Physical Activity and
This pronouncement was written for the American College of
Sports Medicine by Wendy M. Kohrt, Ph.D., FACSM (Chair);Susan A. Bloomfield, Ph.D., FACSM; Kathleen D. Little, Ph.D.;Miriam E. Nelson, Ph.D., FACSM; and Vanessa R. Yingling, Ph.D.
may be indicated even for those postmenopausal women who are habituallyphysically active. Given the current state of knowledge from multiple small
Weight-bearing physical activity has beneficial effects on bone health
randomized, controlled trials and large observational studies, the following
across the age spectrum. Physical activities that generate relatively high-
exercise prescription is recommended to help preserve bone health during
intensity loading forces, such as plyometrics, gymnastics, and high-inten-
sity resistance training, augment bone mineral accrual in children andadolescents. Further, there is some evidence that exercise-induced gains in
weight-bearing endurance activities (tennis; stair climbing;
bone mass in children are maintained into adulthood, suggesting that
jogging, at least intermittently during walking), activities
physical activity habits during childhood may have long-lasting benefits on
that involve jumping (volleyball, basketball), and resistance
bone health. It is not yet possible to describe in detail an exercise program
for children and adolescents that will optimize peak bone mass, because
moderate to high, in terms of bone-loading forces
quantitative dose-response studies are lacking. However, evidence from
weight-bearing endurance activities 3–5 times per week;
multiple small randomized, controlled trials suggests that the following
exercise prescription will augment bone mineral accrual in children and
30 – 60 min⅐dϪ1 of a combination of weight-bearing endur-
ance activities, activities that involve jumping, and resis-tance exercise that targets all major muscle groups
impact activities, such as gymnastics, plyometrics, andjumping, and moderate intensity resistance training; partic-
It is not currently possible to easily quantify exercise intensity in terms
ipation in sports that involve running and jumping (soccer,
of bone-loading forces, particularly for weight-bearing endurance activi-
basketball) is likely to be of benefit, but scientific evidence
ties. However, in general, the magnitude of bone-loading forces increases
in parallel with increasing exercise intensity quantified by conventional
high, in terms of bone-loading forces; for safety reasons,
methods (e.g., percent of maximal heart rate or percent of 1RM).
resistance training should be Ͻ60% of 1-repetition maxi-
The general recommendation that adults maintain a relatively high level
of weight-bearing physical activity for bone health does not have an upper
age limit, but as age increases so, too, does the need for ensuring that
10 –20 min (2 times per day or more may be more effective)
physical activities can be performed safely. In light of the rapid and
During adulthood, the primary goal of physical activity should be to
profound effects of immobilization and bed rest on bone loss, and the poor
maintain bone mass. Whether adults can increase bone mineral density
prognosis for recovery of mineral after remobilization, even the frailest
(BMD) through exercise training remains equivocal. When increases have
elderly should remain as physically active as their health permits to pre-
been reported, it has been in response to relatively high intensity weight-
serve skeletal integrity. Exercise programs for elderly women and men
bearing endurance or resistance exercise; gains in BMD do not appear to be
should include not only weight-bearing endurance and resistance activities
preserved when the exercise is discontinued. Observational studies suggest
aimed at preserving bone mass, but also activities designed to improve
that the age-related decline in BMD is attenuated, and the relative risk for
balance and prevent falls. Maintaining a vigorous level of physical activity
fracture is reduced, in people who are physically active, even when the
across the lifespan should be viewed as an essential component of the
activity is not particularly vigorous. However, there have been no large
prescription for achieving and maintaining good bone health.
randomized, controlled trials to confirm these observations, nor have therebeen adequate dose-response studies to determine the volume of physicalactivity required for such benefits. It is important to note that, although
physical activity may counteract to some extent the aging-related
declinein bone mass, there is currently no strong evidence that even vigorous
In Caucasian, postmenopausal women, osteoporosis is de-
physical activity attenuates the menopause-related
loss of bone mineral in
fined as a bone mineral density (BMD) value more than 2.5
women. Thus, pharmacologic therapy for the prevention of osteoporosis
standard deviations below the young adult mean value (52),with or without accompanying fractures. Whether the samecriteria should apply to premenopausal women, women ofother races, or men remains to be confirmed. In the U.S. and
0195-9131/04/3611-1985MEDICINE & SCIENCE IN SPORTS & EXERCISE®
other developed countries the incidence of osteoporosis is
Copyright 2004 by the American College of Sports Medicine
increasing at rates faster than would be predicted by the
increase in the proportion of aged individuals. Multiple
vertebral fractures and, in particular, hip fractures have a
Currently, BMD is the best surrogate measure of bone
devastating effect on functional abilities and quality of life.
strength in humans and BMD has been estimated to account
The mortality rate for elderly individuals in the first year
for 60% or more of the variance in bone strength (20,125).
