Os teop oros is a nd P hysi cal Acti vity 

Ja net M. S haw

UNI VERS ITY OF UTA H

Ch rist ine Snow 

ORE GON STA TE UNI VERS ITY

ORIGINALLY PUBLISHED AS SERIES 2, NUMBER 3, OF THE PCPFS RESEARCH DIGEST.

HI GHLI GHT

“B on e h e a lt h is p r om ot ed t h rou g h re gu la r we igh t -bea rin g 

p h ysic a l a c t ivi t ies t h a t u s e m u scu l a r st ren g t h a n d p o wer, a n d

ex ert fo rce on t h e sk elet on a b ove n o rm a l a m ou n t s.”

IN TROD UCTI ON

The National Osteoporosis Foundation (NOF) has defined osteoporosis as a disease

characterized by low bone mass and microarchitectural deterioration of bone tissue leading to

enhanced bone fragility and a consequent increase in fracture risk. In other words,

osteoporosis is the loss of bone tissue that makes bones weaker. It has been projected that

over 5 million fractures of the hip, spine, and wrist will occur in women over the age of 45,

which will account for more than $45 billion in direct health care costs over the next ten years

(Chrischilles, Shireman, & Wallace, 1994). This is a conservative estimate of the future public

health impact of osteoporosis, since the projections are based on only three fracture sites and

limited to females. No doubt the estimate would be greater if it included men, who also are at

risk of osteoporosis as they grow older.

In addition to the financial burden attributed to this disease, osteoporosis has a profound

effect on the quality of life of older individuals. Those afflicted typically experience reduced

mobility, pain, loss of independence, and psychological distress associated with postural

disfigurement and the fear of additional fractures. Hip fractures are the most severe fractures

since they carry the highest incidence of morbidity and mortality. The postural abnormality

associated with vertebral fractures results in reduced cardiovascular capacity and affects other

internal organs due to compression of the chest and abdominal regions. In order to define

effective preventive strategies, it is important to determine the lifestyle factors that influence

fracture risk. The two primary determinants of fracture risk are low bone mass and falls.
Physical activity has been proposed as one strategy to reduce fractures by increasing bone

mass and by preventing falls through improved functional ability. Although the mechanism

by which exercise increases bone mass is not clear, it likely influences bone directly through

mechanical forces (loading) transferred to bone. Bone responds to changes in mechanical

loading and the regulation of bone strength is a function of the loads to which the skeleton is

exposed. The most striking examples of this adaptation are the reports that demonstrate

marked bone loss in the absence of weight-bearing activity, such as occurs in space travel and

prolonged bed rest (Mack et al., 1967; Nishimura et al., 1994). Conversely, many reports

have shown that bone mass among physically active individuals and athletes is significantly

higher compared to their nonactive and nonathletic counterparts. Some studies that have

imposed significant mechanical forces via exercise intervention report positive effects on

bone mass, although the magnitude of effect is much less impressive than would be predicted

from studies on athletes and active individuals. Therefore, the ideal exercise program that

maximizes bone response remains elusive. Evidence is accumulating to suggest, however, that

exercise that increases muscle strength, mass, and power, may provide the best osteogenic

stimulus. Activities of this type provide additional skeletal protection in the older adult by

preventing falls, which are highly related to the incidence of fractures, particularly at the hip.

This review presents the recent literature in the field and provides recommendations for

exercise design that may aid in fracture prevention. In this review, the terms “bone mass”

and “bone mineral density (BMD)” will be used as synonyms.

