There are two types of muscle fibers, and depending upon what type of exercise you perform, you mostly only hit one type or the other.

The first of the two, slow-twitch muscle fibers, are called upon mostly during aerobic exercise. 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 essentially take the baton from the slow-twitch fibers and finish the race.

For maximum results, try following a weekly workout regimen that includes exercises targeting both of these types of muscle fibers. One of the best ways to do so is to alternate days of cardio and strength training.