following hip fracture is as high as 15–20% (105). Even
However, studies of animals suggest that changes in BMD
with no change in current incidence rates, it has been esti-
in response to mechanical stress underestimate the effects
mated that the number of hip fractures will double to 2.6
on bone strength. For example, 5– 8% increases in BMD
million by the year 2025, with a greater percentage increase
were associated with increases in bone strength of 64 – 87%
(48,116). The size of bone has a significant contribution to
Because low BMD greatly elevates the risk of fractures
bone strength because the resistance of bone to bending or
with minimal trauma, as with a fall to the floor, strategies
torsional loading is exponentially related to its diameter;
that maximize bone mass and/or reduce the risk of falling
furthermore, bone size may continue to increase during
have the potential of reducing morbidity and mortality from
adulthood (93). Because bone architecture (i.e., geometry) is
osteoporotic fractures. Although bone mass can be increased
an important determinant of strength (104), evaluation of the
through pharmacologic therapy, physical activity is the only
effects of mechanical stress on bone should consider not
intervention that can potentially both 1) increase bone mass
only changes in bone mass, but changes in structural
and strength and 2) reduce the risk of falling in older
strength and material and geometric properties when possi-
populations. There exist other bone health issues associated
with exercise, including the risk of stress fractures with
The two generally accepted strategies to make the skel-
high-volume training and the bone loss associated with
eton more resistant to fracture are to 1) maximize the gain
amenorrhea. However, the focus of this position stand will
in BMD in the first three decades of life and 2) minimize the
be on the effectiveness of physical activity to reduce risk for
decline in BMD after the age of 40 due to endocrine
osteoporotic fracture, without specific reference to nutri-
changes, aging, a decline in physical activity, and other
factors. Because bone strength and resistance to fracture
Well-known principles of exercise training apply to the
depend not only on the quantity of bone (estimated by
effects of physical activity on bone. For example, overload-
BMD) but also bone geometry, methods are being devel-
ing forces must be applied to bone to stimulate an adaptive
oped that enable the assessment of cross-sectional geometry
response, and continued adaptation requires a progressively
with existing DXA technology or with peripheral quantita-
increasing overload. It is important to emphasize that the
tive computed tomography (pQCT) or high-resolution mag-
stimulus to bone is literally physical deformation of bone
netic resonance imaging (MRI). The microarchitecture of
cells, rather than the metabolic or cardiovascular stresses
cancellous, or trabecular, bone (i.e., the lattice-work of bone
typically associated with exercise (e.g., % V
inside vertebral bodies or ends of long bones) is important
cal deformation can be measured by strain gauges on the
to the mechanical strength of the femoral neck, vertebral
bone surface, but is more commonly estimated by such
bodies, and other cancellous bone-rich regions. However,
surrogate measures as ground-reaction forces engendered
microarchitecture of cancellous bone can be assessed at
during weight-bearing activities. Muscle contraction forces
present in humans only by bone biopsy, sophisticated MRI
in the absence of ground-reaction forces (e.g., swimming)
analyses, or the most advanced micro-CT devices not yet
may also stimulate bone formation, but this is more difficult
generally available. Additional valuable information can be
to estimate. A factor that is unique to skeletal adaptations to
gained from mechanical testing of bone samples from hu-
training is the slow turnover of bone tissue. Because it takes
man cadavers and from animals subjected to various train-
3– 4 months for one remodeling cycle to complete the se-
ing protocols, and from histological and gene expression
quence of bone resorption, formation, and mineralization
analyses from trained animals. Recent advances in protocols
(85), a minimum of 6 – 8 months is required to achieve a
that enhance the osteogenic response to mechanical loading
new steady-state bone mass that is measurable.
in animals have not yet been evaluated in humans, but are
The most common outcome measure used to assess the
expected to stimulate new research in this area (116).
effects of physical activity on bone mass in humans is BMD,
The purpose of this position stand is to provide recom-
which describes the amount of mineral measured per unit
mendations for the types of physical activities that are likely
area or volume of bone tissue (51). Dual-energy x-ray ab-
to promote bone health. The current state-of-knowledge
sorptiometry (DXA) is the standard method of measuring
regarding physical activity as it relates to 1) increasing peak
areal BMD in clinical and research settings. The lumbar
bone mass, 2) minimizing age-related bone loss, and 3)
spine and proximal femur are the most common sites of
preventing injurious falls and fractures will be discussed.
measurement by DXA because they are prone to disablingosteoporotic fractures. Other methods of assessing risk for
osteoporosis include computed tomography (CT) measure-ment of spine volumetric BMD, and ultrasonography of the
Various animal models have been utilized to study me-
calcaneus, which provides an index of bone stiffness. Ul-
chanical loading of the skeleton, but this section will focus
trasonography is widely available, easy to perform, and does
mainly on the commonly used rat model. Multiple factors
not involve exposure to ionizing radiation, but should be
characterize the physical activities that are likely to influ-
ence properties of bone, including the type, intensity, dura-
Official Journal of the American College of Sports Medicine
tion, and frequency of the bone-loading activity. Studies of
changes in bone mass (11). High strain rates also increased
animals enable controlled manipulations of these factors to
endocortical bone formation rate in an in vivo
determine their relative contributions to the osteogenic re-
ing protocol (27,50). Such observations emphasize the need
for further studies of the osteogenic effects of exercises thatgenerate high strain magnitude and rate, such as jumping
Type of loading
Mechanical forces have osteogenic effects only if the
Duration and frequency of loading
stress to bone is unique, variable, and dynamic in nature.
