Whenever you start reading things on weight training you come across a bunch of really long and complicated sounding words. One of these is the word hypertrophy. If you want to get bigger, stronger, bulk up and gain weight and muscle, then this is a word you should pay attention to. It’s just another way of saying increase in the size of muscles.
There are two main functions of skeletal muscles. The first is to contract and thereby cause the body to move and the second one is to provide stability for the posture of the body. As mentioned in previous posts, muscles are made up of many smaller units called fibers. It is at this level that hypertrophy occurs. It happens through the increase of muscle mass, as well as the cross-sectional area. This is done through stimulus from the outside. Muscles adapt to work load. Exercise is often used to simulate this work load. Through exercise, stress is applied to the muscle. The muscle then adapts and generates tension. This results in the increase of size and amount of contractile proteins (actin and myosin) which make up the myofibrils in the fibers. So the muscle fibers get bigger. This is hypertrophy.
There are two main types of hypertrophy: sarcoplasmic and myofibrilar. Sometimes a third type, transient, is mentioned as well, however that one is not as important, as it only means the temporary swelling of the muscle (due to fluid accumulation) after exercise.
Sarcoplasmic hypertrophy means an increase in the volume of the sarcoplasm in a muscle cell. Sarcoplasm is the gel-like fluid-like substance that is enclosed within a muscle cell. It contains all kinds of things, with the largest percentage of it being taken up by water. It also contains glycosomes (granules of stored glycogen), as well as myoglobin, which is a protein that binds oxygen. Sarcoplasm accounts for about 25% to 30% of the muscle cell’s size, so if it gets swollen up, the cell increases in size as well. Here the cross-sectional area of the muscle increases, but the actual density of the muscle fibers decreases. So this is mostly non-functional increase in muscle size. This increases the bulk size and look of the muscles. The strength of the muscle does not increase as much.
Myofibrilar hypertrophy means that the muscle fiber itself gets bigger as it gains more myofibrils. These myofibrils can then contract and generate tension in the muscles. This makes the muscle stronger. This is because the density of myofibrils increases. This produces functional muscles and an increase in strength and explosive power. The main type of muscle fiber that gets bigger through training for myofibrilar hypertrophy is the Type IIb muscle fiber. So with myofibrilar hypertrophy the density and strength of the muscle increases, the size not as much.
Growth in muscles can be induced by several factors. The most important is probably progressive tension overload, where a heavier load is applied on the muscles and they get stronger and bigger. This means progressively lifting heavier weights will make your muscles grow bigger. Another factor that can induce growth in muscles is muscle damage. Lifting causes tiny damages to the muscle fibers. After exercise and during your rest period this damage is being repaired, but building blocks made of nutrients are needed for this repair. So the repairing of this damage causes the muscles to grow bigger and stronger. Cellular fatigue is also a stimulus for growth by pushing muscle fibers to their metabolic limits through doing repetitions to muscular failure.
So what type of training do you need to do to stimulate the growth of muscles and to produce the types of hypertrophy described above?
The general consensus is that if you want to go for myofibrilar hypertrophy and increased strength and explosiveness, you should train in the 1-5 rep range. If you want to go for sarcoplasmic hypertrophy and an increase in size, you should train in the 6-12 rep range (with the 6-8 rep range being in the middle of the two types of hypertrophy and you getting a good compromise between both). The weight that you are using in each of the rep ranges needs to be big enough that if you get to the end of the rep range, you should be close to failure (but not at failure). So if you are doing a 5 rep set, then the weight should be around 85% of your 1 rep maximum, while with the 10 rep range, you should be lifting weights close to 75% of your 1 rep maximum. With any rep range above 12, you are going more for muscle endurance and not hypertrophy. You should also remember that after a period you always need to go up in weight in order to induce hypertrophy. So as you progress through your training, the weight you are lifting should get bigger.
I think the main reason why people want to gain weight is to gain size and bulk and thereby look bigger. So here the goal is more sarcoplasmic hypertrophy and that means training in the 6-12 rep range.
PS: There is an interesting side note and that is the discussion of hypertrophy (growth in the size of the cell) vs. hyperplastia (growth in the number of cells). While hypertrophy is well observable in humans, hyperplastia has been observed in scientific experiments only with animals, but so far not with humans (which does not mean that it does not occur in humans, but only that it has not been observed in scientific experiments with human subjects).