Disuse and Aging

Many changes that accompany aging coincide with those associated with physical inactivity and they are generally recognized as effects of disuse. Thus, bed rest as a consequence of disease may be superimposed on aging changes and further accelerates the aging process. Studies of long-term space travel have revealed that weightlessness in space also induces changes resembling those of aging and physical inactivity. The relationship between disuse (due to bed rest, insufficient exercise, or lack of gravity) and aging has some practical implications because the same practices of prevention and rehabilitation may apply to both.

Association of disuse and aging is apparent at all functional levels. A brief list of the functions affected is presented in Table 8. At the top of the list is a reduction in maximum oxygen consumption (VO2 max), an indirect measure of the ability of the organism to transport oxygen from the atmosphere to the tissues. This transport is significantly reduced with old age (at the rate of about 1% per year) and with bed rest; in both cases, a program of physical activity slows the decline. VO2 max depends on cardiac output, which also decreases with both advancing age and bed rest. Cardiac output (5.0 L/min) represents the amount of blood pumped by each ventricle (stroke volume) per unit of time (cardiac rate) (Chapter 20). Simultaneously, blood pressure increases with age and weightlessness, probably due to increased peripheral circulatory resistance.

In all cases, both younger and older subjects are able to benefit from exercise (Chapter 24). Studies of exercise and movement programs for elders (including nonagenarians) have shown significant increase in lean body (muscle) mass, improvement in several joint movements, and subjective perception of improved mobility and well-being. The capacity for improvement in muscle mass and power upon exercise is

TABLE 8 Physiologic Parameters in Aging, Physical Inactivity Weightlessness (In Space)3

Reduced

Maximum oxygen consumption (VO2 max) Resting and maximum cardiac output Stroke volume Sense of balance Body water and sodium Blood cell mass Lean body mass Glucose tolerance test Sympathetic activity and neurotransmission Thermoregulation Immune responses Increased

Systolic blood pressure and peripheral resistance Vestibular sensitivity Serum total cholesterol Urinary nitrogen and creatinine Bone calcium loss Variable

Endocrine changes Altered EEG Altered sleep

Changes in specific senses a Possibly responsive to physical activity. Abbreviation: EEG, electroencephalogram.

quite striking in humans as well as in experimental animals. At all ages, "practice makes perfect" and "use or loose it." However, for the elderly more than for other age groups, activity programs must be tailored to the individual for optimal benefits (Chapter 24).

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