PH YSIC AL A CTIV ITY AND BONE MAS S

Physical activity transmits loads to the skeleton in two ways: by muscle pull and by

gravitational forces from weight-bearing activity. It is generally assumed that a high level of

activity corresponds to a high level of mechanical loading. However, despite the intensity of

muscular activity associated with competitive swimming, studies comparing athletic groups

have demonstrated that swimmers generally have BMD values lower than those of nonathletic

controls (Taaffe et al., 1995). Therefore, activities that require full support of body weight

(i.e., those that are performed on the feet) are recommended if skeletal response is a desired

outcome of exercise participation. Sports with unilateral activity, such as tennis, continue to

provide the best representation of the positive effects of exercise on bone in humans

(Huddleston et al., 1980; Kannus et al., 1994). These studies have demonstrated greater BMD

in the dominant playing arm vs. the nondominant arm across different age groups. However,

most forms of activity are not as easily characterized by such specific, localized loading

patterns.
Other indicators that physical activity exerts a positive influence on the skeleton is the

finding that certain measures of physical fitness are correlated with BMD. Specifically, body

composition and muscular strength exhibit positive associations with bone mass.

Investigations of BMD and body composition have arisen out of the common finding that

body weight is associated with bone density. Research has attempted to specify which aspect

of body composition, lean (muscle) or fat mass, is the best predictor of bone mass. Muscle

directly attaches to bone and may influence the skeletal system via this mechanism, while fat

mass contributes to body weight in a nonspecific manner. It has been proposed that fat mass

has the potential to increase circulating levels of estrogen, although this explanation for its

beneficial influence on bone has yet to be established. Associations between both fat and lean

mass and BMD have been demonstrated (Reid, Plank, & Evans, 1992; Sowers et al., 1992).

Although fat mass has been associated with bone and can provide cushioning in the fallprone elderly, there are known health problems associated with excess body fat (e.g.,

cardiovascular disease, type II diabetes). On the other hand, adequate muscle mass is

necessary for optimal function throughout the life span and muscular atrophy that

accompanies the aging process is associated with falling and fracture. Therefore it is prudent

to recommend that a fracture prevention program include activities that encourage muscle

mass development.

Mechanical forces are directly applied to bone by muscular attachments and individuals

with high muscle strength are able to generate large forces during contraction. Thus, muscle

strength is a measure of physical fitness that has been studied with respect to skeletal health.

Research has shown that the relationship between muscle strength and bone demonstrates sitespecificity. Strength of the hip muscles has been related to hip BMD, and grip strength has

been associated with forearm BMD (Snow-Harter et al., 1993; Snow-Harter et al., 1990). The

contribution of muscle strength to BMD in various cross-sectional studies has ranged from 9

to 38% in nonathletic adults. Since approximately 60–80% of bone mass is estimated to be

genetically determined, the relationship between muscle strength and bone is not trivial and

again points to the importance of the muscular system with respect to bone health.

Research has demonstrated that male and female athletes who participate in sports that

require muscular strength and power (e.g., weight lifting, gymnastics, wrestling) exhibit

higher bone mass than those whose sports involve primarily muscular endurance (e.g.,

distance running, triathlon) (Robinson et al., 1995). Information on the loading

characteristics of various activities suggests that walking and slow running provide loads equal

to or slightly higher than body weight alone at the spine. In comparison, forces at the spine

have been estimated to be five to six times body weight while weight lifting (Granhad, Jonson,

& Hansson, 1987). Jumping associated with gymnastics training may elicit forces as high as

10 to 12 times body weight.

The research on athletes and the size of the load for a specific sport suggests that the

skeleton’s response to mechanical loading depends on the magnitude of the force. In

practical terms, the skeleton must encounter forces that are greater than those it experiences

on a day-to-day basis. Even though walking is a weight-bearing activity, its ability to evoke a

skeletal response is limited to the older adult who was previously bedridden and unable to

ambulate for a period of time. On the other hand, one who performs activities of daily living

without assistance will be in a weight-bearing posture much of the day. For this person,

walking as an exercise will not exceed the loading threshold of daily activities and therefore

will not improve bone mass.
Exercise intervention studies have attempted to introduce various exercise programs in

humans to determine the best exercise prescription for bone health. The results of these

studies are equivocal. While some reports indicate that BMD increases slightly with exercise,

some report no change or slight decreases. In order to detect changes in bone mass, an

intervention must be several months in duration, depending on the age group and type of

program (six months minimum). Relative to other exercise interventions, these are long time

intervals (e.g., muscular strength increases can be observed in eight weeks). Over the life span,

however, these time intervals are relatively short. This may be one reason why remarkable

changes have not been observed within the time frame of training studies. In addition, the

expected magnitude of skeletal response is much less than that observed in the muscular

system. To illustrate, muscular strength improvements on the order of 50–100% during the

course of a resistance training program are not unusual, especially if initial values were low. A