Static loading of bone (i.e., single, sustained force applica-
The seminal work of Rubin and Lanyon (102) using
tion) does not trigger the adaptive response that occurs with
external loading demonstrated that only a few loading cy-
dynamic loading (11). Studies of rats have evaluated the
cles (e.g., 36 per day) of relatively high magnitude were
osteogenic responses to several types of unique (i.e., not
necessary to optimize the bone formation response; increas-
usual cage activity) exercise interventions, including run-
ing the number of loading cycles by 10-fold had no addi-
ning (treadmill and voluntary), swimming, jumping, stand-
tional effect. Similarly, in a more physiologic model of
ing, climbing, and resistance training. Results have been
loading in which rats jumped down from a height of 40 cm,
equivocal, demonstrating that mechanical stress can en-
as few as 5 jumps per day increased bone mass and strength
of the tibia; increasing the number of jumps beyond 10 per
(8,26,92,132) bone mass, formation, and/or mechanical
day did not yield further benefit (118). It should be noted
properties. In general, running and swimming of moderate
that, in these studies, the levels of strain likely exceeded
intensity have been found to have positive effects on bone
those generated during typical human physical activities.
mass and material properties in the cortical and trabecular
The interactions between frequency (repetitions per day and
regions of the tibia and femur in growing and mature rats
sessions per week) and intensity of loading cycles with
(8,26,47,121,127,131). However, decreases in bone mass,
respect to the resulting osteogenic response in humans is not
trabecular thinning, and structural properties have been ob-
served in response to exercise that is very intense and/or
There is intriguing evidence from recent studies that
excessive, particularly in growing animals (26,47,92,132).
applying a given number of loading cycles in multiple daily
Activities that simulate resistance training in humans, in-
sessions is more osteogenic than applying the same number
cluding jumping up to a platform, voluntary tower climbing,
of cycles in a single daily session (116). Rat ulnas that were
and simulated “squat” exercises, have been found to have
loaded 360 times per day in a single session (1ϫ360) for 16
positive effects on both cortical and trabecular bone regions
wk absorbed 94% more energy before failing than the con-
tralateral unloaded ulnas. However, ulnas that received the
Another experimental paradigm that has been used to
same 360 daily loading cycles over 4 sessions (4ϫ90)
evaluate the osteogenic effects of mechanical stress in ani-
absorbed 165% more energy before failing than unloaded
mals is controlled in vivo
external loading, including com-
bones (116). These results suggest that bone cells lose
pression of the ulna and four-point bending of the tibia. This
sensitivity to mechanical stimulation after a certain number
approach has an advantage over physical activity interven-
of loading cycles, and that recovery periods are needed to
tions in that it enables precise control and quantification of
restore sensitivity to loading. It has been estimated that
the mechanical loading forces. Studies of external loading
complete restoration of sensitivity to loading requires a
strongly support favorable adaptations of bone to mechan-
recovery time of 8 h in rats, but recovery times as short as
ical stress (116). For example, the four-point bending model
0.5–1.0 h have been found to be more osteogenic than no
was used in rats to demonstrate that the osteogenic response
recovery period (116). It will be important to determine in
to loading is markedly enhanced when a given number of
humans whether multiple, short daily exercise bouts are
daily loading cycles are partitioned into multiple sessions
more osteogenic than a single, longer daily exercise session.
separated by rest periods (116). It has not yet been deter-mined whether such findings are relevant to humans.
The ability of the skeleton to respond to mechanical
Intensity of loading
loading can be either constrained or enabled by nutritional
The primary mechanical variables associated with load
or endocrine factors. One example of this is calcium insuf-
intensity include strain magnitude and strain rate. Strain is a
ficiency, which diminishes the effectiveness of mechanical
measurement of the deformation of bone that results from an
loading to increase bone mass (66). Another example is
external load and is expressed as a ratio of the amount of
estrogen status. The independent effects of estrogen on bone
deformation to the original length. It has long been recog-
metabolism are well described, but recent studies have de-
nized that strain magnitude is positively related to the os-
termined that the adaptive response of bone cells to me-
teogenic response, but accumulating evidence suggests that
chanical stress involves the estrogen receptor; blocking the
strain rate is also an important factor (11). Increasing strain
estrogen receptor impairs the bone formation response to
rate, while holding loading frequency and peak strain mag-
mechanical stress (133). This observation has led to the
nitude constant, was found to be a positive determinant of
hypothesis that a down-regulation of estrogen receptors as a
Medicine & Science in Sports & Exerciseா
consequence of postmenopausal estrogen deficiency de-
profound when mechanical forces acting on the skeleton are
creases the sensitivity of bone to mechanical loading.
The mechanisms of mechanotransduction in bone (i.e.,
Further research is needed to better understand the inter-
how mechanical forces are translated into metabolic signals)
actions of physical activity with genetics, diet, hormones,
remain to be elucidated, and the discovery of key elements
overuse, and other factors, with respect to the influence on
in the mechanistic pathways will likely reveal factors, po-
bone health. However, due to a paucity of evidence to date,
tentially modifiable, that influence the osteogenic response
to loading. As an example, it has been observed that pros-taglandins and nitric oxide are produced by bone cells in
Role of physical activity in maximizing bone mass
response to mechanical loading, and that blocking their
in children and adolescents
production impairs the bone formation response (16,115).
