A nested case-control.
Table G5.1. Studies Examining the
Association Between Participation in Walking and Risk of Hip/Knee
||Measure of Association OR
|Hart et al., 1999 (68)
1. Joint space narrowing
2. Osteophyte formation
No = less than 5 miles per week
Yes = more than 5 miles per week
|Joint Space Narrowing:
No = 1.0 (referent)
Yes = 0.38 (0.15 – 0.93)
No = 1.0 (referent)
Yes = 0.60
(0.22 – 1.71)
|McAlindon et al., 1999 (69)
||Radiographic knee OA
||Number of city blocks walked per day
||None = 1.0 (referent)
≥4 = 1.2 (0.4 –
|Manninen et al., 2001 (62)
||Knee arthroplasty surgery
||Regularly performed exercise for at least 2 years?
Walking = Yes/No
No = 1.0 (referent)
Yes = 0.17 (0.02
No = 1.0 (referent)
Yes = 0.32 (0.16 –
|Manninen et al., 2002 (70)
||Knee arthroplasty surgery
|Low = 1.0 (referent)
Medium = 1.0 (0.65 –
High = 1.06 (0.68 – 1.64)
|Felson et al., 2007 (67)
||Radiographic, symptomatic knee OA
||Do you walk for exercise?
|No = 1.0 (referent)
<6 = 0.96 (0.57 –
>6 = 0.78 (0.49 – 1.24)
CI, confidence interval; OA, osteoarthritis; OR, odds
* No details were provided on the question used to
determine walking in Hart el al (68). However, another
published paper from the same cohort described the walking variable as less
than versus greater than 5 miles per week.
study (71) did not examine walking in isolation,
but classified physical activity by the amount of joint stress. Women who
participated in activities requiring low joint stress, which included walking,
cycling and swimming, had a 42% (OR 0.58, CI 0.34-0.99) lower risk of hip/knee
OA than did women who were inactive.
However, some select groups of persons may have a moderately elevated
risk of OA due to long-term participation in high-impact activities (Table G5.2).
Table G5.2. Select Individual Sports and
Recreational Activities That Have Been Associated With the Development of
Osteoarthritis in at Least One Study
Associated With Incident OA
Not Associated With Incident OA
Track and Field
Australian Rules Football
. Ice hockey
Tennis (OA in hip/knee)
For example, competitive athletes who
participate and train at high levels (e.g., elite, professional sports,
National Teams, Olympic athletes) in sports requiring high joint impact (e.g.,
football, track and field, soccer) for many years have higher rates of incident
knee or hip OA than do non-athletes (Table G5.A3 [PDF - 150 KB], which
summarizes these studies). Increased risk of OA has been reported in one or
more studies for the following sports: football (Australian rules), soccer,
track and field, basketball, boxing, ice hockey, orienteering running,
wrestling, tennis, ballet, and handball (see Part G. Section 10: Adverse Events or
a discussion of muskuloskeletal injuries related to these sports). The
increased risk of OA in athletes in these sports may be attributed, in part, to
joint injuries, because these sports are also associated with the highest rates
of joint injuries (72;73), which is a
strong risk factor for incident OA (57-59). In addition,
persons who have occupations that require excessive knee bending, kneeling, or
twisting/torsion movements or involve high-load weight bearing (lifting and
carrying heavy loads) and who also participate in moderate or vigorous
recreational activity may have increased risk for lower-extremity OA due to the
additive effects over time (69;74).
Women have a higher prevalence and incidence of most types of OA (57;75). Women also have lower quadriceps
muscle strength, one of the main muscles supporting the hip and knee (76;77), different anatomical and
biomechanical structure (78;79),
higher rates of obesity (80), and participate in different
types of physical activity than do men (81), and have
different risks of injury even in similar sports (72;73). All these factors can influence the risk of OA related
to physical activity, suggesting that the relationship may be sex-dependent.
For example, quadriceps muscle strength has been shown to be an independent
risk factor for the development of hip and knee OA even after controlling for
excess body weight, age, activity level, injury status, and physical fitness
(76). In fact, the weak protective effect of physical
activity participation seems to be stronger among women than men (62;68;70;71;82). Both Rogers and colleagues (71) and Manninen and colleagues (62)
reported that low and high levels of accumulated physical activity were
protective for OA among women (not all were statistically significant due to
small sample sizes), but only high levels were protective among men. A later
study by Manninen and colleagues (70) also reported a
protective effect on severe knee OA among men and women combined. Because that
study was a matched (age and sex) case-control design, the independent effect
of sex could not be estimated.
Excess Body Mass
It has been demonstrated that overweight and obese individuals put
more stress on their lower-extremity joints during normal ambulation than do
normal-weight individuals. This suggests that overweight and obesity would
exaggerate impact forces transmitted to the joint during exercise and
recreational physical activity, potentially increasing the risk of developing
OA. However, evidence suggests that elevated body mass index (BMI)
independently predicts incident OA, and that physical activity does not
contribute significantly to this increased risk (67).
Physical activity plays an integral role in both weight loss and the
maintenance of normal body weight. Currently, no evidence supports the
possibility that promoting activity in the general US population, even among
those who are overweight or obese, will increase risk for OA.
Previous joint injury is a well-established, independent risk factor
for OA. In fact, athletes who sustain major joint injuries, such as anterior
cruciate ligament ruptures, and undergo surgical reconstruction have premature
onset OA (about 10 years early) compared with non‑injured athletes (83-86). Athletes in some sports that involve relatively high
joint impact (e.g., soccer) and who do not suffer a major joint injury do not
seem to have excessive rates of incident OA (87). However,
in other sports (e.g., Australian Rules Football), both players with and
without previous knee injuries had an increased risk of radiographic knee OA
Not all studies included in
Table G5.A3 [PDF - 150 KB]
controlled for previous injury. Three studies that reported an increased risk
of OA associated with the highest level of physical activity (74;82;88) did not
control for previous joint injury, which may explain some of the excess risk.
