Although the loss of muscle strength with aging is multifactorial, the primary factor is the loss of muscle mass. A preferential loss of Type II (fast-twitch) muscle fibers which produce more force than Type I fibers is also observed. The loss of muscle mass may be related to a reduction in the rate of muscle protein synthesis in the old versus the young. Changes in muscle quality and the ability to activate muscle appear to play a minor role in the loss of strength with age. However, co-activation of antagonist muscle groups does appear to reduce muscle force generating capacity in the elderly. Strength gains in response to resistance exercise training in the elderly, although substantial, may be less than in young individuals. Increases in muscle mass appear to be similar in elderly and young individuals as does the muscle protein synthetic response to resistance exercise. Muscle co-activation appears to be substantially and similarly reduced (improved) in young and elderly individuals as a result of resistance training.