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SPECIAL COMMUNICATIONS
Physical Activity and
Bone Health
POSITION STAND
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.
SUMMARY
Weight-bearing physical activity has beneficial effects on bone health
across the age spectrum. Physical activities that generate relatively high-
intensity loading forces, such as plyometrics, gymnastics, and high-inten-
sity resistance training, augment bone mineral accrual in children and
adolescents. Further, there is some evidence that exercise-induced gains in
bone mass in children are maintained into adulthood, suggesting that
physical activity habits during childhood may have long-lasting benefits on
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
quantitative dose-response studies are lacking. However, evidence from
multiple small randomized, controlled trials suggests that the following
exercise prescription will augment bone mineral accrual in children and
adolescents:
Mode: impact activities, such as gymnastics, plyometrics, and
jumping, and moderate intensity resistance training; partic-
ipation in sports that involve running and jumping (soccer,
basketball) is likely to be of benefit, but scientific evidence
is lacking
Intensity: high, in terms of bone-loading forces; for safety reasons,
resistance training should be ⬍60% of 1-repetition maxi-
mum (1RM)
Frequency: at least 3 d䡠wk
⫺1
Duration: 10 –20 min (2 times per day or more may be more effective)
During adulthood, the primary goal of physical activity should be to
maintain bone mass. Whether adults can increase bone mineral density
(BMD) through exercise training remains equivocal. When increases have
been reported, it has been in response to relatively high intensity weight-
bearing endurance or resistance exercise; gains in BMD do not appear to be
preserved when the exercise is discontinued. Observational studies suggest
that the age-related decline in BMD is attenuated, and the relative risk for
fracture is reduced, in people who are physically active, even when the
activity is not particularly vigorous. However, there have been no large
randomized, controlled trials to confirm these observations, nor have there
been adequate dose-response studies to determine the volume of physical
activity required for such benefits. It is important to note that, although
physical activity may counteract to some extent the aging-related decline
in bone mass, there is currently no strong evidence that even vigorous
physical activity attenuates the menopause-related loss of bone mineral in
women. Thus, pharmacologic therapy for the prevention of osteoporosis
may be indicated even for those postmenopausal women who are habitually
physically active. Given the current state of knowledge from multiple small
randomized, controlled trials and large observational studies, the following
exercise prescription is recommended to help preserve bone health during
adulthood:
Mode: weight-bearing endurance activities (tennis; stair climbing;
jogging, at least intermittently during walking), activities
that involve jumping (volleyball, basketball), and resistance
exercise (weight lifting)
Intensity: moderate to high, in terms of bone-loading forces
Frequency: weight-bearing endurance activities 3–5 times per week;
resistance exercise 2–3 times per week
Duration: 30 – 60 min䡠d
⫺1
of a combination of weight-bearing endur-
ance activities, activities that involve jumping, and resis-
tance exercise that targets all major muscle groups
It is not currently possible to easily quantify exercise intensity in terms
of bone-loading forces, particularly for weight-bearing endurance activi-
ties. However, in general, the magnitude of bone-loading forces increases
in parallel with increasing exercise intensity quantified by conventional
methods (e.g., percent of maximal heart rate or percent of 1RM).
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
physical activities can be performed safely. In light of the rapid and
profound effects of immobilization and bed rest on bone loss, and the poor
prognosis for recovery of mineral after remobilization, even the frailest
elderly should remain as physically active as their health permits to pre-
serve skeletal integrity. Exercise programs for elderly women and men
should include not only weight-bearing endurance and resistance activities
aimed at preserving bone mass, but also activities designed to improve
balance and prevent falls. Maintaining a vigorous level of physical activity
across the lifespan should be viewed as an essential component of the
prescription for achieving and maintaining good bone health.
INTRODUCTION
In Caucasian, postmenopausal women, osteoporosis is de-
fined as a bone mineral density (BMD) value more than 2.5
standard deviations below the young adult mean value (52),
with or without accompanying fractures. Whether the same
criteria should apply to premenopausal women, women of
other races, or men remains to be confirmed. In the U.S. and
other developed countries the incidence of osteoporosis is
increasing at rates faster than would be predicted by the
increase in the proportion of aged individuals. Multiple
0195-9131/04/3611-1985
MEDICINE & SCIENCE IN SPORTS & EXERCISE
®
Copyright © 2004 by the American College of Sports Medicine
DOI: 10.1249/01.MSS.0000142662.21767.58
1985
vertebral fractures and, in particular, hip fractures have a
devastating effect on functional abilities and quality of life.
The mortality rate for elderly individuals in the first year
following hip fracture is as high as 15–20% (105). Even
with no change in current incidence rates, it has been esti-
mated that the number of hip fractures will double to 2.6
million by the year 2025, with a greater percentage increase
in men than in women (38).
Because low BMD greatly elevates the risk of fractures
with minimal trauma, as with a fall to the floor, strategies
that maximize bone mass and/or reduce the risk of falling
have the potential of reducing morbidity and mortality from
osteoporotic fractures. Although bone mass can be increased
through pharmacologic therapy, physical activity is the only
intervention that can potentially both 1) increase bone mass
and strength and 2) reduce the risk of falling in older
populations. There exist other bone health issues associated
with exercise, including the risk of stress fractures with
high-volume training and the bone loss associated with
amenorrhea. However, the focus of this position stand will
be on the effectiveness of physical activity to reduce risk for
osteoporotic fracture, without specific reference to nutri-
tional or genetic influences.