Sutton and colleagues (89) reported an increased risk of
knee OA with regular long walks (at least 2 miles at least 1 time per week),
but this association was no longer significant after controlling for previous
knee injury. McAlindon and colleagues (69) reported a
significant effect of more than 3 hours per day of heavy physical activity
(combined occupational, recreational, household and transportation domains) on
symptomatic knee OA incidence, even after controlling for previous joint
injury, BMI, age, sex, and other potential confounders. This finding is
difficult to place into context in today's society. Because of changing job
demands and increased technological advances in high-risk occupations (e.g.,
manufacturing, farming), it is likely that only a small fraction of the current
US population accumulates more than 3 hours of heavy physical activity per
Study Design Issues
It is interesting that the few studies that reported significant
protective effects of physical activity on OA incidence were case-control study
designs (one was a nested case-control within a longitudinal cohort).
Case-control studies are strong and efficient study designs when an outcome is
rare. However, OA is a common condition when compared with the incidence of
some types of cancer or even diabetes. Therefore, some biases inherent to
case‑control studies (e.g., recall bias, lack of representative controls)
(90) may have influenced the findings. This issue remains
unclear, because 2 prospective cohort studies (67;91) also reported measures of association that were below the
referent level, although not statistically significant, suggesting a possible
protective effect for some groups.
Last, observational study designs such as these cannot determine cause
and effect. However, conducting an RCT to investigate the influence of
different exercise participation on the rates of incident OA is not feasible
due to the long incubation period for OA development and the potential ethical
problems of randomizing persons to inactivity.
Some of the inconsistent findings also may be related to the methods
used to collect and analyze self-reported data. Historically, instruments used
to query physical activity behavior were designed to study the relation between
activity and cardiovascular or mortality outcomes. Hence, many instruments are
geared more toward how physical activity may affect the cardiorespiratory
system versus the effects it may have on the musculoskeletal system. As a
result, the bone and joint loading effects of physical activity may be missed
in these studies. For example, jogging and swimming may be rated at the same
MET level based on their cardiovascular effects, yet these two activities are
very different in terms of loading delivered to the muscles, bones and joints.
Hootman and colleagues (91) attempted to address this
issue in part by applying a "joint loading stress score" to the self-reported
data. However, the effects of joint loading physical activity on incident hip
and knee OA were still difficult to identify, even in this relatively large
longitudinal study. Future research should focus on teasing out the
musculoskeletal effects from the cardiovascular effects in an attempt to
identify the types of activities involving high joint loading that may be
associated with increased risk of OA.
Another study design issue is the inconsistent definition of incident
OA. Various outcomes were used across studies including self-reported
doctor-diagnosed OA, radiographically-determined OA (with and without
symptoms), and incident hospitalization for joint replacement surgery. It is
not known how these different definitions may affect the measures of
Consistency of Findings With Other Recommendations
Our findings are not fully consistent with the results of a systematic
review of sporting activities on the development of hip OA (92) or the OASIS group (93), but do
align with the American Gerontological Society Consensus Guidelines for
Lievense and colleagues (92) reported that
moderate evidence exists that participation in a combination of team sport and
running activities is positively associated with the development of hip OA. In
addition, they reported conflicting evidence for ballet and soccer
participation and limited evidence for general athletics. This systematic
review included some of the studies reported in
Table G5.A3 [PDF - 150 KB], but
also included studies published before 1995, the beginning point of this
evidence synthesis. Studies completed before the early 1990s may have included
subjects who were inherently different from more contemporary cohorts. Also,
Lievense and colleagues (92) noted that 4 of the older
studies scored very low in terms of study quality (less than 40 on a 100 point
scale), which may have contributed to the disparate findings.
The OASIS group (93) stated that considerable
scientific evidence indicates that sport is a risk factor for OA of the knee
and hip, and that the risk correlates with frequency, duration, and level of
play. This is consistent with the evidence presented in
Table G5.A3 [PDF - 150 KB].
However, the OASIS group did not specifically address participation in general,
moderate-intensity physical activity. The OASIS summary recommendations also
stated that joint injury and excess body mass are much stronger risk factors
for OA than sports participation. They further recommended that the high-level
athlete should be informed of the risk of OA associated with sports and
counseled regarding protecting joints from trauma and maintaining optimal body
weight. This guidance is an important risk communication message for any person
engaging in high-level sports activity over many years.
In the absence of joint injury, participation in recreational or
leisure physical activities at levels commonly recommended for general health
benefits does not increase the risk of developing OA. However, long-term
high-level participation in select high-impact sports (e.g., football, soccer,
track and field) may be associated with increased risk of OA. As such, health
promotion messages should be developed to inform persons choosing to
participate in such activities that they may have increased risk for OA, and
that modifying other OA risk factors (e.g., maintaining normal body weight,
preventing joint injuries) may help to lower risk.
Question 4. Is Physical Activity Harmful
or Beneficial for Adults With Osteoarthritis or Other Rheumatic
Strong evidence indicates that both endurance and resistance types of
exercise provides considerable disease-specific benefits for persons with OA
and other rheumatic conditions without exacerbating symptoms or worsening
disease progression. Adults with OA can expect significant improvements in
pain, physical function, quality of life and mental health and delayed onset of
disability by engaging in appropriate low-impact physical activity for
approximately 150 minutes per week (3 to 5 times per week for 30 to 60 minutes
per session). No evidence indicates that OA is a contraindication for
participation in physical activity among sedentary populations. However,
patients should be counseled to pursue activities that are low impact, not
painful, and do not have a high risk of joint injury.
More than 46 million adults in the United States have arthritis or
another rheumatic conditions and almost 40% of them are limited in their usual
activities by their condition (95). As a result of the
aging of the population, the prevalence of arthritis is expected to grow to 67
million by the year 2030 (96), and more than 44% of adults
with arthritis are sedentary (97). Because adults with
arthritis make up a significant proportion (21%) of the general US population
(95) and have disease-specific barriers (e.g., pain,
fatigue) to initiating and maintaining physical activity (98-100), Federal authorities should consider this patient
population in the physical activity guideline development process.
