Physical Activity Guidelines Advisory Committee Report
Part C. Key Terms
This section provides definitions for many of the major terms used in
this report and in the scientific literature reviewed during the preparation of
the report. We have attempted to use definitions that have been generally
accepted in the scientific literature and in major reports and recommendations
for physical activity and public health. As scientists, educators, and
practitioners continue to strive to better understand new concepts and explore
the numerous characteristics of physical activity and their relations to
various aspects of health and physical fitness, new terminology is introduced
and existing definitions are modified. As new measurement tools are developed
and new health outcomes are identified, accepted terminology will continue to
evolve as part of the science of physical activity and health.
Included in this section are a number of the terms that pertain to
physical activity, physical fitness, and study design. Definitions for disease
or condition-specific terms are defined within individual chapters in
Part G: The Science
Base. Additional discussion of the terminology used in the
presentation of research results or the development of physical activity and
public health guidelines can be found in the following publications: Public
Health Aspects of Physical Activity and Exercise (1),
Toward Active Living (2), Physical Activity
and Health: A Report of the Surgeon General (3),
Dose-Response Issues Concerning Physical Activity and Health: An
Evidence-Based Symposium (4), American College of
Sports Medicine's Guidelines for Exercise Testing and Prescription (5), and Advancing Physical Activity and Guidelines in
Physical Activity and Exercise
Two terms are widely used to describe human movement: physical
activity and exercise. Although they are often used
interchangeably, their definitions differ.
Physical activity is any bodily movement produced by
the contraction of skeletal muscle that increases energy expenditure above a
basal level. Among the ways physical activity can be categorized is according
to mode, intensity, and purpose (3). Mode and intensity are defined below.
With regard to classification by "purpose," physical activity frequently is
categorized by the context in which it is performed. Commonly used categories
include occupational, leisure-time or recreational, household, self-care, and
transportation or commuting activities. In some studies, sports participation
or "exercise training" is assessed and analyzed separately from other
Exercise is a subcategory of physical activity that is
"planned, structured, and repetitive and purposive in the sense that the
improvement or maintenance of one or more components of physical fitness is the
objective" (7). Exercise and exercise
training frequently are used interchangeably and generally refer to
physical activity performed during leisure time with the primary purpose of
improving or maintaining physical fitness, physical performance, or health.
Other terms that describe additional types of physical activity or
exercise are defined here:
Activities of daily living. Activities required for
everyday living, including eating, bathing, toileting, dressing, getting into
or out of a bed or chair, and basic mobility.
Aerobic exercise (training). Exercise that primarily
uses the aerobic energy-producing systems, can improve the capacity and
efficiency of these systems, and is effective for improving cardiorespiratory
Anaerobic exercise (training). Exercise that uses the
anaerobic energy-producing systems and can improve the capacity of these
systems and increase the tolerance of acid-base imbalance during high-intensity
Balance training. Static and dynamic exercises that are
designed to improve individuals' ability to withstand challenges from postural
sway or destabilizing stimulus caused by self-motion, the environment, or other
Endurance exercise (endurance training). Exercises that
are repetitive and produce dynamic contractions of large muscle groups for an
extended period of time (e.g., walking, running, cycling, swimming).
Flexibility exercise. Exercises that enhance the
ability of a joint to move through its full range of motion.
Instrumental activities of daily living. Activities
related to independent living, including preparing meals, managing money,
shopping for groceries or personal items, performing housework, and using a
Leisure-time physical activity. Physical activities
performed by a person that are not required as essential activities of daily
living and are performed at the discretion of the person. These activities
include sports participation, exercise conditioning or training, and
recreational activities such as going for a walk, dancing, and gardening.
Lifestyle activities. This term is frequently used to
encompass activities that one carries out in the course of one's daily life,
that can contribute to sizeable energy expenditure, e.g., taking the stairs
instead of using the elevator, walking to do errands instead of driving,
getting off one bus stop earlier, or parking further away than usual to walk to
Resistance training (strength training, muscle-strengthening
activities, or muscular strength and endurance exercises). Exercise
training primarily designed to increase skeletal muscle strength, power,
endurance, and mass.