Well-known principles of exercise training apply to the
effects of physical activity on bone. For example, overload-
ing forces must be applied to bone to stimulate an adaptive
response, and continued adaptation requires a progressively
increasing overload. It is important to emphasize that the
stimulus to bone is literally physical deformation of bone
cells, rather than the metabolic or cardiovascular stresses
typically associated with exercise (e.g., % V
˙
O
2max
). Physi-
cal deformation can be measured by strain gauges on the
bone surface, but is more commonly estimated by such
surrogate measures as ground-reaction forces engendered
during weight-bearing activities. Muscle contraction forces
in the absence of ground-reaction forces (e.g., swimming)
may also stimulate bone formation, but this is more difficult
to estimate. A factor that is unique to skeletal adaptations to
training is the slow turnover of bone tissue. Because it takes
3– 4 months for one remodeling cycle to complete the se-
quence of bone resorption, formation, and mineralization
(85), a minimum of 6 – 8 months is required to achieve a
new steady-state bone mass that is measurable.
The most common outcome measure used to assess the
effects of physical activity on bone mass in humans is BMD,
which describes the amount of mineral measured per unit
area or volume of bone tissue (51). Dual-energy x-ray ab-
sorptiometry (DXA) is the standard method of measuring
areal BMD in clinical and research settings. The lumbar
spine and proximal femur are the most common sites of
measurement by DXA because they are prone to disabling
osteoporotic fractures. Other methods of assessing risk for
osteoporosis include computed tomography (CT) measure-
ment of spine volumetric BMD, and ultrasonography of the
calcaneus, which provides an index of bone stiffness. Ul-
trasonography is widely available, easy to perform, and does
not involve exposure to ionizing radiation, but should be
used only as a screening test.
Currently, BMD is the best surrogate measure of bone
strength in humans and BMD has been estimated to account
for 60% or more of the variance in bone strength (20,125).
However, studies of animals suggest that changes in BMD
in response to mechanical stress underestimate the effects
on bone strength. For example, 5– 8% increases in BMD
were associated with increases in bone strength of 64 – 87%
(48,116). The size of bone has a significant contribution to
bone strength because the resistance of bone to bending or
torsional loading is exponentially related to its diameter;
furthermore, bone size may continue to increase during
adulthood (93). Because bone architecture (i.e., geometry) is
an important determinant of strength (104), evaluation of the
effects of mechanical stress on bone should consider not
only changes in bone mass, but changes in structural
strength and material and geometric properties when possi-
ble (120).
The two generally accepted strategies to make the skel-
eton more resistant to fracture are to 1) maximize the gain
in BMD in the first three decades of life and 2) minimize the
decline in BMD after the age of 40 due to endocrine
changes, aging, a decline in physical activity, and other
factors. Because bone strength and resistance to fracture
depend not only on the quantity of bone (estimated by
BMD) but also bone geometry, methods are being devel-
oped that enable the assessment of cross-sectional geometry
with existing DXA technology or with peripheral quantita-
tive computed tomography (pQCT) or high-resolution mag-
netic resonance imaging (MRI). The microarchitecture of
cancellous, or trabecular, bone (i.e., the lattice-work of bone
inside vertebral bodies or ends of long bones) is important
to the mechanical strength of the femoral neck, vertebral
bodies, and other cancellous bone-rich regions. However,
microarchitecture of cancellous bone can be assessed at
present in humans only by bone biopsy, sophisticated MRI
analyses, or the most advanced micro-CT devices not yet
generally available. Additional valuable information can be
gained from mechanical testing of bone samples from hu-
man cadavers and from animals subjected to various train-
ing protocols, and from histological and gene expression
analyses from trained animals. Recent advances in protocols
that enhance the osteogenic response to mechanical loading
in animals have not yet been evaluated in humans, but are
expected to stimulate new research in this area (116).
The purpose of this position stand is to provide recom-
mendations for the types of physical activities that are likely
to promote bone health. The current state-of-knowledge
regarding physical activity as it relates to 1) increasing peak
bone mass, 2) minimizing age-related bone loss, and 3)
preventing injurious falls and fractures will be discussed.
ANIMAL STUDIES
Various animal models have been utilized to study me-
chanical loading of the skeleton, but this section will focus
mainly on the commonly used rat model. Multiple factors
characterize the physical activities that are likely to influ-
ence properties of bone, including the type, intensity, dura-
1986
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
tion, and frequency of the bone-loading activity. Studies of
animals enable controlled manipulations of these factors to
determine their relative contributions to the osteogenic re-
sponse (i.e., bone formation).
Type of loading
Mechanical forces have osteogenic effects only if the
stress to bone is unique, variable, and dynamic in nature.
Static loading of bone (i.e., single, sustained force applica-
tion) does not trigger the adaptive response that occurs with
dynamic loading (11). Studies of rats have evaluated the
osteogenic responses to several types of unique (i.e., not
usual cage activity) exercise interventions, including run-
ning (treadmill and voluntary), swimming, jumping, stand-
ing, climbing, and resistance training. Results have been
equivocal, demonstrating that mechanical stress can en-
hance (26,40,47,48,121,127,131) or compromise
(8,26,92,132) bone mass, formation, and/or mechanical
properties. In general, running and swimming of moderate
intensity have been found to have positive effects on bone
mass and material properties in the cortical and trabecular
regions of the tibia and femur in growing and mature rats
(8,26,47,121,127,131). However, decreases in bone mass,
trabecular thinning, and structural properties have been ob-
served in response to exercise that is very intense and/or
excessive, particularly in growing animals (26,47,92,132).
