Muscle atrophy, weakness, injury, and fatigue are inevitable and immutable concomitants of old age. Atrophy results from a gradual process of fiber denervation with loss of some fibers and atrophy of others. Fast fibers show more denervation and atrophy than slow fibers. Some fast fibers are reinnervated by axonal sprouting from slow fibers resulting in remodeling of motor units. With aging, the decreases in strength and power are greater than expected from the loss in muscle mass. Contraction-induced injury is proposed as a mechanism of the fast fiber denervation. With atrophy and weakness, human beings show a dramatic decrease in endurance and increase in fatigability with aging, but strength and endurance training slows the process.