1.5% increase in bone mass over a period of nine months is meaningful, since average rates

of loss are approximately 0.5–1% per year. To date, most exercise studies have not designed

their exercise training programs according to the principles of training. This may be the main

reason why many studies have observed minimal or no training effects on the skeleton

(Drinkwater, 1994). The application of these principles to bone loading is outlined in Table

12.1.

TA BLE 1 2 .1 

Pr in ci p les of tr ain i n g .

Sp eci f i ci t y Th e im pact of th e tr aini ng sh ould be at th e bo ne si te of in tere st si nce lo adin g se ems to ha ve a

lo cali zed ef fect .

Ov er l o ad Th e tr aini ng st imul us mu st in clud e fo rces mu ch gr eate r th an th at af ford ed by ha bitu al

ac tivi ty.

Re ver s i bi l i t y In th e ab senc e of th e tr aini ng st imul us, th e po siti ve ef fect on bo ne wi ll be lo st.

Initial values In divi dual s wi th lo w BM D wi ll ha ve th e gr eate st po tent ial to ga in fr om in crea sed

me chan ical lo adin g.

Diminishing Each individual’s biological ceiling determines r e tur n s the extent of adaptation to

the training.

PR EVEN TION STR ATEG IES THRO UGHO UT T HE L IFE

SP AN

Bone is a dynamic tissue that is constantly undergoing remodeling activity, a function of

bone cells, during which old bone is removed and new bone is formed. The factors that

determine the level of bone cell activity are mechanical loading, calcium intake, and

reproductive hormones. Strategies to decrease risk for osteoporotic fracture should take these

factors into consideration throughout the life span since bone mass in the older adult is a

product of the amount of bone acquired during growth and subsequent rates of loss during

adulthood.
Physical activity has been shown to be an important contributor to bone mass in children

prior to adolescence (Slemenda et al., 1991). In addition, this group should have adequate

calcium intake so that the necessary blocks for building bone mineral are present during

growth. It has been proposed that young bone may be more responsive to mechanical

loading than old bone (Forwood & Burr, 1993). Given that approximately 60% of the final

skeleton is acquired during adolescence, one preventive strategy is to maximize skeletal

loading during this rapid phase of growth. It is also important to consider reproductive

endocrine status at this time of life. The negative effects of abnormally low estrogen on BMD

in amenorrheic women with a high volume of physical training and very low body weight

(primarily distance running and ballet dancing) are well documented (Drinkwater et al.,

1984). These effects are even more dramatic in amenorrheic women with anorexia nervosa.

Although there is little if any documentation in men, abnormally low testosterone levels are

theoretically detrimental for bone.

Preventive strategies in adults are generally aimed at maintaining bone mass or reducing

the rate of loss. However, recent studies on young adult women indicate that physical activity

may play an important role with respect to the capacity to increase bone mass after growth

has stopped (Recker et al., 1992; Bassey & Ramsdale, 1994). Recker and colleagues observed

increases in spine BMD over a period of five years in a large group of women in their

twenties. The increases were related to self-selected physical activity patterns. Bassey and

Ramsdale (1994) administered high- and low-impact exercise programs to young women for

six months and observed BMD increases at the hip in the high-impact group only. The

authors note that small improvements in bone mass in young to middle adulthood may result

in quite significant reductions in risk for osteoporotic fracture in later years. It is important to

note that BMD increases with physical activity are likely to be most dramatic in young

adulthood when bone appears to be more responsive to mechanical loading. In addition,

loading characteristics must be substantial, as demonstrated by the high-impact activity

administered by Bassey and Ramsdale (1994), which included jumping. Adequate calcium

intake and maintenance of normal circulating levels of reproductive hormones are still

important factors for optimal bone health in adulthood. However, since growth has ceased,

recommendations for calcium intake are slightly lower than in adolescence.

