Muscle groups in the body are comprised of two types of fibers: Slow-twitch and Fast-twitch. Understanding the difference between the two, as well as the training methodologies which lead to successful training of each area, will lead to the recruitment of the highest possible number of fibers, and should be very important to bodybuilders!

Slow-twitch

The first kind is Type I fibers, or slow-twitch muscle fibers. These fibers have very strong aerobic ability for oxidation, they contract very slowly, and they are very useful in endurance activities. These muscle fibers are "hit", or engorged with nitrogen-rich blood, during higher rep training, specifically in sets of 12 to 20 reps. This type of training is often neglected by bodybuilders.

Fast-twitch

This group of muscle fibers is called Type II, and is considered to be of the fast-twitch variety. These fibers assist with short, heavy lift requiring short bursts of power. They are not effective in longer-term training, but are very useful in brief, high-intensity training like we see in bodybuilding or powerlifting. This is the training methodology that most trainers use in the gym.

If you currently train your body using only low- or high-rep schemes, it is very possible that you are neglecting a large number of muscle fibers in each muscle group. Powerlifters who always use rep schemes of 2 to 5 reps are completely ignoring slow-twitch fibers. Likewise, trainers who favor machines and higher rep schemes are neglecting their fast-twitch fibers, which are significantly more important in bodybuilding than slow-twitch fibers.

The lesson here is simple. You must vary your rep ranges in order to recruit the largest possible number of muscle fibers of both types when training. Any bodybuilder or powerlifter who doesn't vary rep ranges is significantly limiting his or her bodybuilding success by leaving millions of muscle fibers untrained during each workout. As stated, bodybuilding training typically involved more fast-twitch fibers. However, as you move closer and closer to your finite potential for muscle building, it suddenly becomes clear that training a large group of untrained fibers might just be a great idea!