Advancing age is associated with profound changes in body composition, including increased fat mass, decreased fat-free mass (particularly muscle), decreased total body water and decreased bone density. Along with these changes in body compositions, and perhaps as a direct result of them, elderly people have lower energy needs, reduced strength and functional capacity and a greatly increased risk for such diseases as noninsulin-dependent diabetes mellitus and osteoporosis. However, the capacity to increase muscle mass through high resistance exercise is preserved even in the oldest subject. Even up to the age of 96 y, men and women can respond to resistance training with a substantial (greater than 200%) increase in strength and muscle size. This review describes the metabolic consequences of strength training in young and old, with particular attention to the eccentric component of this exercise. Eccentric muscle contractions cause muscle damage and delayed-onset muscle soreness. It is our hypothesis that muscle damage is the primary stimulus for muscle hypertrophy, by stimulating an acute phase response that ultimately leads to increased rates of muscle proteolysis and repair.