To evaluate the evidence regarding the disease-specific benefits of PA
among adults with arthritis, the Musculoskeletal Health subcommittee examined
RCTs published since 1995 (Table G5.A4 [PDF - 199 KB], which
summarizes these studies). These studies met the following criteria: 1)
included only patients with arthritis or another rheumatic condition (e.g., OA,
rheumatoid arthritis, fibromyalgia, lupus, gout), 2) compared an exercise group
(i.e., endurance and/or resistance exercise) with a non-exercise control group,
3) reported adequate information on the intervention (e.g., type, frequency,
duration), and 4) reported patient-oriented outcomes such as pain, physical
function, quality of life, and disability. Studies that described a
clinically-delivered exercise intervention (e.g., therapeutic physical or
occupational therapy) were excluded.
Table G5.A4 [PDF - 199 KB] includes
findings of 24 exercise intervention studies (15 endurance,
9 resistance, and 5 combined endurance plus resistance training). Interventions
were included if the exercise program described could feasibly be replicated in
community settings (e.g., group exercise classes, home programs) even if they
were supervised by health care or research professionals such as a nurse,
physical therapist, or exercise physiologist. The 15 endurance exercise studies
represented 17 actual exercise versus non-exercise control comparisons, because
2 studies (101;102) had multiple
endurance exercise groups. Both endurance and resistance exercise training
programs demonstrated effectiveness for reduced pain, improved function, and
additional benefits on quality of life, mental health, self‑efficacy
(confidence), and delayed onset of disability in ADLs.
Components of the Exercise Prescription
Table G5.3 summarizes characteristics of
the exercise RCTs among those with arthritis or other rheumatic conditions.
Many studies did not measure the actual dose of exercise
delivered during the course of the intervention, but prescribed doses
of exercise across all 24 studies averaged 146 minutes per week of
moderate-intensity exercise, such as walking, cycling, tai chi, and water
aerobics. Average frequency (2.8 days per week) and duration of exercise
sessions (51.8 minutes per day) were consistent with current recommendations
for people with arthritis (2003), and with recommendations for the general
adult population in the United States (3;31). The length of the interventions varied considerably,
ranging from 8 to 104 weeks.
Endurance Exercise Versus Control
The 15 endurance exercise studies (17 comparisons) included
participants with OA (n=12), fibromyalgia (n=4) and rheumatoid arthritis (n=1).
The modes of exercise, all moderate intensity, included walking (n=5), tai chi
(n=5), water exercise (n=2), aerobics class (n=2), and cycling (n=1).
Participants exercised in small groups or at home for an average of 2.9 times
per week and 48 minutes per session for a total average of 137 minutes per
week. Endurance interventions lasted an average of 23.9 weeks (range, 8 to 72
weeks). Sample sizes were variable, with an average of 50 subjects in the
exercise arm and 45 in the control arm. Only 1 trial, the Fitness Arthritis and
Seniors Trial (3 separate reports (103-105), had more
than 100 subjects in both the exercise and control arms.
Pain reduction and improvements in physical function were reported in
the majority of studies of endurance exercise. Other benefits included improved
self-efficacy (confidence), quality of life, muscle strength, mental/emotional
health, and physical activity levels. No increases in symptoms (pain, fatigue,
stiffness) or other measures of disease activity (e.g., global rating,
radiographic progression, inflammatory markers) were demonstrated. In fact,
Schachter and colleagues (102) reported decreased disease
severity (physician global rating of severity and Fibromyalgia Impact
Questionnaire total score) in response to exercise training for subjects who
adhered to both long-bout (one 30-minute bout per day) and short‑bout
(two 15-minute bouts per day) programs.
Table G5.3. Summary Descriptive
Characteristics of the Randomized Controlled Trials of Exercise Among Persons
With Arthritis or Other Rheumatic Conditions
||Number of Studies
Characteristics of Interventions
Number of Intervention Subjects
Characteristics of Interventions
Number of Control Subjects
Characteristics of Interventions
Length (Weeks) of Intervention
Characteristics of Interventions
Frequency Per Week
Characteristics of Interventions
Duration (Min) Per Session
Characteristics of Interventions
Total Prescribed Dose (Min/Week)
(Number of Studies/Outcome)
Endurance versus Control
8 ↑ function
1 ↑ quality of
4 ↑ self-efficacy
4 ↑ muscle strength
3 ↓ symptoms (other than pain)
4 ↑ mental
/ emotional health
5 ↑ or no change in symptoms /
Resistance versus Control
5 ↓ pain
5 ↑ function
6 ↑ muscle
3 ↓ stiffness
3 ↓ disease activity
1 ↑ ROM
Combination versus Control
1 ↓ pain
2 ↑ function
2 ↑ muscle
2 ↑ fitness / perceived exertion
2 ↑ no change in
1 ↑ mental health
1 ↓ body weight
* All studies implemented exercise interventions of at
least moderate intensity.
†The endurance group had 15 individual studies,
but 17 actual exercise versus control comparisons.
‡ Review included 24 individual studies, 2
studies compared multiple exercise groups versus a non-exercise control group
and may be counted separately under the rows for the endurance, resistance, and
Resistance Exercise Versus Control
The 9 resistance exercise studies included patients with OA (n= 5),
rheumatoid arthritis (n=3), and fibromyalgia (n=1) who exercised in groups at a
clinic or other exercise facility (n=7) or at home (n=2). Seven studies used
isotonic (i.e., dynamic resistance exercise involving concentric and eccentric
actions) and 2 used isokinetic (i.e., variable resistance, constant velocity)
resistance training modes. Exercise occurred an average of 2.6 times per week
for 52.5 minutes per session, accumulating an average of 145 minutes per week.
The duration of resistance interventions ranged from 8 to 96 weeks (average
50.9 weeks). The average number of subjects in the exercise arms was 54 versus
55 in the control arms. Only one trial, the Fitness Arthritis and Seniors Trial
(3 separate reports (103-105) had more than 100 subjects
in both the intervention and control groups.
