Over the past 3 decades, there has been a continued increase in the number of "older" participants in sporting events such as running, swimming, cycling, rowing, and weightlifting. Some master athletes come from a background with years of training and competition experience, while others have only begun to compete as they approach middle-aged and older. The majority of what we currently know about master athletes and aging has been gained from both cross-sectional and longitudinal testing and re-testing master athletes and recreational athletes. The focus of this paper is on the physiological profile of athletes and individuals performing regular exercise training. Physiological data from elite and non-elite, recreational, sedentary, and senior athletes clearly indicate that human skeletal muscle has a high degree of plasticity that is maintained late into life. Muscle fiber protein expression and single muscle fiber contractile properties are greatly influenced by exercise training. It appears that skeletal muscle can quickly adapt to accommodate a wide range of functionality to meet the demands (or lack of demands) placed upon it.