The percentage of Americans over the age of 65 yr is growing and this trend has heightened interest in aging research. In this review of human studies, comparisons, as a function of age, are made among the declines of VO2max, work endurance, muscle strength, total muscle cross-sectional area, muscle fiber number, spinal motor neuron number, and motor unit number. Declines in VO2max and total cross-sectional area of leg muscle begin in early adulthood. However, an accelerated loss of total muscle area and a decrease in muscle fiber number begins at about 50 yr of age. Losses in spinal motor neurons and motor units become apparent at about 60 yr of age. However, these findings were collected on different subjects. By better defining these temporal relationships in the same subjects, a more accurate cause and effect relationship may be obtained. Although muscle atrophy is attenuated by resistance training with aging, little is known about the effects of resistance training on the loss of spinal motor neurons, motor units, and muscle fiber number. The goal of this research would be to enhance the ability to promote as much function and independence of living as possible, i.e., increase the quality of life in our expanding elderly population.