Benefits of resistance exercise for adults with arthritis included
improvements in muscle strength, symptoms (pain and stiffness), and function.
Reduced risk of incident disability in ADLs and improved measures of disease
activity also were noted. Using two common measures of disease activity
(Disease Activity Score 28 [DAS28], which captures joint tenderness, patient
global rating of health, pain visual analog scale and erythrocyte sedimentation
rate, and the Larsen Score, which measures radiographic damage), 2 studies of
patients with RA reported significant improvements in DAS28 scores in response
to resistance training (106;107)
and no worsening of the Larsen Score (106).
Combined Interventions Versus Control
The 5 studies that examined a combined endurance and resistance
intervention included patients with OA (n=4) and RA/inflammatory arthritis
(n=2) patients. The mode of endurance exercise was walking in 3 studies and
cycling in 2 studies. The mode of resistance exercise was either isotonic (n=3)
or isokinetic (n=1). One study did not report mode. Combined interventions
occurred on average 3 days per week and averaged 55 minutes per session, for a
total average weekly dose of 156 minutes per week. The average duration of the
combined interventions was 44 weeks (range 12 to 104 weeks). The average number
of subjects in the combined exercise arm was 62 versus 64 in the control arm.
Munneke and colleagues (108) and de Jong and colleagues
(109) were the only studies that had more than 100
subjects in each group.
Benefits of intervention programs that included both endurance and
resistance exercise have been similar to those reported for endurance-only and
resistance-only interventions. The benefits include reduced pain and improved
function, muscle strength, fitness, and mental health, with no increase in
disease activity or symptoms. Weight loss and improved satisfaction with
function also were reported benefits. Specifically, the Arthritis, Diet, and
Activity Promotion Trial (ADAPT) (110) noted that the
endurance plus resistance exercise arm reduced body weight by 2.6% compared to
1.3% in the education control arm.
Appropriate Physical Activity Type and Dose
The exercise prescriptions in the reviewed studies varied widely on
the frequency, duration, intensity and type of physical activity. Thus, it is
difficult to define either a minimum dose of activity that results in clinical
benefits for adults with arthritis or a maximum dose that may be associated
with increased symptoms or adverse events. The average minutes per week of
activity prescribed in these studies (146 minutes per week) suggests that a
prescription of 5 days per week for 30 minutes per session is likely
appropriate for most people with arthritis. All reviewed studies prescribed
moderate to vigorous intensity and low-impact activities. However, it is
unclear whether some persons with arthritis can tolerate higher-impact
activities, such as team sports or tennis. It seems appropriate, given the
evidence, to guide persons with arthritis toward low-impact, moderate-intensity
activities, such as walking, cycling, water exercise, and tai chi.
In fact, walking may be a particularly relevant exercise mode for
persons with arthritis, especially in terms of disability prevention and
safety. Walking was the exercise mode of choice for 9 studies (6 endurance and
3 combined), and those studies reported benefits in terms of reduced pain and
improved function among persons with rheumatic conditions. No true
dose-response studies have been conducted, but evidence does suggest that
higher compliance to endurance and/or resistance exercise was associated with
better outcomes, including less disability and pain and improved physical
function. Ettinger and colleagues (103) used walking as
the primary endurance component of the intervention and reported on global ADL
disability, an important patient-oriented outcome measure. The walking group
reported a significant 10% lower ADL disability score and the resistance
training group an 8% lower score compared to the control group. A follow-up of
this study cohort (105) found that endurance exercise
resulted in a 37% reduced risk of incident ADL disability and that resistance
exercise resulted in a 40% reduced risk. These studies are important to
highlight because of several critical design elements that are central to high
study quality (111): 1) large number of subjects
(endurance = 144, resistance = 146, control = 149), 2) use of an appropriate
randomization protocol, 3) concealment of allocation to randomized groups, 4)
low loss-to-follow-up (83% completed study), 5) adequate adherence to the
assigned intervention (approximately 69%), and 6) use of an intent-to-treat
analysis. In addition, Ettinger and colleagues (103)
reported adverse events related to the intervention, including 2 in the
endurance exercise group, 3 in the resistance exercise group, and 1 in the
control group; only 2 of the 6 reported events resulted in injuries (1 in the
endurance group, 1 in the resistance group).
Important Outcome Measures
A recent expert consensus document from the international group,
Osteoarthritis Research International (OARSI), reported 25 evidence-based,
patient-focused, recommendations for the management of knee and hip OA. (112) One of the 11 non-pharmaceutical OARSI recommendations
states that all patients with hip and/or knee osteoarthritis should be
counseled to engage in aerobic, resistance/strengthening, and range-of-motion
exercises. This recommendation was supported by the highest level of evidence
rating (1a — based on meta-analyses of RCTs) and had a ‘strength of
recommendation' rating of 96 (using a 0 ‑ 100 visual analog scale). OARSI
reported the effect of exercise on pain relief as moderate, as pooled effect
sizes reported were 0.52 (95% CI 0.34-0.70) for aerobic exercise and 0.32 (95%
CI 0.23-0.42) for resistance exercise.
Physical Activity Level
Even though the prescribed doses of physical activity in the studies
included in Table G5.3 approached 150 minutes per
week, a dose consistent with current recommendations, only 2 studies measured
actual levels during the intervention (113;114). Both studies suggested that the interventions did,
indeed, increase actual activity levels. However, without monitoring the actual
participation, it is difficult to determine whether the intervention was
ineffective or whether a lack of effect was related to an insufficient increase
in activity. Persons with arthritis are known to have disease-specific
barriers, particularly joint pain, to being physically active (98-100). If an exercise intervention protocol does not
adequately address pain fluctuation during exercise, then persons with joint
pain and stiffness may drop out at high rates, have lower compliance to the
prescribed dose, or not respond to the intervention protocol as expected.
