muscle, and environmental surfaces to absorb and distribute mechanical energy, and the architecture of bone.20,64 A better understanding of biomechanical and other factors affecting the risk of fall injuries is needed.

Older people often have a worse outcome than younger people from the same injury because of impaired tissue regeneration, decreased functional reserves, and poorer immunologic function.18,48 The psychosocial sequelae of falls are a poorly understood but potentially important outcome because they may influence functional recovery from fall injuries as well as the risk of further falls. Very little is known about factors that precipitate a ''postfall syndrome" of extreme fear and anxiety.

PREVENTABILITY OF BURDEN

Research on risk factors for falls and the causes of postural instability suggests many preventive interventions, although at present there is almost no direct evidence of the effectiveness of any approach to preventing falls.46 This situation will soon change as a result of studies now in progress. For the moment, however, one can only speculate about which approaches are most promising. In a few instances, evidence regarding modifications of intermediate variables that are risk factors for falls provides a limited basis for speculation.

Exercise and Physical Activity

Skeletal muscle strength and mass decline with age57,58,71 and immobility.12 Impaired strength is a strong predictor of falls in most studies (Table 15-1) and may also increase the risk of injury from a fall. Exercise might prevent falls and injury by strengthening muscles and increasing endurance; maintaining and improving posture, joint motion, and postural reflexes; stimulating cardiorespiratory function; and improving alertness.41 A growing body of evidence indicates that the elderly respond to exercise training and that this response (which may include increased muscle strength and mass and increased aerobic capacity24) continues at very old ages and extremes of frailty.3,25 Weight-bearing exercise may also help preserve bone mass, although this benefit is uncertain.43,81 Exercise and physical activity are positively associated with physical and mental function in cross-sectional studies, but whether exercise training in the elderly can improve physical function, postural reflexes, mental function, or general health and well-being is uncertain.43

The type, level, intensity, and duration of exercise required to



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