Older adults face multiple challenges with advancing age. Age-related reductions in bone

mass, muscle strength and power, and postural stability make this group at highest risk for

fracture. Impaired musculoskeletal function and dynamic balance associated with aging and

disuse ultimately result in decreased mobility. In addition, these declines in musculoskeletal

function have been associated with an increase in falls and incidence of hip fracture.

Vandervoort et al. (1990) report that once function has declined to the point where mobility

is significantly reduced, older individuals may refuse to ambulate due to a fear of falling,

which is the beginning of a downward spiral that ultimately results in loss of independence.

This situation, in which very few physical attempts are made, leads to marked reductions in

strength and power of the lower extremities, which have been specifically linked to fall risk.

Although bone mass is a major risk factor for osteoporosis, falls and their severity are highly

related to fractures in the elderly. In fact, 90% of all hip fractures occur as a result of a fall

(Melton, 1993).
Falls are caused by many different factors. Epidemiological research has consistently

found that lower limb strength, reaction time, sensory impairment, and postural instability are

important risk factors for falls. Greenspan et al. (1994) have proposed that not all falls are

potentially injurious and that fall severity in combination with bone mass at the hip are the

two primary determinants of hip fracture in ambulatory elderly. Specifically, those who fall to

the side and have no ability to alter fall direction or speed of impact and land directly on the

hip, are more likely to fracture, particularly if BMD at that site is low (Hayes et al., 1993). The

link of muscular strength and power to fall risk is most logically in the stabilization and

control required for voluntary movements as well as for the ability to recover from a stumble.

One study determined that leg extensor (quadriceps) power in older men and women was the

best predictor of functional performance (Bassey et al., 1992).

Strategies to prevent fracture in older adults must target bone mass as well as factors

associated with falls. Several studies have observed beneficial effects of weight training in

older populations including increased bone mass, muscular strength, power, dynamic balance,

and functional independence. Thus, this may be the best choice of exercise training at this

stage in the life span. Most research has focused on machine-based training (e.g., Universal

Gym, Nautilus), which requires a seated posture for lower body exercises. While this isolates

muscle groups in the legs, it effectively reduces loads at the hip and does not require postural

control and balance. To encourage optimal function in a standing posture, older adults

should be encouraged to perform exercises such as stepping and rising from a chair. These

exercises target muscle groups and actions important for everyday function. While it may

seem dangerous for older adults to engage in this type of training, resistance training has

proven successful among nursing-home residents, even among quite old adults. The benefits

of participation clearly outweigh the risks of immobility, decreased function, and increased

likelihood of falls and fracture.

As sess ing Your Ris k fo r Os teop oros is

For each of the following questions, check either yes or no.

Ye

s

N

o

1. Do you have a family history of osteoporosis? (Have any of

your relatives broken a wrist or hip or had a dowager’s hump?)

2. Did you go through menopause or have your ovaries removed

by surgery before age 50?

3. Did your menstrual periods ever stop for more than a year for

reasons other than pregnancy or nursing?

4. Did your ancestors come from England, Ireland, Scotland,

Northern Europe, or Asia, or do you have a small, thin body

frame?

5. Have you had surgery in which a part of your stomach or intestines

was removed?

6. Are you taking or have you taken drugs like cortisone, steroids,

or anticonvulsants over a prolonged period?
7. Do you have a thyroid or parathyroid disorder (hyperthyroidism

or hyperparathyroidism)?