Terms related to patterns
of physical activity or exercise are defined here:
Accumulation. The concept of meeting a specific
physical activity dose or goal by performing activity in short bouts, then
adding together the time spent during each of these bouts. For example, a
30-minute per day goal could be met by performing 3 bouts of 10 minutes each
throughout the day.
Dose. In the field of physical activity, dose refers to
the amount of physical activity performed by the subject or participants. The
total dose or amount is determined by the three components of activity:
frequency, duration, and intensity. Frequency is commonly recorded as sessions,
episodes, or bouts per day or per week. Duration is the length of time for each
bout of any specific activity. Intensity is the rate of energy expenditure
necessary to perform the activity to accomplish the desired function (aerobic
activity) or the magnitude of the force exerted during resistance exercise.
Dose response. The relation between the dose of
physical activity and the health or fitness outcome of interest is considered
the dose response. The dose can be measured in terms of a single component of
activity (e.g., frequency, duration, intensity) or as the total amount. This
concept is similar to the prescription of a medication where the expected
response will vary as the dose of the medication is changed. The dose-response
relation can be linear, exponential, or hyperbolic, and the dose-response
relation is likely to vary depending on the primary measure of interest. For
example, improvements in cardiorespiratory fitness, bone health, or adiposity
are common dose-response measures of interest. A dose of physical activity may
exist below which no effect has been detected as well as a dose above which no
effect has been detected. These seemingly lowest and highest doses of activity
may be called "thresholds," but the term should be used cautiously as these
apparent limits may be more related to limitations of measurement than to true
Duration. The length of time in which an activity or
exercise is performed. Duration is generally expressed in minutes.
Frequency. The number of times an exercise or activity
is performed. Frequency is generally expressed in sessions, episodes, or bouts
Intensity. Intensity refers to
how much work is being performed or the magnitude of the effort required to
perform an activity or exercise. Intensity can be expressed either in
absolute or relative terms.
- Absolute. The
absolute intensity of an activity is determined by the rate of work being
performed and does not take into account the physiologic capacity of the
individual. For aerobic activity, absolute intensity typically is expressed as
the rate of energy expenditure (e.g., milliliters per kilograms per minute of
oxygen being consumed, kilocalories per minute, METs) or, for some activities,
simply as the speed of the activity (e.g., walking at 3 miles per hour, jogging
at 6 miles per hour), or physiologic response to the intensity (e.g., heart
rate). For resistance activity or exercise intensity frequently is expressed as
the amount of weight lifted or moved.
- Relative. Relative intensity takes into
account or adjusts for a person's exercise capacity. For aerobic exercise,
relative intensity is expressed as a percent of a person's aerobic capacity
(VO2max) or VO2 reserve, or as a percent of a person's
measured or estimated maximum heart rate (heart rate reserve). It also can be
expressed as an index of how hard the person feels he or she is exercising. A
person's subjective assessment of how hard he or she is working relative to
his/her own capacity is called rating of perceived exertion. The Borg
Scale is a commonly used numerical scale for rating perceived exertion (8).
Rating of perceived exertion is used for both aerobic and muscle-strengthening
types of activities.
MET. MET refers to metabolic equivalent and 1
MET is the rate of energy expenditure while sitting at rest. It is taken by
convention to be an oxygen uptake of 3.5 milliliters per kilogram of body
weight per minute. Physical activities frequently are classified by their
intensity, using the MET as a reference (see
Table D.3 in Part D: Background).
Mode. The type of activity or
exercise that is being performed. Biking, walking, rowing, and weight lifting
are all examples of different modes of activity.
Progression. The process of increasing the intensity,
duration, frequency, or amount of activity or exercise as the body adapts to a
given activity pattern.
During the 20th century, physical fitness has been
defined in a variety of ways, but a generally accepted definition is "the
ability to carry out daily tasks with vigor and alertness, without undue
fatigue and with ample energy to enjoy leisure-time pursuits and meet
unforeseen emergencies" (3, p.20). It has been defined by
the World Health Organization as "the ability to perform muscular work
satisfactorily" (9, p.6). Physical fitness includes a
number of components consisting of cardiorespiratory endurance (aerobic power),
skeletal muscle endurance, skeletal muscle strength, skeletal muscle power,
flexibility, balance, speed of movement, reaction time, and body composition.
Because these attributes differ in their importance to athletic performance
versus health, a distinction has been made between performance-related
fitness and health-related fitness (7).
