This review discusses the physiologic age-related changes in both contractile and neural aspects of the motor unit (MU), summarizing mainly studies on human muscles. The substantial weakness, slowed movements, and reduction in force control seen in old age cannot be accounted for solely by loss of muscle mass. Factors such as changes in contractile quality, specific tension, and neurologic mechanisms must be considered. The only commonality to date is that loss of neuromuscular function is specific to the muscle group, the task, history of use, and possibly gender. Since there is a functional relationship between contractile and neural properties of the MU, these aspects need to be studied together and their age-related changes evaluated and correlated to better understand neuromuscular weakness with age. Although there are many reports of the positive benefits of strength training for aged humans, the relative contributions of the different aspects of the MU to improved function are not well understood. One goal in this field is to better understand the mechanisms associated with strength improvements in order to develop the most appropriate rehabilitation programs for aged humans.