8. Are you allergic to milk products or are you lactose intolerant?

9. Do you smoke cigarettes?

10. Do you drink wine, beer, or other alcoholic beverages daily?

11. Do you do less than one hour of exercising such as aerobics,

walking, or jogging per week?

12. Have you ever exercised so strenuously that you had irregular

periods or no periods at all?

13. Have you ever had an eating disorder (bulimia or anorexia

nervosa)?

If you answered “yes” to many of these questions, you may be at an increased risk

for osteoporosis.

Source: From Wellness: Choices for Health and Fitness, by R. Donatelle, C. Snow-Harter, and A.

Wilcox. Copyright © 1995 Benjamin/Cummings Publishing Company. By permission of Brooks/Cole

Publishing Company, Pacific Grove, CA 93950, a division of International Thompson Publishing Inc.

C ON CL US IO NS 

Exercise may benefit the skeleton and reduce osteoporosis and fracture risk in the following

ways: (1) increase bone mass up to and through adolescence, which will result in higher BMD

levels across the life span; (2) improve and maintain bone density during early adulthood;

and (3) reduce or slow the rate of age-related loss during middle and older age. In order to

have an effect on bone, exercise must be different from daily activities, that is, an overload

must be applied to the skeleton. It is important to remember that physical activity has not

been shown to offset the transient increase in bone loss resulting from estrogen deficiency

that is observed in the first five to seven years past menopause. Although still uncertain, the

gap is beginning to narrow with respect to the types and amount of activities that confer the

best osteogenic stimulus. Participation in activities of high load and low repetitions, which

increase muscle strength and power, may ultimately prove to be the most beneficial to bone

mass. Research to quantify forces from activities that promote strength and power will

substantiate these predictions and the models should be evaluated in populations at different

stages of skeletal development.

The importance of building lower extremity strength and cardiovascular health cannot be

over- emphasized with respect to fall prevention and general health. Low bone mass is a

primary risk factor for fractures, with 90% of hip fractures occurring as the result of a fall.

Perhaps the most significant benefit of participation in exercise relates to improvements in

neuromuscular function. Sound neuromuscular function is essential for both static and

dynamic postural stability. The ability to avoid an obstacle, recover from a stumble, or alter

the direction of a fall may significantly reduce fall severity (Greenspan et al., 1994). Muscle

mass, strength, and power decline with age, particularly in the lower extremities (Annianson et

al., 1984) and this has been attributed, in part, to a decrease in physical activity. As a result, it

is more difficult for the elderly to perform activities of daily living, particularly ambulation.

Resistance training programs have demonstrated significant improvement in neuromuscular

function in the elderly through the tenth decade, which translates to reduced risk of fallrelated fractures.
“The foundation for bone health begins early in life. Physical activity that places a load

on the bones is essential throughout childhood and the adolescent years.”

Editors

R EF ER EN CE S

Annianson, A.C., Zitterberg, C., Hedberg, C., et al. (1984). Impaired muscle function with aging. Clinical Orthopaedics and Related Research, 191, 193–210.

Bassey, E.J., & Ramsdale, S.J. (1994). Increase in femoral bone density in young women following high-impact

exercise. Osteoporosis International, 4, 72–75.

Bassey, E.J., Fiatarone, M.J., O’Neill, E.F., et al. (1992). Leg extensor power and functional performance in very

old men and women. Clinical Science, 82, 321–327.

Cavanaugh, D.J., & Cann, C.E. (1988). Brisk walking does not stop bone loss in postmenopausal women. Bone,

9, 201–204.

Chrischilles, E., Sherman, T., & Wallace, R. (1994). Cost and health effects of osteoporotic fractures. Bone, 15,

377–386.

Donatelle, Snow-Harter, C., & Wilcox (1995). Wellness: Choices for health and fitness. Pacific Grove, CA:

Brooks/Cole.

Drinkwater, B.L. (1994). C.H. McCloy Research Lecture: Does physical activity play a role in preventing

osteoporosis? Research Quarterly for Exercise and Sport, 65, 197–206.

