There are two types of muscle fiber types: type I and type II. Type I fibers are slow twitch fibers that resist muscle fatigue. Endurance athletes have more type I than type II muscle fibers. Type II muscle fibers shorten and develop tension faster than type I fibers. Type II fibers are good for activities that require power and greater speed.

You are born with a set amount of each type of muscle fibers. You can't change that with exercise or training. However, you can stimulate muscle fibers already present and develop them to their maximum potential. Training to increase speed or improve endurance is possible, but only within the confines of your body type and muscle type. There are some studies that suggest prolonged exercise training can alter fiber type, but this hasn't been proven yet.