Activities that simulate resistance training in humans, in-
cluding jumping up to a platform, voluntary tower climbing,
and simulated “squat” exercises, have been found to have
positive effects on both cortical and trabecular bone regions
of the tibia and femur (91,92,126).
Another experimental paradigm that has been used to
evaluate the osteogenic effects of mechanical stress in ani-
mals is controlled in vivo external loading, including com-
pression of the ulna and four-point bending of the tibia. This
approach has an advantage over physical activity interven-
tions in that it enables precise control and quantification of
the mechanical loading forces. Studies of external loading
strongly support favorable adaptations of bone to mechan-
ical stress (116). For example, the four-point bending model
was used in rats to demonstrate that the osteogenic response
to loading is markedly enhanced when a given number of
daily loading cycles are partitioned into multiple sessions
separated by rest periods (116). It has not yet been deter-
mined whether such findings are relevant to humans.
Intensity of loading
The primary mechanical variables associated with load
intensity include strain magnitude and strain rate. Strain is a
measurement of the deformation of bone that results from an
external load and is expressed as a ratio of the amount of
deformation to the original length. It has long been recog-
nized that strain magnitude is positively related to the os-
teogenic response, but accumulating evidence suggests that
strain rate is also an important factor (11). Increasing strain
rate, while holding loading frequency and peak strain mag-
nitude constant, was found to be a positive determinant of
changes in bone mass (11). High strain rates also increased
endocortical bone formation rate in an in vivo impact-load-
ing protocol (27,50). Such observations emphasize the need
for further studies of the osteogenic effects of exercises that
generate high strain magnitude and rate, such as jumping
activities.
Duration and frequency of loading
The seminal work of Rubin and Lanyon (102) using
external loading demonstrated that only a few loading cy-
cles (e.g., 36 per day) of relatively high magnitude were
necessary to optimize the bone formation response; increas-
ing the number of loading cycles by 10-fold had no addi-
tional effect. Similarly, in a more physiologic model of
loading in which rats jumped down from a height of 40 cm,
as few as 5 jumps per day increased bone mass and strength
of the tibia; increasing the number of jumps beyond 10 per
day did not yield further benefit (118). It should be noted
that, in these studies, the levels of strain likely exceeded
those generated during typical human physical activities.
The interactions between frequency (repetitions per day and
sessions per week) and intensity of loading cycles with
respect to the resulting osteogenic response in humans is not
known.
There is intriguing evidence from recent studies that
applying a given number of loading cycles in multiple daily
sessions is more osteogenic than applying the same number
of cycles in a single daily session (116). Rat ulnas that were
loaded 360 times per day in a single session (1⫻360) for 16
wk absorbed 94% more energy before failing than the con-
tralateral unloaded ulnas. However, ulnas that received the
same 360 daily loading cycles over 4 sessions (4⫻90)
absorbed 165% more energy before failing than unloaded
bones (116). These results suggest that bone cells lose
sensitivity to mechanical stimulation after a certain number
of loading cycles, and that recovery periods are needed to
restore sensitivity to loading. It has been estimated that
complete restoration of sensitivity to loading requires a
recovery time of8hinrats, but recovery times as short as
0.5–1.0 h have been found to be more osteogenic than no
recovery period (116). It will be important to determine in
humans whether multiple, short daily exercise bouts are
more osteogenic than a single, longer daily exercise session.
Other considerations
The ability of the skeleton to respond to mechanical
loading can be either constrained or enabled by nutritional
or endocrine factors. One example of this is calcium insuf-
ficiency, which diminishes the effectiveness of mechanical
loading to increase bone mass (66). Another example is
estrogen status. The independent effects of estrogen on bone
metabolism are well described, but recent studies have de-
termined that the adaptive response of bone cells to me-
chanical stress involves the estrogen receptor; blocking the
estrogen receptor impairs the bone formation response to
mechanical stress (133). This observation has led to the
hypothesis that a down-regulation of estrogen receptors as a
PHYSICAL ACTIVITY AND BONE HEALTH Medicine & Science in Sports & Exercise
姞
1987
consequence of postmenopausal estrogen deficiency de-
creases the sensitivity of bone to mechanical loading.
The mechanisms of mechanotransduction in bone (i.e.,
how mechanical forces are translated into metabolic signals)
remain to be elucidated, and the discovery of key elements
in the mechanistic pathways will likely reveal factors, po-
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
response to mechanical loading, and that blocking their
production impairs the bone formation response (16,115).
The translation of such information generated from studies
of animals and cultured bone cells will be critical in finding
strategies to maximize the osteogenic effects of physical
activity in humans.
HUMAN STUDIES
In humans, physical activity appears to play an important
role in maximizing bone mass during childhood and the
early adult years, maintaining bone mass through the fifth
decade, attenuating bone loss with aging, and reducing falls
and fractures in the elderly. The benefits of physical activity
on bone health have typically been judged by measuring
associations of physical activity level with bone mass and,
in fewer studies, incidence of fractures, or by evaluating
changes in bone mass that occur in response to a change in
physical activity level or to a specific exercise training
program. In evaluating the osteogenic effects of exercise
training programs, the following principles should be noted:
Specificity. Only skeletal sites exposed to a change in
daily loading forces undergo adaptation.
Overload. An adaptive response occurs only when the
loading stimulus exceeds usual loading conditions; contin-
ued adaptation requires a progressively increasing overload.
