Physical Activity: A Strong Stimulant for Hormesis During Aging.

In the animal world physical exercise is an intimate part of life cycle to pursue food, escape predators, and ensure reproduction. Aged individuals with reduced fitness and mobility are subjected to natural selection. In rodents, volunteer wheel runners show an increase in both maximal life span and 50% survival rate compared to their sedentary counterparts, indicating physical activity can change aging process (Holloszy 1993). In human population, morbidity is concentrated in the last 2 decades of life, beginning on the average at age 55 and increasing in frequency until the average age of death at 75. The benefit of exercise is highlighted by the increase of approximately 2 years in longevity in physically active people as compared to less active people (Paffenbarger et al. 1993). Disability levels in a vigorously exercising population are below that of non-exercisers and age-related increases in disability are delayed by approximately 15 years (Fries 1996). These data indicate that engaging in regular physical activity would increase the age of onset of chronic illness and shorten the time between the onset of morbidity and death. Furthermore, this compression of the period of morbidity as a result of physical exercise would represent a significant improvement in the quality of life and result in major reductions in the health care for the elderly. Despite these clear benefits of participating in physical exercise, there is a concern that aged individuals are more susceptible to some of the harmful effects of rigorous exercise as a result of increased exposure to reactive oxygen species (ROS) (Davies et al. 1982). The free radical theory of aging (Harman 1956) has allowed for the establishment of a powerful link between exercise and aging research. A fundamental premise for this theory is that ROS generated in normal metabolic processes are the underlying reason for cell and tissue oxidative damage seen throughout the aging process. Since exercise increases metabolic rate reflected by a greater amount of oxygen uptake, ROS production is increased during physical exertion. One may naturally ask this question: is physical exercise more beneficial or harmful to the elderly population? A complete and unequivocal conclusion is still forthcoming; however, there is enough evidence to demonstrate that a mild oxidative stress associate with exercise may not be a bad thing during aging. Although aging may cause increased ROS generation and oxidative stress probably in all cell types, which could be influenced by physical activity overall, in this chapter the author chose to focus on skeletal muscle for the obvious reasons that (a) skeletal muscle health is vital for mobility and normal life; (b) deterioration of skeletal muscle mass and functionality (sarcopenia) is an important issue in medical gerontology (Thomas 2007); and (c) skeletal muscle has displayed some unique characteristics during aging. [ABSTRACT FROM AUTHOR]