The translation of such information generated from studies
A primary factor associated with risk for osteoporosis is
of animals and cultured bone cells will be critical in finding
the peak bone mass developed during childhood and the
strategies to maximize the osteogenic effects of physical
early adult years. Cross-sectional data suggest that trabec-
ular bone loss begins as early as the third decade, whereascortical bone increases or remains constant until the fifthdecade (74,100). One longitudinal study found that
both cortical and trabecular bone mass continued to
In humans, physical activity appears to play an important
increase slightly in healthy young women well into the third
role in maximizing bone mass
during childhood and the
early adult years, maintaining bone mass
through the fifth
It has been observed that bone mass is higher in children
decade, attenuating bone loss
with aging, and reducing falls
who are physically active than in those who are less active
in the elderly. The benefits of physical activity
(108), and higher in children who participate in activities
on bone health have typically been judged by measuring
that generate high impact forces (e.g., gymnastics and bal-
associations of physical activity level with bone mass and,
let) than in those who engage in activities that impart lower
in fewer studies, incidence of fractures, or by evaluating
impact forces (e.g., walking) or are not weight bearing (e.g.,
changes in bone mass that occur in response to a change in
swimming) (12,19,58). Recent studies have focused on
physical activity level or to a specific exercise training
jumping and other high-impact activities based on the the-
program. In evaluating the osteogenic effects of exercise
ory that high-intensity forces, imposed rapidly, produce
training programs, the following principles should be noted:
greater gains in bone mass than low- to moderate-intensity
Only skeletal sites exposed to a change in
forces (29,70,72,78,83,96). Ground-reaction forces during
daily loading forces undergo adaptation.
jumping can reach 6 – 8 times body weight and some gym-
An adaptive response occurs only when the
nastics maneuvers generate forces that are 10 –15 times
loading stimulus exceeds usual loading conditions; contin-
body weight; in contrast, ground-reaction forces during
ued adaptation requires a progressively increasing overload.
walking or running are 1–2 times body weight (79). Most of
The benefits of exercise on bone may not
the intervention studies of children were implemented as
persist if the exercise is markedly reduced. However, the
part of school programs and lasted between 7 and 20 months
rate at which bone is lost when an exercise program is
(29,70,72,78,83,96). These studies uniformly found that
discontinued, and whether this is different in young vs older
children who participated in the experimental high-impact
individuals, is not well understood.
jumping and calisthenics programs increased bone mass to
The associations of physical activity and specific types of
a greater extent than children who participated in usual
exercise with bone mass have been assessed in a variety of
activities. One study that added weight lifting to other high-
research paradigms. As reviewed previously (51,123), the
impact loading exercises found robust increases in bone
majority of studies have been cross-sectional, comparing
mass of the hip, spine, and total body (83). Based on this
nonathletes with athletes who participate in a variety of
evidence, it is recommended that physical activity for chil-
sports, or comparing people who report being sedentary
dren should include activities that generate relatively high
with those who report varying levels of physical activity.
ground-reaction forces, such as jumping, skipping, and run-
Because of the numerous confounding factors inherent to
ning and, possibly, strengthening exercises.
cross-sectional studies, these will be discussed only briefly.
Peak bone mineral accrual rate has been reported to occur
The response of bone to changes in physical activity and
at puberty (2), with 26% of adult total body bone mineral
exercise training has also been assessed, including prospec-
accrued within a 2-yr period of this time (3). Thus, the
tive studies (e.g., athletes followed through peak and off-
peri-pubertal period may represent a relatively short win-
season training cycles) and controlled intervention studies in
dow of time in which to maximize peak bone mass. Cross-
which physical activity is increased (e.g., exercise training)
sectional studies indicate that male and female adolescent
or decreased (e.g., bed rest). Perhaps the most compelling
athletes have higher, site-specific BMD when compared
evidence that mechanical loading is essential to bone integ-
with nonathletic adolescents (123). The effect is most pro-
rity comes from studies of bed rest, space flight, and spinal
nounced in athletes who participate in sports that generate
cord injury, which demonstrate that bone loss is rapid and
high-intensity ground- or joint-reaction forces (e.g., gym-
Official Journal of the American College of Sports Medicine
nastics, weight lifting) and less pronounced in athletes who
portant to determine the influence of exercise on bone
participate in sports that generate lower-intensity loading
geometry in children and adolescents.
There have been few exercise intervention studies of
Role of physical activity in young adults
adolescents, all involving girls only, with contradictory re-sults. No significant changes in BMD were found in re-
Because peak bone mass is thought to be attained by the
sponse to 6 months of resistance training (7), 9 months of
end of the third decade, the early adult years may be the final
resistance training and plyometrics with weighted vests
opportunity for its augmentation. Numerous cross-sectional
(129), or 9 months of step aerobics and plyometrics (44). In
studies of male and female athletes representing a variety of
contrast, significant increases in BMD occurred in response
sports suggest that athletes have higher, site-specific BMD
to 3 yr of artistic gymnastics (65), or 15 months of resistance
values when compared with nonathletes (123). BMD values
training (89). The most obvious difference between the
tend to be highest in athletes who participate in sports that
studies that elicited an effect of exercise and those that failed
involve high-intensity loading forces, such as gymnastics,
to do so was the duration of the intervention. However, these
weight lifting, and body building, and lowest in athletes who
studies involved a very small number of participants and
participate in non–weight bearing sports such as swimming.
must be interpreted cautiously. There have been no well-
As noted previously, inherent limitations of cross-sectional
controlled studies that isolated the effects of exercise train-
studies include confounding variables such as genetics, self-
ing duration on the bone response, independent of changes
selection, diet, hormones, and other factors.