Reversibility. The benefits of exercise on bone may not
persist if the exercise is markedly reduced. However, the
rate at which bone is lost when an exercise program is
discontinued, and whether this is different in young vs older
individuals, is not well understood.
The associations of physical activity and specific types of
exercise with bone mass have been assessed in a variety of
research paradigms. As reviewed previously (51,123), the
majority of studies have been cross-sectional, comparing
nonathletes with athletes who participate in a variety of
sports, or comparing people who report being sedentary
with those who report varying levels of physical activity.
Because of the numerous confounding factors inherent to
cross-sectional studies, these will be discussed only briefly.
The response of bone to changes in physical activity and
exercise training has also been assessed, including prospec-
tive studies (e.g., athletes followed through peak and off-
season training cycles) and controlled intervention studies in
which physical activity is increased (e.g., exercise training)
or decreased (e.g., bed rest). Perhaps the most compelling
evidence that mechanical loading is essential to bone integ-
rity comes from studies of bed rest, space flight, and spinal
cord injury, which demonstrate that bone loss is rapid and
profound when mechanical forces acting on the skeleton are
markedly diminished (31).
Further research is needed to better understand the inter-
actions of physical activity with genetics, diet, hormones,
overuse, and other factors, with respect to the influence on
bone health. However, due to a paucity of evidence to date,
these issues will not be addressed.
Role of physical activity in maximizing bone mass
in children and adolescents
A primary factor associated with risk for osteoporosis is
the peak bone mass developed during childhood and the
early adult years. Cross-sectional data suggest that trabec-
ular bone loss begins as early as the third decade, whereas
cortical bone increases or remains constant until the fifth
decade (74,100). One longitudinal study found that
both cortical and trabecular bone mass continued to
increase slightly in healthy young women well into the third
decade (99).
It has been observed that bone mass is higher in children
who are physically active than in those who are less active
(108), and higher in children who participate in activities
that generate high impact forces (e.g., gymnastics and bal-
let) than in those who engage in activities that impart lower
impact forces (e.g., walking) or are not weight bearing (e.g.,
swimming) (12,19,58). Recent studies have focused on
jumping and other high-impact activities based on the the-
ory that high-intensity forces, imposed rapidly, produce
greater gains in bone mass than low- to moderate-intensity
forces (29,70,72,78,83,96). Ground-reaction forces during
jumping can reach 6– 8 times body weight and some gym-
nastics maneuvers generate forces that are 10 –15 times
body weight; in contrast, ground-reaction forces during
walking or running are 1–2 times body weight (79). Most of
the intervention studies of children were implemented as
part of school programs and lasted between 7 and 20 months
(29,70,72,78,83,96). These studies uniformly found that
children who participated in the experimental high-impact
jumping and calisthenics programs increased bone mass to
a greater extent than children who participated in usual
activities. One study that added weight lifting to other high-
impact loading exercises found robust increases in bone
mass of the hip, spine, and total body (83). Based on this
evidence, it is recommended that physical activity for chil-
dren should include activities that generate relatively high
ground-reaction forces, such as jumping, skipping, and run-
ning and, possibly, strengthening exercises.
Peak bone mineral accrual rate has been reported to occur
at puberty (2), with 26% of adult total body bone mineral
accrued within a 2-yr period of this time (3). Thus, the
peri-pubertal period may represent a relatively short win-
dow of time in which to maximize peak bone mass. Cross-
sectional studies indicate that male and female adolescent
athletes have higher, site-specific BMD when compared
with nonathletic adolescents (123). The effect is most pro-
nounced in athletes who participate in sports that generate
high-intensity ground- or joint-reaction forces (e.g., gym-
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Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
nastics, weight lifting) and less pronounced in athletes who
participate in sports that generate lower-intensity loading
forces.
There have been few exercise intervention studies of
adolescents, all involving girls only, with contradictory re-
sults. No significant changes in BMD were found in re-
sponse to 6 months of resistance training (7), 9 months of
resistance training and plyometrics with weighted vests
(129), or 9 months of step aerobics and plyometrics (44). In
contrast, significant increases in BMD occurred in response
to 3 yr of artistic gymnastics (65), or 15 months of resistance
training (89). The most obvious difference between the
studies that elicited an effect of exercise and those that failed
to do so was the duration of the intervention. However, these
studies involved a very small number of participants and
must be interpreted cautiously. There have been no well-
controlled studies that isolated the effects of exercise train-
ing duration on the bone response, independent of changes
in exercise volume or intensity.
Three studies have attempted to determine at what point
in the peri-pubertal period the skeleton is most responsive to
the benefits of physical activity or exercise training. One
study determined the effect of 9 months of step aerobics and
plyometrics on bone mineral content (BMC) in premenar-
cheal and postmenarcheal girls; control subjects were
matched on menarche status. BMC increased in response to
exercise in premenarcheal girls only (44). Another study
assessed the effect of 7 months of plyometrics on BMC and
BMD in prepubertal (Tanner stage I) and early pubertal
(Tanner stages II and III) girls. Significant bone gains were
observed in the early pubertal, but not the prepubertal, girls
when compared with controls (71). A cross-sectional study
evaluated humeral BMD of both the dominant and non-
dominant arms of female junior tennis players matched with
controls for Tanner stage of maturity (39). Bilateral differ-
ences in BMD were similar in athletes and controls at
Tanner stage I (9.4 yr), but became progressively larger in
athletes at Tanner stages II (10.8 yr), III (12.6 yr), and IV
(13.5 yr) with a plateau at stage V (15.5 yr). Based on these
observations, bone appears to be most responsive to me-
chanical stress during Tanner stages II through IV, corre-
sponding to the 2-yr window that has been identified (3) for
peak bone mineral accrual around the time of puberty.
