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Physical Activity Guidelines

February 28 — 29, 2008 Advisory Committee Meeting Minutes

Subcommittee Report: Musculoskeletal Health

Wendy Kohrt, Ph.D., reviewed the work of the musculoskeletal health subcommittee. The group researched the following 6 questions and came to the associated conclusions:

  1. Is there evidence that physical activity reduces the incidence of osteoporotic fractures?

    Conclusions
    There is evidence from prospective cohort and case-control studies for an inverse association between physical activity and fracture risk, particularly for hip fractures.

    The evidence also supports a dose-response relationship between physical activity and fracture risk, such that a greater volume of physical activity confers greater risk reduction.

    There was discussion among the subcommittee as to whether there is actual scientific evidence to support the notion that physical activity reduces the risk of bone fracture. The subcommittee will need to review the precise wording of the statement as to whether it can say there is "some" evidence that supports their position.

    From the studies that were reviewed which included 11 prospective cohort, 1 retrospective cohort, 6 case-controlled, 1 RCT and 2 cross-sectional studies, all supported an association of physical activity with reduced risk. The limitation of the data is that the studies do not isolate physical activity as the sole cause for risk reduction. The evidence is most consistent in hip fracture risk; however, less consistent evidence in other fractures should not be considered as evidence that physical activity plays a reduced role in these fractures. Studies that isolated sex were consistent but studies with mixed sex groups were less consistent.

    The data demonstrates, based on different exposures of activity, that bone fracture risk is reduced by 36 – 41%. Higher levels of physical activity showed a risk reduction of 36 – 68%. Standing 40 hours or more a week was associated with a 34% reduction; however, one cannot discriminate volume versus intensity. There is evidence that decreases in activity increase risk.

  2. Is there evidence that physical activity reduces the risk for osteoporosis by increasing or slowing the decline in bone mass density (BMD)?

    Conclusions
    Exercise training can increase, or attenuate the decrease, in BMD at clinically relevant skeletal sites. The magnitude of the effect is approximately 1-2% per year for studies up to 1 year in duration. Both weight bearing endurance training (ET) and resistance training (RT) can be effective in this area.

    The evidence base for this question included 15 meta-analyses of which 4 found no significant effects on BMD. 10 of the 13 reports showed significant changes in LS BMD. 2 of 8 reported changes in FN BMD. 3 of 6 reported significant changes in BMD of any hip region. 4 of 5 studies found significant effects of exercise on BMD in premenopausal women as opposed to 10 of 12 studies showing significant effects in postmenopausal women. Meta-analysis of studies of men appeared to find larger effects of exercise on BMD than observed in men.

    Regarding type of exercise there were significant effects on LS and hip BMD in ET. Walking only meta-analysis found effect on LS but not FN BMD. 3 out of 4 studies found significant effects on LS BMD in RT. There were no significant effects on FN or other hip regions.

    There is no evidence from meta-analyses for dose response effects. One relatively large RCT on resistance training found a positive association of total weight lifted with change in BMD.

  3. Is there evidence that physical activity reduces or increases the incidence of OA?

    Conclusions
    In the absence of major joint injury, there is no evidence that regular moderate activity increases the risk of developing OA. There is also little evidence to suggest that low to moderate levels of physical activity may provide limited protection against the development of OA.

    Certain groups may have a moderately elevated risk of OA as a result of participation in high-impact activities and long-term participation. There may also be additive effects of recreational and occupational physical activity on OA, for occupations that require excessive knee bending, kneeling, or twisting/torsional movements.

    Females may have increased risk of physical activity related OA. Individuals with excessive body mass may have risk of OA but some evidence suggests that physical activity does not augment that risk. Previous joint injuries are a risk factor for OA but it is not clear if activity further increases the risk.

  4. Is there evidence that physical activity is harmful or beneficial for adults with OA or other rheumatic conditions?

    Conclusions
    There is clear evidence that participation in both endurance and resistance types of exercise provide disease-specific benefits for persons with OA without exacerbating symptoms or worsening disease progression. Patients with OA should pursue activities that are relatively low impact, not painful, and do not have a high risk of joint injury. The data suggests that a prescription of 30 minutes per day for 5 days per week is appropriate.

  5. Is there evidence that physical activity increases or preserves muscle mass throughout the lifespan?

    and

  6. Is there evidence that physical activity improves skeletal muscle quality, defined as changes in intrinsic or extrinsic force-generating capacity?

    Conclusions
    Specific modes and intensities of activity can preserve or increase skeletal muscle mass, strength, power, and intrinsic neuromuscular activation. Benefits appear to be similar in women and men and pervasive across the lifespan. Regular and progressive high-intensity RT can increase muscle size, strength, power and neuromuscular function. Endurance types of activity do not increase muscle mass or quality, but may attenuate the rate of loss with aging. Muscle quantity and quality are not closely linked with a specific health outcome, but are important components of physical fitness. There is strong evidence that progressive, high-intensity RT can increase muscle mass and strength in a dose-dependent manner and are similar in both men and women. There is also evidence that both RT and ET results in improvements in muscle strength.

    For question 6 the evidence suggests muscle power may play a more important role in physical function and fall prevention than muscle strength.

 

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