Quality of Life
Thirteen studies measured quality of life outcomes using various
instruments, and 9 of those reported benefits, mostly in terms of the function
component of quality of life. Quality of life, a concept that includes
physical, mental, and emotional elements, is particularly important for people
with arthritis. Arthritis is not typically associated with excess mortality, as
are cardiovascular and other chronic diseases. However, it is associated with
pain, functional limitation, work disability, and loss of participation in
valued life activities, which severely affect quality of life. These results
suggest that adequately measuring quality of life as a primary outcome measure
in arthritis interventions should be a priority.
Only 2 of 24 studies (103;105) included a measure of disability, as defined by the
authors. In terms of self-reported disability outcomes, the OARSI
recommendations report pooled effect sizes for self-reported disability of 0.46
(95% CI 0.25-0.67) for aerobic exercise and 0.32 (95% CI 0.23-0.41) for
resistance exercise (112). The International
Classification of Functioning and Disability model purports participation
restriction as an important concept to capture in health studies. Participation
restriction goes beyond limitation in specific activities (e.g., climbing a
flight of stairs, rising from a chair) by placing the activity limitation in
the context of a social role (115). For example, not
being able to play the piano (activity limitation) would be a significant
disability (participation restriction) for a concert pianist (social role), but
not for someone who does not play the piano. Therefore, it is equally important
to include reliable and valid measures of function/activity limitation
(self‑report or performance-based), as well as measures of participation
restriction in studies of arthritis treatment interventions. Participation
restriction was not an outcome measure in any of the reviewed studies.
Few studies reported adverse events, even though the CONSORT
guidelines state it is important to report even minor adverse events from RCTs
(111). However, at least 14 studies reported that
arthritis symptoms (pain and/or stiffness) were improved, or at least not
worsened, with exercise and at least 4 studies reported improvement or no
increase in disease activity. Of the 2 studies that did report
intervention-related adverse events, Ettinger and colleagues (103) reported that only 2 of 6 events resulted in injury, 1
each in the endurance and resistance exercise groups, and Coleman and
colleagues (116) reported no major musculoskeletal
adverse events. In addition, Fransen and colleagues (101)
reported that 4 participants dropped out of the study, 2 due to aggravation of
knee pain (both in the tai chi group) and 2 due to low back pain (1 each in the
tai chi and hydrotherapy groups). These reviewed studies, as well as others (117), noted that the frequency of study-related adverse
events were low among arthritis patients and older adults in general. This
suggests that the promotion of moderate physical activity, such as walking,
cycling, and water exercise, is likely safe in patients with arthritis.
However, risk communication messages geared for this population should include
concepts such as "start low and go slow."
Consistency of Findings with Other Recommendations
The above recommendations agree with the OARSI expert consensus
guidelines (112), the OASIS statement (93), the American Geriatrics Association Consensus Practice
Statement (94), and the MOVE Consensus (118). All 4 of these consensus documents recommended that
adults with OA participate in moderate-intensity, low-impact exercises with low
risk of injury. Both endurance and resistance exercises are recommended,
accumulating approximately 150 minutes per weeks, delivered either in group or
home settings, 3 to 5 times per week for 30 to 60 minutes per session. The
recommendations also are aligned with disease management guidelines of the
American College of Rheumatology and the European League Against Rheumatism
(EULAR) (119-121). At least 9 systematic reviews provide
additional support to the recommendations in the current report (122-130).
Current scientific evidence indicates that physical activity has
important health benefits for adults with arthritis, including reduced pain,
improved function, and a reduced risk of disability. Such benefits have been
observed in adults with arthritis who participate in moderate-intensity,
low-impact activities (e.g., walking, cycling, water exercise), 3 to 5 times
per week for 30 to 60 minutes per session (i.e., accumulate approximately 150
minutes per week). Both endurance and resistance exercise, performed in group
or home settings, has been found to be effective.
Question 5. Does Physical Activity
Increase or Preserve Muscle Mass Throughout the Lifespan? Does Physical
Activity Improve Skeletal Muscle Quality, Defined as Changes in Intrinsic and
Extrinsic Measures of Force-Generating Capacity, Such as Strength or
Specific modes and intensities of physical activity can preserve or
increase skeletal muscle mass, strength, power, and intrinsic neuromuscular
activation. Such effects appears to be similar in women and men and pervasive
throughout the lifespan, although some evidence indicates that the magnitude of
the increases in skeletal muscle mass with resistance training may be
attenuated in advanced age. Specific types of activity can effectively increase
fat‑free mass (i.e., lean mass), strength, and power. Specifically,
performance of regular (i.e., 2 to 4 times per week), high-intensity (i.e., 60%
to 80% of the 1 repetition maximum [1RM]), progressive resistance exercise can
result in significant increases in muscle size, strength, and neuromuscular
function. Endurance activities have not been shown to increase muscle mass or
quality, but may be associated with an attenuation of loss. Muscle power output
may be a critical determinant of physical functioning in the elderly, and
evidence is emerging that resistance training performed at high velocity and
low external resistance to maximize muscle power output may have important
beneficial effects on physical function in older adults.
Evidence indicates that the preservation of fat-free mass and, in
particular, skeletal muscle mass is associated with favorable health outcomes
with advancing age. Cross-sectional studies have reported that sarcopenia, the
age-associated loss of muscle mass, is associated with muscle weakness,
functional limitations, and disability (131;132). Emerging evidence for the effects of increasing
adiposity on disability risk also have raised questions regarding the relative
importance of sarcopenia on age-associated disability (133-135). Despite these observations, evidence remains for
an important role of fat-free mass in maintaining physical functioning and
preventing disability with advancing age (131;136;137). Physical activity and
exercise interventions that have the potential to increase or preserve skeletal
muscle mass also may have important therapeutic benefits on improving physical
functioning and preventing disability, particularly in older adults (see
Part G. Section 6: Functional
Health for a detailed discussion of this issue). Muscle mass
also has been reported to be a significant reserve of energy and a critical
tissue for metabolic homeostasis during stress and chronic disease. Thus,
physical activity interventions designed to increase or preserve muscle mass
may be important for several health outcomes across the lifespan (5).
