By age twenty-five, men and women begin to lose skeletal muscle mass (skeletal muscles are the muscles that move your body, such as biceps, abs, and hamstrings) at a rate of up to 1 percent per year. That adds up. And once a muscle cell is gone, it’s gone forever. The same process occurs with your stride length (the distance each running stride carries you), which, barring intervention, will shorten up to 40 percent by the time you reach your seventies. Proper training can drastically curtail both these losses.