With aging, several functions related to the locomotor system will deteriorate. Still, there is a rather well-maintained adaptability for increasing physical activity and for training in the elderly with respect to strength as well as endurance. Part of the observed reduction in muscle function with age could therefore be caused by inactivity. Evidently, however, there is a reduction in muscle mass with age due to loss of motor units. Muscle strength and aerobic power will fall fairly proportionally to the reduction in muscle mass. The muscle changes are, at least up to around the age of 70, more quantitative than qualitative. There is a reduction in the size of the fast-twitch (type II) fibers in the quadriceps, but not in other muscles such as the biceps brachii, presumably due to differences in activity pattern and motor unit recruitment. Aging per se may not reduce oxidative enzymatic capacity in the skeletal muscle. There is maintained potential for increase with endurance training. Thus, as an extreme example in very well trained elderly endurance athletes, high activity of citrate synthase is found and, in connection with that, also low plasma insulin values at baseline as well as after glucose intake. Limitations to aerobic capacity besides the reduced muscle mass are mainly located in the central circulatory and respiratory systems. Comparing the oxygen cost of activities in daily living, such as walking, and the aerobic power of the elderly, suggestions will be given on the choice of training activities.