The effects of physical activity on muscle mass may mediate observed
changes in muscle strength and, as such, are important to men and women of all
ages. For example, exercise-induced increases in muscle strength are associated
with improved muscular fitness in formerly sedentary obese individuals (138;139). This is particularly noteworthy because sedentary
overweight and obese individuals have a limited exercise capacity (140), which may impair physical function. In older
individuals, the age-related loss of muscle mass is accompanied by losses in
voluntary muscle strength (141). Consequently, in those
at risk of sarcopenia, functional capacity and mobility are likely to be
comprised. Studies conducted in older adults indicate that increases in lower
body strength are associated with improvements in gait parameters (142;143), functional capacity (144-147),
and bone health (51;148;149). Strength adaptations also have been suggested to
mediate increased endurance (150).
Given the current scope of physical inactivity in the United States
and the declines in muscle quality parameters that begin in early adulthood,
interventions designed to prevent declines in muscle quantity and quality
through physical activity should be focused on all ages of the population.
However, because the percentage of older Americans is increasing rapidly and
the associated detriments in function may similarly escalate, a special
emphasis on the importance of musculoskeletal health should be placed in this
population to prevent the substantial economic costs associated with decreased
physical functioning that result from the loss of muscle mass and muscle
Physical Activity and Muscle Mass
Many studies have examined the role of physical activity on changes in
body composition. Because of the association between muscle strength, power,
and muscle mass and the well described age-related declines in skeletal muscle
mass, we examined the literature on the influence of exercise training
interventions, in particular resistance training interventions, on changes in
muscle and fat-free mass. Studies that were evaluated included trials conducted
in young, middle-aged, and older men and women. Very few studies, if any,
examined subgroups of different ethnic populations to evaluate variations in
The effects of progressive resistance training in young healthy men
and women have been well described (151). As reviewed by
Kraemer and colleagues, high-intensity progressive resistance training in young
adults results in significant increases in dynamic strength, explosive power,
and muscle mass. More recent studies have confirmed these findings. Short-term
studies of both lower- and upper-extremity resistance training have
demonstrated increases in muscle cross-sectional area (CSA) in men (152-154) and women (155), with
corresponding increases in muscle strength.
Sex-specific changes in muscle mass or CSA in response to resistance
exercise training have been investigated. Short-term studies of progressive
resistance training noted similar increases in muscle adaptations of men and
women (156). Increases in muscle CSA by computed
tomography (CT) have also been shown to be similar in men (17.5%) and women
(20.4%) in response to 16 week of upper- and lower-extremity high-intensity
resistance training (157). However, one study employing
elastic bands for resistance training noted significant increases in muscle
fiber CSAs in men, but not women, in response to 8 week of training, with 2 - 3
sessions per week (158). Interestingly, one RCT of
adolescent girls demonstrated that a 5 day per week mixed mode endurance
training program (running, aerobic dance, competitive sports) induced a
significant (4%) increase in mid-thigh muscle volume (159). More recently, assessment of fat-free mass by
dual-energy x-ray absorptiometry (DXA) and serial CT scans to measure muscle
volume confirmed that similar increases in muscle mass and volume occurred in
young men and women in response to a 6-month whole-body program of progressive
resistance exercise training (160). These results suggest
that resistance exercise training can increase muscle strength and mass to
similar relative extent in men and women. Other modes of physical activity may
increase fat-free mass during adolescence.
Several studies have assessed combinations of the number of
repetitions and intensity of resistance training required to maximize gains in
muscle strength and mass in young adults. Campos and colleagues compared the
responses to 8 weeks of 3 different regimens of progressive resistance training
(161). Young healthy men were randomized to perform
intermediate-repetition/moderate-intensity, or high-repetition/ low-intensity
progressive resistance training of the lower extremities (leg press, squat, and
knee extension). Increases in muscle fiber hypertrophy and muscle strength were
greater in the low-repetition/high-intensity and
intermediate-repetition/moderate-intensity groups than in the
high-repetition/low-intensity group. In contrast, Hisaeda and colleagues
observed similar gains in peak torque and muscle CSA in young women in response
to 8 weeks of either high-intensity/low-repetition or
high-repetition/low-intensity resistance training (155).
The influence of the number of sets performed at each training session on
changes in muscle strength and mass in response to resistance training also has
been studied. Ronnestad and colleagues demonstrated that 3 sets of lower-body
resistance exercise per session was more effective than 1 set in increasing
muscle strength and CSA, suggesting that the volume of training may drive the
gains in muscle strength and mass (162). In support of
this, varying the number of training days per week and the number of training
sets performed to control the total volume of work performed per week resulted
in similar gains in muscle strength and CSA in young men and women (163). The evidence from these trials suggests that muscle
hypertrophy from resistance training occurs in a dose-dependent manner that is
primarily dependent on the intensity of the resistance.
As reviewed by Fielding, a number of early studies demonstrated the
positive effects of progressive resistance training on muscle mass in healthy
older men and women (164). More recent short duration
randomized trials have confirmed these initial findings (165-168), and one study has demonstrated that muscle mass
can continue to increase in older adults throughout 2 years of resistance
The influence of age, per se, on changes in muscle mass in response to
training also has been investigated. Although resistance exercise training
interventions can increase both whole muscle and fiber CSA in older men and
women, some evidence indicates that this hypertrophic response is attenuated in
old age. Cross-sectional studies of older bodybuilders who had been performing
resistance training for 12 to 17 years were reported to have mid-thigh muscle
CSAs that were similar to young sedentary controls, suggesting that the ability
to stimulate muscle growth is diminished with age (169).
