This compilation discusses how low levels of physical activity and excess body weight are considered key health risks in modern societies. This may be attributed to changes in the social and built environment, along with technical advances that reduced the requirement for physical activity in daily living. The authors investigate population aging, the physiology of aging, and the prescription of physical activity for the elderly. In addition, the effects of oxidative stress on skeletal muscle during high-intense resistance training are studied, particularly focusing on reactive oxygen species.