There remains a need for further research to elucidate the
best type and duration of exercise to augment bone accrual
and the time during the growth period when loading is most
effective. The evidence to date supports the same prescrip-
tion noted previously for children (i.e., relatively high im-
pact and strengthening activities, such as plyometrics, gym-
nastics, soccer, volleyball, and resistance training). These
activities appear to be most effective in promoting bone
mineral accrual when started before or in the early pubertal
period. Further, because measures of bone geometry may
emerge as important determinants of bone strength that are
independent of BMD (96), and because it seems plausible
that geometric factors could be particularly responsive to
mechanical stress during periods of growth, it will be im-
portant to determine the influence of exercise on bone
geometry in children and adolescents.
Role of physical activity in young adults
Because peak bone mass is thought to be attained by the
end of the third decade, the early adult years may be the final
opportunity for its augmentation. Numerous cross-sectional
studies of male and female athletes representing a variety of
sports suggest that athletes have higher, site-specific BMD
values when compared with nonathletes (123). BMD values
tend to be highest in athletes who participate in sports that
involve high-intensity loading forces, such as gymnastics,
weight lifting, and body building, and lowest in athletes who
participate in non–weight bearing sports such as swimming.
As noted previously, inherent limitations of cross-sectional
studies include confounding variables such as genetics, self-
selection, diet, hormones, and other factors.
A handful of prospective, controlled studies of athletes
have monitored changes in bone mass through periods of
training or detraining. Bilateral differences in arm BMC of
national level male tennis players (13–25%) were signifi-
cantly greater than in controls (1–5%) and persisted after 4
yr of retirement (63). Studies of runners, rowers, power
athletes, and gymnasts, ranging in duration from 7 months to
2 yr all showed significant increases (1–5%) in either BMC
or BMD of skeletal regions loaded by the specific type of
exercise performed during periods of training (123). In
competitive gymnasts followed for 2 yr (111), BMD in-
creased during the competitive seasons (2– 4%) and de-
creased during the off-seasons (1%).
A number of intervention studies ranging in duration
from 6 to 36 months have evaluated the effects of exercises
that generate relatively high ground-reaction and/or joint-
reaction forces (e.g., resistance training, plyometrics) on
bone mass of previously sedentary women. The majority of
these studies found significant increases in femoral neck
and/or lumbar spine BMD (1–5%) (4,5,28,43,68,77,
112,128). In two of three studies of resistance training that
failed to elicit a significant effect on BMD, exercise inten-
sity was only low to moderate (i.e., 60% or less of 1-repe-
tition maximum, 1RM) (34,107). Exercise intensity was
high in the third study (i.e., 80% 1RM; 5 sets; 10 repetitions;
4 d·wk
⫺1
) (122), but only the unilateral leg press exercise
was performed and this exercise may have lacked site-
specificity for adaptation of the spine and femoral neck
because it was performed in a seated position (109). Two
studies found an unexpected decrease in BMD in response
to relatively high-impact exercise. In one (101), there was
no change in femoral neck BMD but a 4% decrease in
lumbar spine BMD after 9 months of resistance training;
exercise intensity was moderate (i.e., 70% 1RM). In the
other (124), there was a significant increase in total body
BMC (1–2%), a nonsignificant increase in spine BMD
(1%), and a significant decrease in femoral neck BMD
(1.5%) in response to 2 yr of resistance training and rope
skipping; however, exercise compliance was poor (i.e.,
45%). Thus, although there is evidence that exercise training
PHYSICAL ACTIVITY AND BONE HEALTH Medicine & Science in Sports & Exercise
姞
1989
can increase BMD in young adult women, a number of
factors such as intensity of loading forces, site-specificity of
the exercise, and adherence to the program may be impor-
tant determinants of the relative effectiveness.
Exercise training that generates high-intensity loading
forces (i.e., high strain magnitude) may also induce changes
in body composition (i.e., fat and fat-free mass) and mus-
cular strength. This has stimulated interest in the potential
additive and interactive effects of changes in body compo-
sition and strength with the direct effects of mechanical
loading on BMD. Significant correlations of body mass, fat
mass, fat-free mass, and strength with total and regional
BMD have been found in several studies, with these factors
accounting for up to 50% of the variance in BMD (109,113).
Weight lifters typically have high levels of fat-free mass and
strength compared with other athletes and BMD also tends
to be highest in these athletes. For exercises, such as weight
lifting, that introduce loading forces to the skeleton primar-
ily through joint-reaction forces (i.e., muscle contractions)
rather than ground-reaction forces, it seems likely that in-
creases in bone mass will occur only if the exercise is of
sufficient intensity to cause an increase in muscle mass.
Although physical activities that involve high-intensity
skeletal loading are recommended to optimize and maintain
bone mass in young adults, the benefits may not be realized
in the presence of hormonal or dietary deficiencies or an
overuse syndrome. The Female Athlete Triad, consisting of
disordered eating, amenorrhea, and osteoporosis, is an ex-
ample of the ineffectiveness of exercise to fully counteract
the deleterious effects of other factors on bone health; this is
reviewed in an ACSM Position Stand on this topic (94).
Calcium and other nutritional deficiencies that can limit the
osteogenic effects of exercise have been reviewed previ-
ously (67), as have overuse syndromes such as stress frac-
tures resulting from extreme, repetitive loading forces (10).