A handful of prospective, controlled studies of athletes
Three studies have attempted to determine at what point
have monitored changes in bone mass through periods of
in the peri-pubertal period the skeleton is most responsive to
training or detraining. Bilateral differences in arm BMC of
the benefits of physical activity or exercise training. One
national level male tennis players (13–25%) were signifi-cantly greater than in controls (1–5%) and persisted after 4
study determined the effect of 9 months of step aerobics and
yr of retirement (63). Studies of runners, rowers, power
plyometrics on bone mineral content (BMC) in premenar-
athletes, and gymnasts, ranging in duration from 7 months to
cheal and postmenarcheal girls; control subjects were
2 yr all showed significant increases (1–5%) in either BMC
matched on menarche status. BMC increased in response to
or BMD of skeletal regions loaded by the specific type of
exercise in premenarcheal girls only (44). Another study
exercise performed during periods of training (123). In
assessed the effect of 7 months of plyometrics on BMC and
competitive gymnasts followed for 2 yr (111), BMD in-
BMD in prepubertal (Tanner stage I) and early pubertal
creased during the competitive seasons (2– 4%) and de-
(Tanner stages II and III) girls. Significant bone gains were
creased during the off-seasons (1%).
observed in the early pubertal, but not the prepubertal, girls
A number of intervention studies ranging in duration
when compared with controls (71). A cross-sectional study
from 6 to 36 months have evaluated the effects of exercises
evaluated humeral BMD of both the dominant and non-
that generate relatively high ground-reaction and/or joint-
dominant arms of female junior tennis players matched with
reaction forces (e.g., resistance training, plyometrics) on
controls for Tanner stage of maturity (39). Bilateral differ-
bone mass of previously sedentary women. The majority of
ences in BMD were similar in athletes and controls at
these studies found significant increases in femoral neck
Tanner stage I (9.4 yr), but became progressively larger in
and/or lumbar spine BMD (1–5%) (4,5,28,43,68,77,
athletes at Tanner stages II (10.8 yr), III (12.6 yr), and IV
112,128). In two of three studies of resistance training that
(13.5 yr) with a plateau at stage V (15.5 yr). Based on these
failed to elicit a significant effect on BMD, exercise inten-
observations, bone appears to be most responsive to me-
sity was only low to moderate (i.e., 60% or less of 1-repe-
chanical stress during Tanner stages II through IV, corre-
tition maximum, 1RM) (34,107). Exercise intensity was
sponding to the 2-yr window that has been identified (3) for
high in the third study (i.e., 80% 1RM; 5 sets; 10 repetitions;
peak bone mineral accrual around the time of puberty.
4 d·wkϪ1) (122), but only the unilateral leg press exercise
There remains a need for further research to elucidate the
was performed and this exercise may have lacked site-
best type and duration of exercise to augment bone accrual
specificity for adaptation of the spine and femoral neck
and the time during the growth period when loading is most
because it was performed in a seated position (109). Two
effective. The evidence to date supports the same prescrip-
studies found an unexpected decrease in BMD in response
tion noted previously for children (i.e., relatively high im-
to relatively high-impact exercise. In one (101), there was
pact and strengthening activities, such as plyometrics, gym-
no change in femoral neck BMD but a 4% decrease in
nastics, soccer, volleyball, and resistance training). These
lumbar spine BMD after 9 months of resistance training;
activities appear to be most effective in promoting bone
exercise intensity was moderate (i.e., 70% 1RM). In the
mineral accrual when started before or in the early pubertal
other (124), there was a significant increase in total body
period. Further, because measures of bone geometry may
BMC (1–2%), a nonsignificant increase in spine BMD
emerge as important determinants of bone strength that are
(1%), and a significant decrease in femoral neck BMD
independent of BMD (96), and because it seems plausible
(1.5%) in response to 2 yr of resistance training and rope
that geometric factors could be particularly responsive to
skipping; however, exercise compliance was poor (i.e.,
mechanical stress during periods of growth, it will be im-
45%). Thus, although there is evidence that exercise training
Medicine & Science in Sports & Exerciseா
can increase BMD in young adult women, a number of
the interaction between use of hormone therapy and phys-
factors such as intensity of loading forces, site-specificity of
ical activity with respect to relative risk for hip fracture. Hip
the exercise, and adherence to the program may be impor-
fracture risk was reduced by 60 –70% in women on hormone
tant determinants of the relative effectiveness.
therapy, regardless of physical activity level, when com-
Exercise training that generates high-intensity loading
pared with sedentary women not on hormone therapy.
forces (i.e., high strain magnitude) may also induce changes
Among women not on hormone therapy, those in the highest
in body composition (i.e., fat and fat-free mass) and mus-
quintile of physical activity (Ͼ24 MET⅐h⅐wkϪ1) also had a
cular strength. This has stimulated interest in the potential
67% reduction in hip fracture risk, suggesting that a high
additive and interactive effects of changes in body compo-
level of physical activity may prevent fractures even if it
sition and strength with the direct effects of mechanical
does not attenuate bone loss. Fat-free mass remains a stron-
loading on BMD. Significant correlations of body mass, fat
ger determinant of bone mass with aging than either total
mass, fat-free mass, and strength with total and regional
mass or fat mass, although fat mass may also be an inde-
BMD have been found in several studies, with these factors
pendent determinant (1,6). Thus, physical activities that help
accounting for up to 50% of the variance in BMD (109,113).