Drinkwater, B.L., Nilson, K., Chestnut, C.H., Bremner, W.J., Shainholtz, S., et al. (1984). Bone mineral content

of amenorrheic and eumenorrheic athletes. New England Journal of Medicine, 311, 277–281.

Forwood, M.R. & Burr, D.B. (1993). Physical activity and bone mass: Exercises in futility? Bone Mineral, 21,

89–112.

Granhad, H., Jonson, R., & Hansson, T. (1987). The loads on the lumbar spine during extreme weight lifting.

Spine, 12, 146–149.

Greenspan, S.L., Meyers, E.R., Maitland, L.A., et al. (1994). Fall severity and bone mineral density as risk

factors for hip fracture in ambulatory elderly. Journal of the American Medical Association, 271, 128–133.

Hayes, W.C., Meyers, E.R., Morris, J.N., et al. (1993). Impact near the hip dominates fracture risk in elderly

nursing home residents who fall. Calcified Tissue International, 52, 192–198.

Huddleston, A.L., Rockwell, D., Kulund, D.N., et al. (1980). Bone mass in lifetime tennis players. Journal of the

American Medical Association, 244, 1107–1109.

Kannus, P., Haapasalo, H., Sievanen, H., et al. (1994). The site-specific effects of long-term unilateral activity on

bone mineral density and content. Bone, 15, 279–284.

Mack, P.B., LaChance, P.A., Vose, G.P., et al. (1967). Bone demineralization of foot and hand of Gemini Titan

IV, V and VII astronauts during orbital flight. American Journal of Roentgenology: Radium Therapy and

Nuclear Medicine, 100, 503–511.

Melton, L. J. (1993). Hip fractures: A worldwide problem today and tomorrow. Bone, 14, S1–S8.

Nishimura, H., Fukuoka, H., Kiriyama, M., et al. (1994). Bone turnover and calcium metabolism during 20 days

bed rest in young healthy males and females. Acta Physiol Scand, 150, Suppl 616, 27–35.

Recker, R.R., Davies, K.M., Hinders, S.M., et al. (1992). Bone gain in young adult women. Journal of the

American Medical Association, 268, 2403–2408.

Reid, I.R., Plank, L.D., & Evans, M.C. (1992). Fat mass is an important determinant of whole body bone density

in premenopausal women but not in men. Journal of Clinical Edocrinal Metabolism, 75, 779–782.

Robinson, T.L., Snow-Harter, C., Taaffe, D.R., et al. (1995). Gymnasts exhibit higher bone mass than runners

despite similar prevalence of amenorrhea and oligomenorrhea. Journal of Bone Mineral Research, 10, 26–35.

Slemenda, C.W., Miller, J.Z., Hui, S.L., et al. (1991). Role of physical activity in the development of skeletal

mass in children. Journal of Bone Mineral Research, 6, 1227–1233.

Snow-Harter, C., Bouxsein, M., Lewis, B., et al. (1990). Muscle strength as a predictor of bone mineral density in

young women. Journal of Bone Mineral Research, 5, 589–595.

Snow-Harter, C., Robinson, T., Shaw, J., et al. (1993). Determinants of femoral neck mineral density in pre- and

postmenopausal women. Medicine and Science in Sports and Exercise, 25, Suppl Sl53.
Sowers, M.R., Kshirsagar, A., Crutchfield, M.M., & Updike, S. (1992). Joint influence of fat and lean body

composition compartments on femoral bone mineral density in premenopausal women. American Journal of

Epidemiology, 136, 257–265.

Taaffe, D.R., Snow-Harter, C., Connolly, D.A., et al. (1995). Differential effects of swimming versus weight

bearing activity on bone mineral status of eumenorrheic athletes. Journal of Bone Mineral Research, 10, 586–

593.

Vandervoort, A., Hill, K., Sandrin, M., et al. (1990). Mobility impairment and failing in the elderly. Physical

Therapy Canada, 42, 99–107.