Fast- and slow-twitch muscle fibers

Depending upon which type of exercise you perform, you mostly use one of the two types of muscle fibers.

The first of the two, slow-twitch muscle fibers, are called upon mostly during aerobic exercise (aerobic exercise in this case means the use of oxygen in the body's metabolic or energy-generating process, not a bunch of people jumping around in leotards). Slow-twitch muscle fibers react to slow, long-duration contractions. And, they're typically used at the beginning of a movement.

The second of the two, fast-twitch muscle fibers, are used more often during anaerobic exercise. These muscle fibers are utilized for rapid, short-duration contractions. Once you've started an exercise, the fast-twitch muscle fibers take the baton from the slow-twitch fibers.

If you're looking to add lean and toned muscle to your frame and drop some body fat, follow a workout program that incorporates the use of both types of the aforementioned muscle fibers.