preserve muscle mass (e.g., resistance exercise) may also be
Weight lifters typically have high levels of fat-free mass and
strength compared with other athletes and BMD also tends
The effect of exercise intervention on bone mass of post-
to be highest in these athletes. For exercises, such as weight
menopausal women has received considerable attention
lifting, that introduce loading forces to the skeleton primar-
over the past three decades; exercise programs have in-
ily through joint-reaction forces (i.e., muscle contractions)
cluded brisk walking, jogging, stair climbing/descending,
rather than ground-reaction forces, it seems likely that in-
rowing, weight lifting, and/or jumping exercises. The gen-
creases in bone mass will occur only if the exercise is of
eral conclusion from meta-analyses of published studies is
sufficient intensity to cause an increase in muscle mass.
that a variety of types of exercise can be effective in pre-
Although physical activities that involve high-intensity
serving bone mass of older women (54,55).
skeletal loading are recommended to optimize and maintain
Walking exercise programs of up to 1 yr have yielded
bone mass in young adults, the benefits may not be realized
only modest effects (88), if any (13,88), on the preservation
in the presence of hormonal or dietary deficiencies or an
of bone mass. This is not surprising as walking does not
overuse syndrome. The Female Athlete Triad, consisting of
generate high-intensity loading forces, nor does it represent
disordered eating, amenorrhea, and osteoporosis, is an ex-
a unique stimulus to bone in most individuals. These find-
ample of the ineffectiveness of exercise to fully counteract
ings do not rule out the possibility that habitual walking for
the deleterious effects of other factors on bone health; this is
many years helps to preserve bone. Studies that included
reviewed in an ACSM Position Stand on this topic (94).
activities with higher intensity loading forces, such as stair
Calcium and other nutritional deficiencies that can limit the
climbing and jogging, generally found a more positive skel-
osteogenic effects of exercise have been reviewed previ-
ously (67), as have overuse syndromes such as stress frac-
Exercise intervention trials that included high-intensity
tures resulting from extreme, repetitive loading forces (10).
progressive resistance training have found increases in hipand spine BMD in estrogen-deficient women (22,56,57,60,82,87) and in women on hormone therapy (HT) (35,82).
Role of physical activity in middle-aged and older
Moderate-intensity resistance training has not been found to
generate the same increases in hip BMD as high-intensity
Bone mass decreases by about 0.5% per year or more
training (56,57). In one study, the increase in BMD was
after the age of 40, regardless of sex or ethnicity. In this
linearly related to the total amount of weight lifted in a
context, it is important to recognize that benefits of exercise
progressive resistance exercise training program (22).
in middle-aged and older people may be reflected by an
The osteogenic response to jumping exercise (i.e., per-
attenuation in the rate of bone loss, rather than an increase
forming vertical jumps from a standing position) appears to
in bone mass. The rate of loss varies by skeletal region and
be less robust in postmenopausal women than in children
is likely influenced by such factors as genetics, nutrition,
and young adults. Jumping exercise that increased hip BMD
hormonal status, and habitual physical activity, making it
of premenopausal women was not effective in postmeno-
difficult to determine the extent to which the decline in bone
pausal women not on HT, even when the duration of the
mass is an inevitable consequence of the aging process. In
exercise program was extended (5). Although not signifi-
women, estrogen withdrawal at the menopause results in
cant, the response of postmenopausal women on HT was
rapid bone loss that is distinct from the slower age-related
intermediate to that of the pre- and postmenopausal women
bone loss. Comparisons of pre- and postmenopausal athletes
not on HT. It should be noted that the exercise stimulus in
suggest that even very vigorous levels of physical activity
the study was constant, rather than progressive as would
do not prevent the menopause-induced loss of bone mineral
typically be prescribed. In a 5-yr study of a small group of
(32,41,59,81,103). There have been no intervention studies
postmenopausal women, exercisers who wore weighted
of perimenopausal women to determine whether exercise
vests averaging 5 kg during jumping activity preserved hip
can attenuate the loss of bone during the menopausal tran-
BMD to a greater extent than control subjects (110). There
sition. However, the Nurses’ Health Study (24) examined
is preliminary evidence that combining exercise with
Official Journal of the American College of Sports Medicine
bisphosphonate therapy may be effective in preventing os-
and long period of observation that would be required.
There is encouraging evidence from a study conducted on a
Recent findings that estrogen receptor antagonists impair
small sample of postmenopausal women that a 2-yr trial of
the response of bone cells to mechanical stress (15) have
back strengthening exercises reduced the incidence of ver-
raised the possibility that a down-regulation of estrogen
tebral fractures over the subsequent 8 yr (106). However,
receptors as a consequence of postmenopausal estrogen
little other evidence exists from prospective trials that phys-
deficiency decreases the sensitivity of bone to mechanical
ical activity reduces the incidence of vertebral or wrist
loading (49). Indeed, there is evidence that exercises that
generate high-intensity loading forces are more effective in
There is considerable evidence from epidemiologic stud-
increasing BMD in postmenopausal women on HT than in
ies that physical inactivity is a risk factor for hip fracture.
women not on HT (61,62,82,90), although this is not a
The incidence of hip fracture has been found to be 20 – 40%
uniform finding (42). It is also not clear whether the effects
lower in individuals who report being physically active than
of mechanical stress and HT are independent, or whether
in those who report being sedentary (37,75). Elderly women
HT modulates the response of bone to mechanical stress.
