Muscle hypertrophy is a term for the growth and increase of the size of muscle cells. The most common type of muscular hypertrophy occurs as a result of physical exercise such as weightlifting, and the term is often associated with weight training.

When you start exercising a muscle there is first an increase in the nerve impulses that cause muscle contraction. This alone often results in strength gains without any noticeable change in muscle size. As you continue to exercise, there is a complex interaction of nervous system responses that result in an increase in protein synthesis over months and the muscle cells begin to grow larger and stronger.

There are two essential components necessary for the growth of muscles—stimulation and repair. Stimulation occurs during the contraction of the muscle, or during the actual exercising of the muscle. Each time that a muscle is exercised, contraction occurs. This repeated contraction during a workout causes damage to the internal muscle fibers. These muscle fibers are broken down throughout the course of a workout. Once damaged, these fibers are then ready to be repaired. This is where muscle growth occurs.1

Muscle fiber repair occurs after the workout while the muscles are in resting mode. New muscle fibers are produced to help replace and repair the damaged ones. More fibers are produced to make up for the damaged ones, and this is where the actual muscle growth takes place.

Researchers are beginning to identify a potential third important component—peripheral fatigue.2 Peripheral fatigue occurs when you are unable to complete exercises and it may occur at the end of strenuous activity. Studies are ongoing but scientists believe that the more peripheral fatigue you can induce, the harder the muscles have to work and the more the muscle that is stimulated, the more hypertrophy occurs. However, more research needs to be conducted to fully understand the relationship

Genetic Impact on Hypertrophy

Although the process of hypertrophy is the same for everyone, your results are likely to be different from others doing the same workouts. This variance in results is due directly to the genetic make-up of the muscles of the individual. For some people, the process of hypertrophy occurs to a much larger degree or at a faster rate than it does for others.

If you won the genetic lottery, you may grow bigger muscles or see results sooner than others at your gym. Unfortunately, the opposite may also be true.

The shape and appearance of a muscle are other factors that vary based on genetics. As badly as you may want a nice, round, plump bicep, it may not always be in the cards. The shape of a muscle is determined by the length of the tendons of the muscle. Tendon length is a genetic factor.

For bigger muscles, it is better to have shorter muscle tendons. Someone with extremely long muscle tendons may see less growth and shaping of their muscles than someone with much shorter muscle tendons, despite doing the same amount (or more) weight lifting.

Lastly, your muscles are made up different muscle fibers: type 1 (slow-twitch) and type 2 (fast-twitch). Different muscles have different ratios of type 1 and type 2 fibers.

For maximum hypertrophy you want to train both muscle fiber types with different types of exercises. This is why athletes in sports like football have large muscles because their training involves different types of activities that target both fast and slow twitch fibers. For example, lifting heavy weights targets slow twitch fibers and sprinting targets the fast twitch fibers.