Role of Exercise in Preventing and Treating Osteoporosis

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Bone mass at any time of life reflects the totality of events that have impinged on the skeleton to that point. For adults, these events include those that have influenced the acquisition of bone during years of growth, resulting in the achievement of skeletal maturity, or “peak bone mass, ” as well as those that have subsequently influenced bone losses. For each limb of this trajectory, physical activity has been implicated as a powerful and independent factor. This article reviews current evidence regarding the relation of habitual physical activity to bone acquisition and maintenance, the skeletal consequences of exercise training, and the clinical value of exercise for patients with skeletal frailty.

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TRAJECTORY OF BONE ACQUISITION, MAINTENANCE, AND LOSS

A brief recapitulation of the gain and loss of bone with age is provided here to assist the reader in placing the role of physical activity into perspective. After gradual fairly linear acquisition of bone during childhood, growth acceleration with puberty stimulates an intense gain in bone over a period lasting, in any individual, about 3 to 4 years. Controversy remains over the precise time at which peak bone mass is achieved, but the process seems to be at least 95% complete by the age of 17

PHYSICAL ACTIVITY AND BONE

A primary function of the skeleton is to provide a strong resilient structure that permits resistance against gravitational and other forces while providing structural rigidity for locomotion. To carry out this function, bone adapts to the mechanical demands that are placed on it. According to the principle known as Wolf's law, bone accommodates the loads imposed on it by altering its mass and distribution of mass. When habitual loading increases, bone is gained; when loading decreases, bone is

BONE MASS IN ATHLETES

It is frequently observed that the bone mineral density (BMD) of trained athletes exceeds that of nonathletic controls, an observation that holds true across a wide variety of sports, with the largest effects being seen with power events requiring high muscle forces.35 Interpretation of this literature is confounded by the fact that the musculoskeletal characteristics of elite athletes likely differ from those of the general population even before the initiation of training (i.e., ascertainment

PHYSICAL ACTIVITY AND BONE STATUS OF NONATHLETES

A critical issue is whether the skeletal benefits enjoyed by elite athletes also extend to the general population. In healthy normal children and adolescents, observational studies as well as exercise intervention programs indicate a direct relation between habitual physical activity and increases in BMD (Fig. 2).2, 3, 31, 34 In those studies, however, BMD measurements reflect not a true volumetric BMD but are reported as an “areal” density, that is, the amount of mineral contained within a

Children

The relation of physical activity to gains in bone during childhood have been alluded to previously. In addition, childhood represents a unique opportunity not only to gain bone density but to modify skeletal dimensions and architecture in response to mechanical loads. Vigorous exercise during growth years seems to increase the accumulation of bone5, 21 and to be associated with permanently increased cross-sectional areas of the trained region such as the dominant forearm of tennis players.14

SPECIAL ASPECTS OF EXERCISE AND BONE: AMENORRHEA, GYMNASTS, AND SWIMMERS

Skeletal maintenance requires an adequate hormonal, mechanical, and nutritional milieu, and deficits in one sphere are not adequately compensated by overzealous attention to the others. Amenorrheic women athletes lose bone and have increased risk for fracture despite Herculean training schedules.10, 18, 22 Despite initial views that cortical bone is spared in these women, significant deficits are found at all appendicular sites, with the possible exception of the forearm.22 Elite women gymnasts

CONTRIBUTION OF EXERCISE TO BONE HEALTH OF OLDER PEOPLE

The modest BMD results with exercise should not trivialize the importance of physical activity for protecting older people against falls. Although the primary focus of this article is bone, the relation of age-associated declines in muscle strength to skeletal disability is of more than tangential relevance. More than 90% of hip fractures occur as the immediate consequence of a fall onto the hip. Among the important risk factors for falls, muscle strength is relatively more susceptible to

SPECIAL CONSIDERATIONS FOR PATIENTS WITH OSTEOPOROSIS

Minimal attention has been devoted to the role of exercise for osteoporotic patients who have already sustained fractures. In fact, it is frequently the case that patients and physicians show reluctance to participate in an exercise program because of concerns for additional injury. Predictably, avoidance of activity aggravates bone loss and places the skeleton at even greater jeopardy. Health professionals who work with osteoporotic patients recognize that back-strengthening exercise

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    Address reprint requests to Robert Marcus, MD, GRECC 182-B, Veterans Affairs Medical Center, 3801 Miranda Avenue, Palo Alto, CA 94304

    *

    Geriatrics Research, Education, and Clinical Center, Veterans Affairs Medical Center, Palo Alto; and the Department of Medicine, Stanford University School of Medicine, Stanford, California

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    Copyright © 2001 W. B. Saunders Company. Published by Elsevier Inc. All rights reserved.