and men who were chronically inactive (i.e., rare stair
The vast majority of osteoporosis prevention research has
climbing, gardening, or other weight-bearing activities)
focused on women because the incidence of osteoporotic
were more than twice as likely to sustain a hip fracture as
fractures does not increase markedly in men until the eighth
those who were physically active, even after adjusting for
or ninth decade (21). Research on the effectiveness of phys-
differences in body mass index, smoking, alcohol intake,
ical activity to preserve bone health of men is therefore
and dependence in daily activities (18). A prospective study
sparse, but is becoming increasingly important due to the
of more than 30,000 Danish men and women found that the
incidence of hip fracture in active people who became
A strong association between BMD and jogging was
sedentary was twice as high as in those who remained
observed in 4254 men, aged 20 –59 yr (86). Men who jogged
physically active (45). In the Finnish Twin Cohort, men who
nine or more times per month had higher BMD levels than
reported participation in vigorous physical activity had a
men who jogged less frequently. In a 5-yr prospective study
62% lower relative risk of hip fracture than men who indi-
of middle-aged and older runners (81), the rate of bone loss
cated they did not participate in vigorous physical activity
was attenuated in runners compared with controls. Among
(64). The Nurses’ Health Study of more than 61,000 post-
the runners, decreases in BMD were most pronounced in
menopausal women suggested that the relative risk of hip
men who substantially decreased their running volume. The
fracture was reduced by 6% for every 3 MET⅐h⅐wkϪ1 of
general conclusion from a meta-analysis of published exer-
physical activity, which is roughly equivalent to 1 h of
cise intervention studies was that exercise can improve or
walking per week (24). Interestingly, women who reported
walking at least 4 h⅐wkϪ1 had a 41% lower risk of hip
Several studies have evaluated the effects of resistance
fracture compared with sedentary peers who walked less
training on bone mass in older men (9,73,76,80,130). The
than 1 h⅐wkϪ1. This suggests that even low-intensity weight-
duration of exercise ranged from 3 to 24 months and exer-
bearing activity, such as walking, may be beneficial in
cise intensity was moderate to high. All but one (76) of the
lowering fracture risk, even though minimal changes in
studies found beneficial effects of resistance training on
BMD, most commonly at the femur; the study that did not
Regular physical activity may help to prevent fractures by
find a benefit used a moderate exercise intensity. In general,
preserving bone mass and/or by reducing the incidence of
the improvements in BMD in response to exercise were of
injurious falls. Many factors contribute to falling, including
the same relative magnitude as has been observed in
diminished postural control, poor vision, reduced muscle
women, although much larger increases were observed in
strength, reduced lower limb range of motion, and cognitive
male heart transplant patients who performed 6 months of
impairment, as well as such extrinsic factors as psychotropic
resistance exercise training (9). Thus, the types of exercise
medications and tripping hazards. Exercise interventions
programs that help to preserve bone mass in older women
will be effective in reducing falls only if they are directed to
individuals in whom the cause of falling involves factorsthat are amenable to improvement with exercise (e.g., poormuscle strength, balance, or range of motion). Reviews and
Physical activity and fracture risk
meta-analyses of randomized trials (14,30,37) suggest that
Osteoporotic fractures occur with minimal trauma in
exercise trials that included balance, leg strength, flexibility,
bones weakened because of low BMD or unfavorable ge-
and/or endurance training effectively reduced risk of falling
ometry (e.g., length or angle of the neck region of the
proximal femur). The most common sites of osteoporotic
It must be noted that some studies have found little or no
fractures are the distal radius, spine, and the neck and
effect of exercise interventions on the incidence of falls
trochanteric regions of the femur. There have been no ran-
(69,84). A recent Cochrane database review concluded that
domized, controlled trials of the effectiveness of exercise to
exercise alone does not reduce fall risk in elderly women
reduce fractures, and such a trial would be extremely chal-
and men (33). One reason forwarded for the lack of a
lenging to conduct, in part because of the large sample size
positive effect was that studies frequently targeted very frail
Medicine & Science in Sports & Exerciseா
nursing home residents, who likely had multiple risk factors
ing; participation in sports that involve running and jumping
for falling that would not be expected to be ameliorated by
(soccer, basketball) is likely to be of benefit, but scientific
exercise (e.g., poor vision). Further, if the exercise intensity
is too low (common in studies of the frail elderly), only
high, in terms of bone-loading forces; for
minimal gains in muscle strength that might help reduce
safety reasons, resistance training should be Յ60% of 1RM
falling risk are achieved. Lastly, it must be recognized that
at least 3 d⅐wkϪ1
the opportunity for falling probably increases as people
10 –20 min (2 times per day or more may be
become more physically active, particularly in community-
During adulthood, the primary goal of physical activity
The type of exercise regimen most likely to reduce falls
should be to maintain bone mass. Whether adults can in-
remains unclear (14), because studies with positive and
crease BMD significantly through exercise training remains
negative findings overlap a great deal in the type of activity
equivocal. When increases have been reported, it has been in
utilized (i.e., oriented to strength, endurance, balance, or
response to relatively high intensity weight-bearing endur-
flexibility), duration of exercise, and frequency of training
ance or resistance exercise; gains in BMD do not appear to
sessions (51). It appears that balance training is a critical
be preserved when the exercise is discontinued. Observa-
component of these programs and should be included in
tional studies suggest that the age-related decline in BMD is
exercise interventions for older individuals at risk of falling.