Role of physical activity in middle-aged and older
adults
Bone mass decreases by about 0.5% per year or more
after the age of 40, regardless of sex or ethnicity. In this
context, it is important to recognize that benefits of exercise
in middle-aged and older people may be reflected by an
attenuation in the rate of bone loss, rather than an increase
in bone mass. The rate of loss varies by skeletal region and
is likely influenced by such factors as genetics, nutrition,
hormonal status, and habitual physical activity, making it
difficult to determine the extent to which the decline in bone
mass is an inevitable consequence of the aging process. In
women, estrogen withdrawal at the menopause results in
rapid bone loss that is distinct from the slower age-related
bone loss. Comparisons of pre- and postmenopausal athletes
suggest that even very vigorous levels of physical activity
do not prevent the menopause-induced loss of bone mineral
(32,41,59,81,103). There have been no intervention studies
of perimenopausal women to determine whether exercise
can attenuate the loss of bone during the menopausal tran-
sition. However, the Nurses’ Health Study (24) examined
the interaction between use of hormone therapy and phys-
ical activity with respect to relative risk for hip fracture. Hip
fracture risk was reduced by 60 –70% in women on hormone
therapy, regardless of physical activity level, when com-
pared with sedentary women not on hormone therapy.
Among women not on hormone therapy, those in the highest
quintile of physical activity (⬎24 MET䡠h䡠wk
⫺1
) also had a
67% reduction in hip fracture risk, suggesting that a high
level of physical activity may prevent fractures even if it
does not attenuate bone loss. Fat-free mass remains a stron-
ger determinant of bone mass with aging than either total
mass or fat mass, although fat mass may also be an inde-
pendent determinant (1,6). Thus, physical activities that help
preserve muscle mass (e.g., resistance exercise) may also be
effective in preserving bone mass.
The effect of exercise intervention on bone mass of post-
menopausal women has received considerable attention
over the past three decades; exercise programs have in-
cluded brisk walking, jogging, stair climbing/descending,
rowing, weight lifting, and/or jumping exercises. The gen-
eral conclusion from meta-analyses of published studies is
that a variety of types of exercise can be effective in pre-
serving bone mass of older women (54,55).
Walking exercise programs of up to 1 yr have yielded
only modest effects (88), if any (13,88), on the preservation
of bone mass. This is not surprising as walking does not
generate high-intensity loading forces, nor does it represent
a unique stimulus to bone in most individuals. These find-
ings do not rule out the possibility that habitual walking for
many years helps to preserve bone. Studies that included
activities with higher intensity loading forces, such as stair
climbing and jogging, generally found a more positive skel-
etal response (17,23,60,90,95,98).
Exercise intervention trials that included high-intensity
progressive resistance training have found increases in hip
and spine BMD in estrogen-deficient women (22,56,57,60,
82,87) and in women on hormone therapy (HT) (35,82).
Moderate-intensity resistance training has not been found to
generate the same increases in hip BMD as high-intensity
training (56,57). In one study, the increase in BMD was
linearly related to the total amount of weight lifted in a
progressive resistance exercise training program (22).
The osteogenic response to jumping exercise (i.e., per-
forming vertical jumps from a standing position) appears to
be less robust in postmenopausal women than in children
and young adults. Jumping exercise that increased hip BMD
of premenopausal women was not effective in postmeno-
pausal women not on HT, even when the duration of the
exercise program was extended (5). Although not signifi-
cant, the response of postmenopausal women on HT was
intermediate to that of the pre- and postmenopausal women
not on HT. It should be noted that the exercise stimulus in
the study was constant, rather than progressive as would
typically be prescribed. In a 5-yr study of a small group of
postmenopausal women, exercisers who wore weighted
vests averaging 5 kg during jumping activity preserved hip
BMD to a greater extent than control subjects (110). There
is preliminary evidence that combining exercise with
1990
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
bisphosphonate therapy may be effective in preventing os-
teoporotic fractures (119).
Recent findings that estrogen receptor antagonists impair
the response of bone cells to mechanical stress (15) have
raised the possibility that a down-regulation of estrogen
receptors as a consequence of postmenopausal estrogen
deficiency decreases the sensitivity of bone to mechanical
loading (49). Indeed, there is evidence that exercises that
generate high-intensity loading forces are more effective in
increasing BMD in postmenopausal women on HT than in
women not on HT (61,62,82,90), although this is not a
uniform finding (42). It is also not clear whether the effects
of mechanical stress and HT are independent, or whether
HT modulates the response of bone to mechanical stress.
The vast majority of osteoporosis prevention research has
focused on women because the incidence of osteoporotic
fractures does not increase markedly in men until the eighth
or ninth decade (21). Research on the effectiveness of phys-
ical activity to preserve bone health of men is therefore
sparse, but is becoming increasingly important due to the
growing numbers of elderly men.
A strong association between BMD and jogging was
observed in 4254 men, aged 20 –59 yr (86). Men who jogged
nine or more times per month had higher BMD levels than
men who jogged less frequently. In a 5-yr prospective study
of middle-aged and older runners (81), the rate of bone loss
was attenuated in runners compared with controls. Among
the runners, decreases in BMD were most pronounced in
men who substantially decreased their running volume. The
general conclusion from a meta-analysis of published exer-
cise intervention studies was that exercise can improve or
maintain BMD in men (53).
