Deconditioning

Many people, and even some health care professionals, often see much of the disease and loss of function found with aging as a normal and inevitable consequence of aging. However, it is now clear that such changes are in fact due to a combination of true age-related decline, disease (whether overt or hidden), and disuse. It is often difficult in practice to determine the relative contribution of each of these factors.

Muscle mass declines steadily with increasing age. This results in a loss of muscle strength of 1 to 2 percent per year. It is important to note, however, that reduced muscle mass (or sarcopenia) with age cannot be entirely due to inactivity, as it is also found in highly trained elderly athletes. The patterns of muscle loss do appear to differ between the two. While aging results in a reduced number of muscle fibres, disuse primarily causes a reduction in muscle fibre size. Sustained muscular activity requires the delivery of an adequate supply of oxygen to the muscles and utilization of the oxygen in the mitochondria of the muscle cells. Cross-sectional studies (comparing people of different ages) and longitudinal studies (serial examinations of people as they age) have found a decline in maximal oxygen uptake with age. The most important factor in this decline may be the changes in muscle mass due to either age or disuse, since maximal oxygen uptake is almost independent of age when expressed relative to fat-free mass (mass mainly composed of muscle). Similarly, cardiac output shows little change with age when related to fat-free mass.

The degree to which decreased physical activity with increasing age is a cause, rather than an effect, of reduced muscle bulk is uncertain. Jackson and colleagues have argued that about half of the decline in exercise capacity over the adult life span can be attributed to chronic physical inactivity and the resultant changes in body composition (increased body fat and reduced muscle bulk). The beneficial effects of exercise training programs on muscle strength, exercise capacity, and balance suggest, but do not prove, that the decline in physical fitness with old age is at least partially preventable.