Performance-related fitness includes those attributes that significantly
contribute to athletic performance and places emphasis on aerobic endurance or
power, muscle strength and power, speed of movement, and reaction time.
Health-related fitness includes cardiorespiratory fitness, muscular strength
and endurance, body composition, flexibility, and balance. The relative
importance of any one attribute depends on the specific performance or health
The following terms relate
to specific aspects of physical fitness.
Adaptation. The body's response to exercise or
activity. Some of the body's structures and functions favorably adjust to the
increase in demands placed on them whenever physical activity of a greater
amount or higher intensity is performed than what is usual for the individual.
It is these adaptations that are the basis for much of the improved health and
fitness associated with increases in physical activity.
Agility. A performance-related component of physical
fitness that is the ability to change position of the entire body in space with
speed and accuracy.
Balance. A performance-related component of physical
fitness that involves the maintenance of the body's equilibrium while
stationary or moving.
Body composition. A health-related component of
physical fitness that applies to body weight and the relative amounts of
muscle, fat, bone, and other vital tissues of the body. Most often, the
components are limited to fat and lean body mass (or fat-free mass).
Cardiorespiratory fitness (endurance). A health-related
component of physical fitness that is the ability of the circulatory and
respiratory systems to supply oxygen during sustained physical activity.
Usually expressed as measured or estimated maximal oxygen uptake
Coordination. A performance-related component of
physical fitness that is the ability to use the senses, such as sight and
hearing together with body parts in carrying out motor tasks smoothly and
Flexibility. A health and performance-related component
of physical fitness that is the range of motion possible at a joint.
Flexibility is specific to each joint and depends on a number of specific
variables, including but not limited to the tightness of specific ligaments and
Maximal oxygen uptake (VO2max). The body's
capacity to transport and use oxygen during a maximal exertion involving
dynamic contraction of large muscle groups, such as during running or cycling.
It is also known as maximal aerobic power and cardiorespiratory endurance
capacity. Peak oxygen consumption (VO2peak) is the highest rate of
oxygen consumption observed during an exhaustive exercise test.
Power. A performance-related component of physical
fitness that describes the rate (or speed) at which work can be applied.
Speed. A performance-related component of physical
fitness that is the ability to perform movements rapidly or within a short
period of time.
Strength. A health and performance-component of
physical fitness that is the ability of a muscle or muscle group to exert
Numerous definitions of health exist and, in this
report, we have adopted the following: "Health is a human condition with
physical, social and psychological dimensions, each characterized on a
continuum with positive and negative poles. Positive health is associated with
a capacity to enjoy life and to withstand challenges; it is not merely the
absence of disease. Negative health is associated with morbidity, and in the
extreme, with premature mortality" (10, p.100).
Health-related quality of life is an individual's
overall sense of well being and includes such factors as pain, mood, energy
level, family and social interactions, sexual function, ability to work, and
ability to keep up with routine daily activities.
Study Design and Measurement
Absolute risk. The percentage of subjects in a group
that experiences a discrete negative outcome, such as death or hospital
Case-control study. A type of epidemiologic study
design in which subjects are selected based on the presence or absence of a
specific outcome of interest, such as cancer or diabetes. The individual's past
physical activity practices are assessed, and the association between past
physical activity and presence of the outcome is determined.
Case report. This includes single case reports of
individual patients and published case series.
Confidence interval. When
relative risk (see definition below) is calculated, one
can also calculate a confidence interval, or a band of uncertainty, around the
estimate of the relative risk. Typically, 95% confidence intervals are used in
epidemiologic studies. For example, if the estimated relative risk for colon
cancer associated with physical activity, compared with inactivity, is 0.5 with
a 95% confidence interval of 0.3 to 0.8, this means that we are 95% certain
that the true estimate of the relative risk lies between 0.3 and 0.8.
Cross-sectional study. Studies that compare and
evaluate specific groups or populations at a single point in time.
Observational studies. Studies in which outcomes are
measured but no attempt is made to change the outcome. The two most commonly
used designs for observational studies are case-control studies and prospective
Odds ratio. A measure of probability used in
epidemiologic studies. It measures the chances of an event (or disease)
occurring in one group of people as compared to another group with different
characteristics. For example, an odds ratio of 0.5 for high blood pressure in
people who participate in physical activity, compared with people who are
inactive, indicates that active persons are 0.5 times (50%) less likely to have
high blood pressure, compared with those who are inactive (see also confidence interval).