Several studies have evaluated the effects of resistance
training on bone mass in older men (9,73,76,80,130). The
duration of exercise ranged from 3 to 24 months and exer-
cise intensity was moderate to high. All but one (76) of the
studies found beneficial effects of resistance training on
BMD, most commonly at the femur; the study that did not
find a benefit used a moderate exercise intensity. In general,
the improvements in BMD in response to exercise were of
the same relative magnitude as has been observed in
women, although much larger increases were observed in
male heart transplant patients who performed 6 months of
resistance exercise training (9). Thus, the types of exercise
programs that help to preserve bone mass in older women
also appear to be effective in men.
Physical activity and fracture risk
Osteoporotic fractures occur with minimal trauma in
bones weakened because of low BMD or unfavorable ge-
ometry (e.g., length or angle of the neck region of the
proximal femur). The most common sites of osteoporotic
fractures are the distal radius, spine, and the neck and
trochanteric regions of the femur. There have been no ran-
domized, controlled trials of the effectiveness of exercise to
reduce fractures, and such a trial would be extremely chal-
lenging to conduct, in part because of the large sample size
and long period of observation that would be required.
There is encouraging evidence from a study conducted on a
small sample of postmenopausal women that a 2-yr trial of
back strengthening exercises reduced the incidence of ver-
tebral fractures over the subsequent 8 yr (106). However,
little other evidence exists from prospective trials that phys-
ical activity reduces the incidence of vertebral or wrist
fractures (36).
There is considerable evidence from epidemiologic stud-
ies that physical inactivity is a risk factor for hip fracture.
The incidence of hip fracture has been found to be 20 – 40%
lower in individuals who report being physically active than
in those who report being sedentary (37,75). Elderly women
and men who were chronically inactive (i.e., rare stair
climbing, gardening, or other weight-bearing activities)
were more than twice as likely to sustain a hip fracture as
those who were physically active, even after adjusting for
differences in body mass index, smoking, alcohol intake,
and dependence in daily activities (18). A prospective study
of more than 30,000 Danish men and women found that the
incidence of hip fracture in active people who became
sedentary was twice as high as in those who remained
physically active (45). In the Finnish Twin Cohort, men who
reported participation in vigorous physical activity had a
62% lower relative risk of hip fracture than men who indi-
cated they did not participate in vigorous physical activity
(64). The Nurses’ Health Study of more than 61,000 post-
menopausal women suggested that the relative risk of hip
fracture was reduced by 6% for every 3 MET䡠h䡠wk
⫺1
of
physical activity, which is roughly equivalent to1hof
walking per week (24). Interestingly, women who reported
walking at least 4 h䡠wk
⫺1
had a 41% lower risk of hip
fracture compared with sedentary peers who walked less
than 1 h䡠wk
⫺1
. This suggests that even low-intensity weight-
bearing activity, such as walking, may be beneficial in
lowering fracture risk, even though minimal changes in
BMD would be expected.
Regular physical activity may help to prevent fractures by
preserving bone mass and/or by reducing the incidence of
injurious falls. Many factors contribute to falling, including
diminished postural control, poor vision, reduced muscle
strength, reduced lower limb range of motion, and cognitive
impairment, as well as such extrinsic factors as psychotropic
medications and tripping hazards. Exercise interventions
will be effective in reducing falls only if they are directed to
individuals in whom the cause of falling involves factors
that are amenable to improvement with exercise (e.g., poor
muscle strength, balance, or range of motion). Reviews and
meta-analyses of randomized trials (14,30,37) suggest that
exercise trials that included balance, leg strength, flexibility,
and/or endurance training effectively reduced risk of falling
in older adults.
It must be noted that some studies have found little or no
effect of exercise interventions on the incidence of falls
(69,84). A recent Cochrane database review concluded that
exercise alone does not reduce fall risk in elderly women
and men (33). One reason forwarded for the lack of a
positive effect was that studies frequently targeted very frail
PHYSICAL ACTIVITY AND BONE HEALTH Medicine & Science in Sports & Exercise
姞
1991
nursing home residents, who likely had multiple risk factors
for falling that would not be expected to be ameliorated by
exercise (e.g., poor vision). Further, if the exercise intensity
is too low (common in studies of the frail elderly), only
minimal gains in muscle strength that might help reduce
falling risk are achieved. Lastly, it must be recognized that
the opportunity for falling probably increases as people
become more physically active, particularly in community-
dwelling elderly (97,114).
The type of exercise regimen most likely to reduce falls
remains unclear (14), because studies with positive and
negative findings overlap a great deal in the type of activity
utilized (i.e., oriented to strength, endurance, balance, or
flexibility), duration of exercise, and frequency of training
sessions (51). It appears that balance training is a critical
component of these programs and should be included in
exercise interventions for older individuals at risk of falling.
Improving muscle strength has been posited as potentially
one of the most effective means of reducing falls and frac-
ture incidence in the elderly because of its beneficial effects
on multiple risk factors for fracture, such as low BMD, slow
walking speed, low levels of energy-absorbing soft tissue,
and immobility (75). There is further evidence that the gains
in functional abilities after a course of resistance training
lead to an increase in voluntary physical activity in older
adults (46) as well as in the very elderly living in nursing
homes (25). The capacity of even frail elderly to exercise at
relatively high intensities may be habitually underestimated,
though the feasibility of establishing community programs
that utilize the intensive training that has been found to
increase muscle strength and improve functional ability (25)
is likely limited by the challenges of implementing such
programs outside a research setting.
