Skeletal muscle contraction applies force to bone. Because the architecture of bone remodeling involves osteoblast and osteoclast activation in response to loading and unloading, physical activity is a major site-specific influence on bone mineral density and geometry. Repetitive physical activity can create excessive strain, leading to inefficiency in bone remodeling and stress fracture,- however, extreme inactivity allows osteoclast dominance and bone loss.
The forces applied to bone during exercise are related both to the weight borne by the bone during activity and to the strength of the involved muscles. Consequently, bone strength and density appear to be closely related to applied gravitational forces and to muscle strength. This suggests that exercise programs to prevent or treat osteoporosis should emphasize weight-bearing activities and strength as well as endurance training. Adequate dietary calcium is essential for any exercise effect: weight-bearing activity enhances spinal bone mineral density in post-
Exercise and bone density. This graph shows spine bone density in young adult women who are nonathletes (controls), distance runners with regular menstrual cycles (cyclic runners), and distance runners with amenorrhea (amenorrheic runners). Differences from controls indicate the roles that exercise and estrogen play in determination of bone mineral density.
menopausal women only when calcium intakes exceed 1 g/day. Because exercise may also improve gait, balance, coordination, proprioception, and reaction time, even in older and frail persons, the risk of falls and osteoporosis are reduced by chronic activity. In fact, the incidence of hip fracture is reduced nearly 50% when older adults are involved in regular physical activity. However, even when activity is optimal, it is apparent that genetic contributions to bone mass are greater than exercise. Perhaps 75% of the population variance is genetic, and 25% is due to different levels of activity. In addition, the predominant contribution of estrogen to homeostasis of bone in young women is apparent when amenorrhea occurs secondary to chronic heavy exercise. These exceptionally active women are typically very thin and exhibit low levels of circulating estrogens, low trabecular bone mass, and a high fracture risk (Fig. 30.5).
Exercise also plays a role in the treatment of osteoarthritis. Controlled clinical trials find that appropriate, regular exercise decreases joint pain and degree of disability, although it fails to influence the requirement for antiinflammatory drug treatment. In rheumatoid arthritis, exercise also increases muscle strength and functional capacity without increasing pain or medication requirements. Whether or not exercise alters disease progression in either rheumatoid arthritis or osteoarthritis is not known.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.