attenuated, and the relative risk for fracture is reduced, in
Improving muscle strength has been posited as potentially
people who are physically active, even when the activity is
one of the most effective means of reducing falls and frac-
not particularly vigorous. However, there have been no
ture incidence in the elderly because of its beneficial effects
large randomized, controlled trials to confirm these obser-
on multiple risk factors for fracture, such as low BMD, slow
vations, nor have there been adequate dose-response studies
walking speed, low levels of energy-absorbing soft tissue,
to determine the volume of physical activity required for
and immobility (75). There is further evidence that the gains
such benefits. Animal research has demonstrated that me-
in functional abilities after a course of resistance training
chanical loading generates improvements in bone strength
lead to an increase in voluntary physical activity in older
(i.e., resistance to fracture) that are disproportionately larger
adults (46) as well as in the very elderly living in nursing
than the increases in bone mass. This supports the concept
homes (25). The capacity of even frail elderly to exercise at
that physical activity can reduce fracture risk even in the
relatively high intensities may be habitually underestimated,
absence of changes in BMD. Confirmation of this in humans
though the feasibility of establishing community programs
will require large randomized, controlled trials of the effects
that utilize the intensive training that has been found to
of physical activity on fracture incidence, although further
increase muscle strength and improve functional ability (25)
advancements in technology to enable the in vivo
is likely limited by the challenges of implementing such
ment of bone strength will provide insight regarding
programs outside a research setting.
whether this occurs. Evidence from multiple small random-ized, controlled trials of the effectiveness of exercise toincrease or maintain BMD suggests that the bone health of
adults will be favorably influenced by the maintenance of a
Weight-bearing physical activity has beneficial effects on
high level of daily physical activity, as recommended by the
bone health across the age spectrum. There is evidence that
U.S. Surgeon General (117), if the activity is weight-bearing
physical activities that generate relatively high-intensity
in nature. It is important to note that, although physical
loading forces, such as plyometrics, gymnastics, and high-
activity may counteract to some extent the aging-related
intensity resistance training, augment bone mineral accrual
decline in bone mass, there is currently no strong evidence
in children and adolescents. This is compatible with the
that even vigorous physical activity attenuates the meno-
findings from studies of animals that the osteogenic re-
pause-related loss of bone mineral in women. Thus, phar-
sponse to mechanical stress is maximized by dynamic load-
macologic therapy for the prevention of osteoporosis may
ing forces that engender a high strain magnitude and rate.
be indicated even for those postmenopausal women who are
Further, there is some evidence that exercise-induced gains
habitually physically active. Given the current state of
in bone mass in children are maintained into adulthood,
knowledge from multiple small randomized, controlled tri-
suggesting that physical activity habits during childhood
als and epidemiological studies, the following exercise pre-
may have long-lasting benefits on bone health. It is not yet
scription is recommended to help preserve bone health
possible to describe in detail an exercise program for chil-
dren and adolescents that will optimize peak bone mass,
weight-bearing endurance activities (tennis; stair
because quantitative dose-response studies are lacking.
climbing; jogging, at least intermittently during walking),
However, evidence from multiple small randomized, con-
activities that involve jumping (volleyball, basketball), and
trolled trials suggests that the following exercise prescrip-
tion will augment bone mineral accrual in children and
moderate to high, in terms of bone-loading
impact activities, such as gymnastics, plyomet-
weight-bearing endurance activities 3–5
rics, and jumping, and moderate intensity resistance train-
times per week; resistance exercise 2–3 times per week
Official Journal of the American College of Sports Medicine
30 – 60 min⅐dϪ1 of a combination of weight-
sistance activities aimed at preserving bone mass, but also
bearing endurance activities, activities that involve jumping,
activities designed to improve balance and prevent falls
and resistance exercise that targets all major muscle groups
Maintaining a vigorous level of physical activity across
It is not currently possible to easily quantify exercise
the lifespan should be viewed as an essential component of
intensity in terms of bone-loading forces, particularly for
the prescription for achieving and maintaining optimal bone
weight-bearing endurance activities. However, in general,
health. Further research will be required to define the type
the magnitude of bone-loading forces increases in parallel
and quantity of physical activity that will be most effective
with increasing exercise intensity quantified by conven-
in developing and maintaining skeletal integrity and mini-
tional methods (e.g., percent of maximal heart rate or per-
The general recommendation that adults maintain a rela-
tively high level of weight-bearing physical activity forbone health does not have an upper age limit, but as age
This pronouncement was reviewed for the American
increases so, too, does the need for ensuring that physical
College of Sports Medicine by members-at-large; the
activities can be performed safely. In light of the rapid and
Pronouncements Committee; and by Debra Bemben, Ph.D.,
profound effects of immobilization and bed rest on bone
FACSM; Patricia Fehling, Ph.D., FACSM; Scott Going,
loss, and the poor prognosis for recovery of mineral after
Ph.D.; Heather McKay, Ph.D.; Charlotte Sanborn, Ph.D.,
remobilization, even the frailest elderly should remain as
FACSM; and Christine Snow, Ph.D., FACSM.
physically active as their health permits to preserve skeletal
This Position Stand replaces the 1995 ACSM Position
integrity. Exercise programs for elderly women and men
Stand, “Osteoporosis and Exercise,” Med. Sci. Sports Exerc.
should include not only weight-bearing endurance and re-
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