Prospective cohort study. A type of epidemiologic study
in which the physical activity practices of the enrolled subjects are
determined and the subjects are followed (or observed) for the development of
selected outcomes. It differs from clinical trials in that the exposure, in
this case physical activity, is not assigned by the researchers.
Randomized controlled trial (also known as a randomized clinical
trial). A type of study design in which participants are grouped on
the basis of an investigator-assigned exposure of interest, such as physical
activity. For example, among a group of eligible participants, investigators
may randomly assign them to exercise at three levels: no activity, moderate
activity, and vigorous activity. These participants are then followed over time
to assess the outcome of interest, such as change in abdominal fat. Randomized
controlled trials are often considered the "gold standard" of human
intervention study designs. However, because of the cost and issues regarding
compliance with an assigned activity level, it may not always be feasible, or
even desirable, to conduct this type of trial.
Relative risk. A measure of
association used in epidemiologic studies. It measures the magnitude of
association between the exposure (such as physical activity) and the disease
(such as colon cancer). A relative risk of 0.5 for colon cancer associated with
physical activity, compared with inactivity, indicates that active persons have
0.5 times (or 50%) the risk of developing colon cancer compared to inactive
Retrospective study. A study in which the outcomes have
occurred before the study has begun.
Cochrane Collaboration. An internationally organized
effort to bring existing clinical studies into systematic reviews to facilitate
the process of bringing clinical evidence to bear on decisionmaking in patient
Meta-analysis. A review of a focused question that
follows rigorous methodological criteria and uses statistical techniques to
combine data from studies on that question.
Systematic review. A review of a clearly defined
question that uses systematic and explicit methods to identify, select, and
critically evaluate relevant research, and to collect and analyze data from the
studies to include in the review.
- Powell KE, Paffenbarger RS, Jr. Workshop on
Epidemiologic and Public Health Aspects of Physical Activity and Exercise: a
summary. Public Health Rep. 1985 Mar;100(2):118-26.
- Quinney HA, Gauvin L, Wall AE. Toward active
living: proceedings of the International Conference on Physical Activity,
Fitness, and Health. Champaign, IL: Human Kinetics; 1994.
- United States Public Health Service, Office of the
Surgeon General, National Center for Chronic Disease Prevention and Health
Promotion, President's Council on Physical Fitness and Sports. Physical
activity and health: a report of the Surgeon General. Atlanta, Ga.: U.S. Dept.
of Health and Human Services, Centers for Disease Control and Prevention,
National Center for Chronic Disease Prevention and Health Promotion;
President's Council on Physical Fitness and Sports; 1996.
- Kesaniemi YK, Danforth E Jr, Jensen MD, Kopelman
PG, Lefebvre P, Reeder BA. Dose-response issues concerning physical activity
and health: an evidence-based symposium. Med.Sci.Sports Exerc. 2001 Jun;33(6
- Whaley MH, Brubaker PH, Otto RM, Armstrong LE.
ACSM's guidelines for exercise testing and prescription. Philadelphia, Pa.:
Lippincott Williams & Wilkins; 2006.
- Shephard RJ, ed. Advancing physical activity
measurement and guidelines in Canada: a scientific review and evidence-based
foundation for the future of Canadian physical activity guidelines.
Can.J.Public Health 2007;98(Suppl. 2e):s1-s224.
- Caspersen CJ, Powell KE, Christenson GM. Physical
activity, exercise, and physical fitness: definitions and distinctions for
health-related research. Public Health Rep. 1985 Mar;100(2):126-31.
- Borg G. Borg's perceived exertion and pain scales.
Champaign, IL: Human Kinetics; 1998.
- Exercise tests in relation to cardiovascular
function. Report of a WHO meeting. World Health Organ Tech.Rep.Ser.
- Preamble to the Constitution of the World Health
Organization as adopted by the International Health Conference, New York, 19-22
June, 1946; signed on 22 July 1946 by the representatives of 61 states
(Official Records of the World Health Organization, no. 2, p. 100) and entered
into force on 7 April 1948. 1946.
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