In young men and women, the change in mid-thigh CSA after 4 months of
high-intensity resistance training is typically 16% to 23 % (157), compared to a 2.5% to 9.0% increase in
institutionalized or frail older individuals in response to similar resistance
Few studies have directly compared increases in muscle hypertrophy in
young and older subjects using a similar standardized training intervention;
comparisons across studies are prohibitive due to differences in subject
selection criteria, the specific training intervention employed, and the
techniques implemented to assess muscle mass. Welle and colleagues reported
impaired responses of both knee and elbow flexors, but not knee extensors,
after a whole-body resistance training program in older compared to young men
and women (173). Hakkinen and colleagues reported a
decline in the adaptive response of the vastus lateralis from middle to old age
of approximately 40% (174). Lemmer and colleagues
reported a significant increase in thigh muscle CSA in both young and older
adults following resistance training; the magnitude of the increase was greater
in the young (175). Similar results also were observed by
Dionne and colleagues following 6 months of resistance training in young and
older non-obese women (176). In contrast, resistance
training studies of similar intensity and duration also have been reported to
generate similar changes in thigh CSA in young and old (160;177). These findings suggest that
progressive resistance training-induced increases in muscle mass can occur in
older individuals, but that the magnitude of the response may be attenuated,
particularly in the oldest old.
Whether the anabolic response to resistance training among older
adults is sex-specific remains equivocal. Several studies have reported similar
increases in muscle mass in older men and women in response to resistance
training (52;160;178;179). In contrast, men were found
to have larger increases than women in muscle volume after 9 weeks of
high-intensity resistance training (177) and larger
increases in fat-free mass after 12 weeks of high-intensity resistance training
(180). At the cellular level, Bamman and colleagues found
a greater degree of hypertrophy of both type I and II fibers in older men than
in older women in response to 26 weeks of high-intensity resistance training
(181). However, in contrast to these reports, Hakkinen
and colleagues found a smaller increase in muscle CSA in older men than in
older women (174). Despite some lack of agreement, the
majority of studies evaluated suggested that sex plays a relatively small role
in the magnitude of the hypertrophic response to resistance exercise training
in older adults.
Physical Activity and Strength
Several studies have documented gains in strength as a direct result
of resistance training regimens throughout the lifespan (182;183). In young men, a 2-week
isokinetic resistance training program increased isokinetic and isometric
quadriceps muscle peak torque at both 60 and 240 degrees (184). In another study of men, a 12-week high-intensity
resistance training program resulted in an increase in isokinetic concentric
(quadriceps) knee extension strength at a velocity of 30 degrees and eccentric
(hamstring) knee joint strength at velocities of 30, 120 and 240 degrees (185). The hamstring/quadriceps ratio also increased. A
dynamic resistance training protocol of similar duration resulted in isometric
torso rotation strength gains in men and women who exercised twice weekly for
12 weeks (186). Significant gains in both upper- and
lower-body strength have also been reported for longer studies (6 months) (138). Although the preferential mode for strength gains has
been dynamic resistance training (139;187;188), with inclusion of some
amount of eccentric contractions (189), some studies
indicate that other modes also may be effective, including nordic training (190), circuit weight training (153),
balance training (191), and a combination of strength and
endurance or endurance-only protocols (188;192).
In middle-aged men and women subjected to short-duration physical
activity interventions, strength gains also have been observed after
progressive resistance (150), endurance (193), and multi-modal aerobic/weight (194) training protocols. Gains in strength are evident in
longer duration studies (4 to 6 months) in this age group (195;196), and further demonstrate that
greater gains in strength begin to occur after 8 weeks of a combined resistance
and endurance exercise protocol (196).
In older adults, investigators have used relatively long duration (4
to 12 months) resistance training alone (142;143;145) or in combination with
endurance training (144;146;197-199), endurance/balance (200), or endurance/strength/balance/coordination/flexibility
(201) regimens to successfully increase strength in an
effort to counteract the late-life decline in physical functioning. Although
resistance training induces muscle strength gains, functional-task exercises
may be more effective at counteracting declines in function (202). It has been suggested that gains in isometric and
dynamic muscle strength (199) and in isokinetic muscle
strength (145) are associated with improved physical
functioning. However, the gains in strength may be muscle-specific and
translate into improvements only in select parameters of physical functioning,
as indicated in both long- (146;203;204) and short-duration exercise
interventions (205). The results of these studies are in
agreement with a large systematic review (206) of 62 RCTs
of resistance training in older men and women (older than age 60 years), which
found that resistance training increased muscle strength and had a modest
significant effect on some measures of physical functioning (e.g., gait
Strength gains also have been reported for shorter (8 to 12 week)
duration studies of older adults. These studies have employed dynamic training
(179;207), exclusively eccentric
resistance training (147), an integration of resistance,
endurance and balance types of activities (208-210), or
endurance-only activities (211). A progressive resistance
training protocol in older adults resulted in a linear increase in dynamic
strength at different time points of a 12-week study (212). Other intervention paradigms for functional
improvements have been explored. In an 8-week comparison between a combined
resistance training/functional training regimen (1 day per week of each) and
resistance training only (2 days per week), both programs resulted in
significant gains in dynamic strength (213). However,
others report a dose-response relationship between high-intensity progressive
resistance training and functional capacity that may explain the preponderant
use of this type of resistance training (145;214). Gains in strength also occur with low- (215) and variable-intensity resistance training (6 months)
Physical Activity and Muscle Power
Although physical activity interventions that increase or maintain
muscle strength have important health implications, emerging evidence suggests
that muscle power (the rate at which muscle force can be generated) may play a
more important role in functional independence and fall prevention,
particularly among older adults. Muscle power has been shown to decline more
precipitously with aging than does dynamic and isometric strength (218). Lower extremity muscle power also is a strong
predictor of physical performance, functional mobility, and risk of falling
among older adults (219;220).
Muscle power has been found to be inversely associated with self-reported
disability status in community-dwelling older adults with mobility limitations
(221;222) and is a better
discriminator of mobility limitations than muscle strength (220).
Most trials that have evaluated the effects of progressive resistance
training on muscle strength and mass have traditionally involved relatively
slow movement velocities. Some of these have examined changes in lower
extremity power output. In a study of nursing home residents, progressive
resistance training resulted in an increase in muscle strength of more than
100%, but only a 28% increase in stair climbing power, suggesting a
disproportionate and specific rise in strength versus power with traditional
resistance training (171). Skelton and colleagues also
examined changes in peak leg extensor power in response to 12 weeks of
traditional resistance training in older women (223).