CONCLUSIONS
Weight-bearing physical activity has beneficial effects on
bone health across the age spectrum. There is evidence that
physical activities that generate relatively high-intensity
loading forces, such as plyometrics, gymnastics, and high-
intensity resistance training, augment bone mineral accrual
in children and adolescents. This is compatible with the
findings from studies of animals that the osteogenic re-
sponse to mechanical stress is maximized by dynamic load-
ing forces that engender a high strain magnitude and rate.
Further, there is some evidence that exercise-induced gains
in bone mass in children are maintained into adulthood,
suggesting that physical activity habits during childhood
may have long-lasting benefits on bone health. It is not yet
possible to describe in detail an exercise program for chil-
dren and adolescents that will optimize peak bone mass,
because quantitative dose-response studies are lacking.
However, evidence from multiple small randomized, con-
trolled trials suggests that the following exercise prescrip-
tion will augment bone mineral accrual in children and
adolescents:
Mode: impact activities, such as gymnastics, plyomet-
rics, and jumping, and moderate intensity resistance train-
ing; participation in sports that involve running and jumping
(soccer, basketball) is likely to be of benefit, but scientific
evidence is lacking
Intensity: high, in terms of bone-loading forces; for
safety reasons, resistance training should be ⱕ60% of 1RM
Frequency: at least 3 d䡠wk
⫺1
Duration: 10–20 min (2 times per day or more may be
more effective)
During adulthood, the primary goal of physical activity
should be to maintain bone mass. Whether adults can in-
crease BMD significantly through exercise training remains
equivocal. When increases have been reported, it has been in
response to relatively high intensity weight-bearing endur-
ance or resistance exercise; gains in BMD do not appear to
be preserved when the exercise is discontinued. Observa-
tional studies suggest that the age-related decline in BMD is
attenuated, and the relative risk for fracture is reduced, in
people who are physically active, even when the activity is
not particularly vigorous. However, there have been no
large randomized, controlled trials to confirm these obser-
vations, nor have there been adequate dose-response studies
to determine the volume of physical activity required for
such benefits. Animal research has demonstrated that me-
chanical loading generates improvements in bone strength
(i.e., resistance to fracture) that are disproportionately larger
than the increases in bone mass. This supports the concept
that physical activity can reduce fracture risk even in the
absence of changes in BMD. Confirmation of this in humans
will require large randomized, controlled trials of the effects
of physical activity on fracture incidence, although further
advancements in technology to enable the in vivo assess-
ment of bone strength will provide insight regarding
whether this occurs. Evidence from multiple small random-
ized, controlled trials of the effectiveness of exercise to
increase or maintain BMD suggests that the bone health of
adults will be favorably influenced by the maintenance of a
high level of daily physical activity, as recommended by the
U.S. Surgeon General (117), if the activity is weight-bearing
in nature. It is important to note that, although physical
activity may counteract to some extent the aging-related
decline in bone mass, there is currently no strong evidence
that even vigorous physical activity attenuates the meno-
pause-related loss of bone mineral in women. Thus, phar-
macologic therapy for the prevention of osteoporosis may
be indicated even for those postmenopausal women who are
habitually physically active. Given the current state of
knowledge from multiple small randomized, controlled tri-
als and epidemiological studies, the following exercise pre-
scription is recommended to help preserve bone health
during adulthood:
Mode: weight-bearing endurance activities (tennis; stair
climbing; jogging, at least intermittently during walking),
activities that involve jumping (volleyball, basketball), and
resistance exercise (weight lifting)
Intensity: moderate to high, in terms of bone-loading
forces
Frequency: weight-bearing endurance activities 3–5
times per week; resistance exercise 2–3 times per week
1992
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
Duration: 30– 60 min䡠d
⫺1
of a combination of weight-
bearing endurance activities, activities that involve jumping,
and resistance exercise that targets all major muscle groups
It is not currently possible to easily quantify exercise
intensity in terms of bone-loading forces, particularly for
weight-bearing endurance activities. However, in general,
the magnitude of bone-loading forces increases in parallel
with increasing exercise intensity quantified by conven-
tional methods (e.g., percent of maximal heart rate or per-
cent of 1RM).
The general recommendation that adults maintain a rela-
tively 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 physical
activities can be performed safely. In light of the rapid and
profound effects of immobilization and bed rest on bone
loss, and the poor prognosis for recovery of mineral after
remobilization, even the frailest elderly should remain as
physically active as their health permits to preserve skeletal
integrity. Exercise programs for elderly women and men
should include not only weight-bearing endurance and re-
sistance activities aimed at preserving bone mass, but also
activities designed to improve balance and prevent falls.
Maintaining a vigorous level of physical activity across
the lifespan should be viewed as an essential component of
the prescription for achieving and maintaining optimal bone
health. Further research will be required to define the type
and quantity of physical activity that will be most effective
in developing and maintaining skeletal integrity and mini-
mizing fracture risk.
ACKNOWLEDGMENT
This pronouncement was reviewed for the American
College of Sports Medicine by members-at-large; the
Pronouncements Committee; and by Debra Bemben, Ph.D.,
FACSM; Patricia Fehling, Ph.D., FACSM; Scott Going,
Ph.D.; Heather McKay, Ph.D.; Charlotte Sanborn, Ph.D.,
FACSM; and Christine Snow, Ph.D., FACSM.
This Position Stand replaces the 1995 ACSM Position
Stand, “Osteoporosis and Exercise,” Med. Sci. Sports Exerc.
27(4):i-vii, 1995.
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