In order to gain mass, the muscles must hypertrophy. There are two types of hypertrophy; myofibrillar hypertrophy, and sarcoplasmic hypertrophy. These terms may or may not mean anything to you at all, depending on your background, but they are two terms that are vital to understand if you choose to begin lifting weights.

Myofibrillar hypertrophy is the hypertrophy ("enlarging") of the actual contractile units in the muscle, the muscle fibers. Imagine a muscle cell as a jelly donut, if you would. The myofibrillar hypertrophy range packs more jelly into that donut, but doesn't make the donut itself any bigger until no more jelly can be held. When more "donut" is needed, you gain a little bit of mass, and continue filling up the "donut" with "jelly". The "jelly" is what makes your muscles able to contract harder and stronger. Myofibrillar hypertrophy is best stimulated with low reps, usually between 5 and 8. Myofibrillar hypertrophy will keep your muscles hard and strong, but deceptively small. Most athletes participating in strength-based sports work in this range. Some powerlifters work in this range, and that is, in part, along with other forms of training, are able to lift such huge amounts of weight and often be relatively small.

Sarcoplasmic hypertrophy is the hypertrophy of the "donut". The "donut" gets bigger, but no extra "jelly" is added.
Sarcoplasmic hypertrophy is what bodybuilders do to get their godlike physiques. It's important to note, however, that sarcoplasmic hypertrophy does not yield much for strength gains, and is just about all mass and no "umph", depending on the exact range you use. Sarcoplasmic hypertrophy makes the muscles bigger, but not much stronger, if at all stronger. Sarcoplasmic hypertrophy is generally thought of to be found between rep ranges of 10 and 15. Training more on the 15-end of the spectrum will yield some muscular endurance, however. It is important to note, though, that there are much more efficient ways to gain muscular endurance than resorting to the 10 to 15 sarcoplasmic range.

Well, that's all well and good, you may be thinking. But what about the range you haven't mentioned yet, the 5-and-below range?

The 5-and-below range stimulates, mostly neuromuscular facilitation. Take a moment to soak that word in. It's beautiful, isn't it? But what does it mean?

Before my explaining of what exactly the term "neuromuscular facilitation" means, it is vital for you to understand in some detail what makes muscles strong.

Muscles are strong by two means: the muscle itself, and the neural connections between the muscle and the brain. Now, the muscle itself has clear-cut limits, for the most part. But the limits of neural connections between the muscle and the brain are vague at best. Most of the strength in the muscle actually comes from neural connections; some estimates are as high as 80%. Just imagine--your muscles can be outstandingly stronger, and not much bigger, simply because of a mighty connection between the brain and the neural factors in the muscle. This considered, strength training need not only exist in hypertrophy range. One could become extremely strong yet remain small, lean, and compact.

Now that that is out of the way, I can better explain neuromuscular facilitation. Neuromuscular facilitation is essentially the physical incarnation of the old axiom that "practice makes perfect". When we lift weights, both our muscles and our nervous system are affected in positive ways. By working in the 1-5 range, the neuromuscular connection has to be pushed along to help the muscle contract safely. When the body realizes that the neuromuscular connection successfully prevented you from, well, dying, it strengthens that neuromuscular connection for next time, thus making you stronger than before. Also, the 1-5 range has minimal effect on the ATP. ATP is adenosine triphosphate, the fuel of your body. ATP is made in the body, in the mitochondria, through the breakdown of glucose.

When ATP is depleted, your muscles may feel weak, heavy, and tired. ATP depletion is at its most after you complete a long endurance exercise, such as performing 100 consecutive pushups, or something like that. Naturally, then, ATP depletion is at its least when you lift very low reps, such as in the 1-5 range.

This means that you could lift every day in the 1-5 range and continue to get stronger, if you wanted. You are able to do this in the 1-5 range and in no other range because in other ranges, ATP is depleted and takes more than few hours to recover (as it does with the neuromuscular facilitation method) and the muscles hypertrophy some, and need to repair themselves. As there is no hypertrophy (or absolutely minimal) hypertrophy involved in the neuromuscular facilitation method, it becomes possible to train daily.

But what if you want to train low-rep, for maximal strength, and still gain mass?

Volume is the answer. Volume is the total number of reps performed. If one were to perform, say, 3 sets of 10 benchpresses, that's a total of 30 reps. If one were to perform 5 sets of 6 benchpresses, that's also a total of 30 reps. If one were to perform 10 sets of 3 benchpresses, that's also a total of 30 reps, and all three methods of adding volume will result in similar hypertrophic effects, though the 3x10 would result in sarcomplasmic hypertrophy mostly, the 5x6 would result in myofibrillar hypertrophy mostly for maximal strength, and the 10x3 would result in myofibrillar hypertrophy also, but with even more of a focus on maximal strength.

That said, get out there and lift some weights. Or, if you don't like weights, or don't have any around, or a gym, there are bodyweight strength training methods that you could use...but that's another article.

Source: http://www.associatedcontent.com/article/104845/fitness_tips_rep_ranges_...