Type I muscle fibers These also called "slow-twitch" muscle. It is composed of small fibers. Type I muscle is used to carry light loads over long distances. A marathon runner has an abundance of type I muscle fibers. Picture the marathon runner - lean and toned. Type II muscle fibers These are called "fast-twitch" muscle fibers. Type II muscles are large and used for lifting heavy loads. Type II muscles are powerful but fatigue quickly. Sprinters have an abundance of type II muscle... Read More