An excellent, detailed, and relatively non-technical introduction to the topic: PHYSIOLOGIC AND LONG-TERM ADAPTATIONS TO EXERCISE.
One quote:
The deterioration in physiological function normally associated with aging is, in fact, caused by a combination of reduced physical activity and the aging process itself. By maintaining an active lifestyle, or by increasing levels of physical activity if previously sedentary, older persons can maintain relatively high levels of cardiovascular and metabolic function, including ˙VO2max (Kohrt et al. 1991), and of skeletal muscle function (Rogers and Evans 1993). For example, Fiatarone and colleagues (1994) found an increase of 113 percent in the strength of elderly men and women (mean age of 87.1 years) following a 10-week training program of progressive resistance exercise. Cross-sectional thigh muscle area was increased, as was stair-climbing power, gait velocity, and level of spontaneous activity. Increasing endurance and strength in the elderly contributes to their ability to live independently.
The saddest part of the story is how quickly training gains are lost during periods without regular (meaning 3+ days/wk) strenuous activity. There is measurable decline in cardio functioning and muscle strength in weeks, and training gains can be lost in a few months. Use it or lose it. That is just how it is after age 30 or 35.
Physical fitness, like mental fitness, is a life-style choice.