They observed increases in strength of 22% to 27% with a non-significant
increase in leg extensor power. A randomized trial by Joszi and colleagues also
noted a modest improvement (30%) in leg extensor power in response to 12 weeks
of progressive resistance training in healthy older men and women (224). More recently, Delmonico and colleagues examined the
effects of moderate-velocity resistance training on changes in peak power in
older men and women (225). They observed similar changes
in absolute peak power in response to 10 weeks of resistance training in both
older men and women. However, the relative improvements in peak power were
greater in women (16%) compared to the men (11%). Similar results have also
been reported by Newton and colleagues employing a "periodized" resistance
training intervention in healthy young and older men (226). These studies suggest that traditional slow velocity
resistance training results in minimal improvements in peak power, that
adaptations may be sex-dependent, and that resistance training performed at
relatively slow velocities may lack the specificity to improve peak power,
particularly in older individuals.
Early randomized trials that examined high-velocity resistance
training to increase muscle power in older subjects compared the effects
against walking exercise (227), slow velocity resistance
training (228), or slow velocity isokinetic training, (229). In general, these studies all demonstrated that
interventions designed to maximize muscle power are feasible, well tolerated,
and can dramatically improve lower-extremity muscle power in healthy older men
and women and older women with self-reported disability. Earles and colleagues
reported a 50% to 141% increase in leg power in older women and men following
12 weeks of high-velocity resistance training in combination with
moderate-intensity non-resistance exercise compared to a structured walking
program (227). Fielding and colleagues compared
high-velocity lower-extremity resistance training with traditional
slow-velocity resistance training in older women with self-reported disability
(228). They observed an 84% greater increase in leg press
power in the high-velocity training group. Similar results were reported by
Signorile and colleagues in healthy older men and women in response to 12 weeks
of high‑velocity isokinetic training (229). All of
these studies employed high-velocity training at a relatively high external
resistance. Only one study to date has examined high-velocity training at
varying levels of external resistance (measured as a percent of the 1 RM) (230). Older adults were randomized to 12 weeks of
high-velocity resistance training at 20%, 50%, or 80% of 1 RM. Peak power
output improved similarly across all training intensities, suggesting that
speed of movement is a key factor in generating improvements in power output.
A small number of studies have evaluated different types of exercise
interventions that did not depend on specific resistance training equipment or
isokinetic dynamometry, but emphasized explosive power. These have included
modified calisthenics and plyometric (i.e., jumping) exercises (231), stair climbing (232), and
weighted-vest exercise (233). Bean and colleagues
compared 12 weeks of a weighted stair climbing program (i.e., stair climbing
while wearing a weighted vest) to a walking program in older adults with
baseline mobility limitations (232). When compared with
walking, the stair climbing intervention increased leg power by 17% with a
corresponding 12% increase in stair climbing power. The same group also
examined the effects of a program of weighted vest exercise performed at a high
velocity (InVEST) compared to a slow-velocity training program (233). Lower-extremity power and chair rise time were
increased more in the InVEST group. Surakka and colleagues examined the effects
of a group exercise intervention that consisted of leg and trunk exercise that
emphasized both strength and power training (231). They
observed that the explosive power training intervention resulted in improved
perceived fitness compared to non-exercising controls. These studies confirm
that several types of exercise programs that can be performed at high velocity
can improve muscle power and improve physical functioning.
A few studies have evaluated the influence of power training on
changes in physical functioning in older adults (234-237). Sayers and colleagues compared 16 weeks of
slow-velocity resistance training to high-velocity power training in older
women with self-reported disability (234). They noted
significant improvements in dynamic balance and stair climbing performance in
both groups, but no differential effects of the two programs. Recent studies
have evaluated low-resistance (40% to 60% 1 RM) high-velocity power training on
measures of physical functioning (235-237). Orr and
colleagues reported improvements in measures of dynamic balance in older women
and men in response to lower-intensity power training when compared with a
no-exercise control group (236). Both Miszko and
colleages and Bottaro and colleagues. found that lower-intensity power training
improved physical functioning composite scores when compared with traditional
slow-velocity resistance training (235;237).
Exercise interventions targeted at improving lower-extremity muscle
power in the elderly have been well-tolerated, safe, and effective.
Improvements in muscle power were generally greater with interventions that
emphasized high- versus low-velocity resistance training. In addition, emerging
evidence indicates that higher-velocity, lower-intensity resistance training
may improve physical functioning in older adults to a greater extent than
traditional slow-velocity resistance training.
As this chapter amply demonstrates, physical activity has many
benefits for musculoskeletal health (for a detailed summary of these benefits,
see Table E.1 in Section E: Integration and Summary of the
Science). Briefly, physical activity is inversely associated
with risk of hip and spine fracture. Exercise training can increase, or slow
the decrease, in spine and hip BMD, and can increase skeletal muscle mass,
strength, power, and intrinsic neuromuscular activation. In the absence of
major joint injury, regular moderate-intensity physical activity does not
appear to promote the development of OA. In fact, physical activity may provide
protection against the development of OA, but there is limited evidence for
this. In adults with OA, participation in moderate-intensity, low-impact
physical activity has disease-specific benefits (e.g., pain, function, quality
The musculoskeletal benefits of physical activity have been observed
in adult women and men across a wide age range, but information on race and
ethnic specificity is lacking. Moderate evidence supports a dose-response
association of volume of physical activity with hip fracture risk, and muscle
mass and strength increase in an exercise intensity-dependent manner.
High-intensity and/or high-velocity resistance exercise may be particularly
effective in increasing BMD and muscle strength and power. Endurance exercise,
even when high‑intensity in nature, has little effect on muscle mass and
strength, but may preserve BMD if the activities are weight-bearing. In the
absence of major prior joint injury, regular moderate- and vigorous-intensity
physical activity in amounts that are commonly recommended for general health
benefits does not appear to increase the risk of developing OA.
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