Muscle weakness in the elderly is prevalent and morbid, closely linked to the frailty, functional decline, immobility, falls, and injuries in this population. The marked decrease in skeletal muscle strength and size with aging is a multifactorial syndrome which may be attributable in part to: (a) biological changes of aging itself; (b) the accumulation of acute and chronic diseases; (c) the assumption of a sedentary life style, and (d) selective or generalized nutritional inadequacies. Inactivity and undernutrition are potentially at least partially reversible with appropriate interventions, and therefore the delineation of the attributable risk of these two factors to the muscle weakness of aging is a critical research goal. Similarly, identification of appropriate modalities of physical activity and nutrition which have positive effects on muscle physiology in the aged is the focus of major investigations currently.