Hypertrophy by definition is the increase in organ size due to an increase in cellular size, rather than an increase in cell numbers. Hyperplasia, in contrast, is the increase in size due to an increase in the number of cells. Hyperplasia is the dominant growth pattern during the years from conception to adulthood, while hypertrophy is the dominant pattern seen in muscular growth after adulthood.
Hypertrophy, in a little more detail, is the increase in size of a single muscle fiber (cell) due to the incorporation of other nuclei from satellite cells to stabilize and strengthen the muscle after trauma. True muscle growth is believed to be a combination of hypertrophy and hyperplasia.
When a muscle is subjected to an overload (resistance training) it causes trauma to the muscle fibers. These fibers are damaged and need repair. The body quickly reacts by multiplying satellite cells on the muscles and then these daughter cells are pulled into the fibers and the nuclei and sarcoplasm are then incorporated into the new cell. This results in an increase in muscle fiber size. The muscles are also reinforced with additional myofibrils, containing the contractile proteins actin and myosin, to increase the strength of the individual fibers. The body takes these steps to decrease the damage done the next time the muscle is subjected to that same load. However, there is a limit to how much the fiber can be increased, and the amount of myofibril increase, in one session.
In order to stimulate hypertrophy in the muscles it is important to cause this trauma and create an environment within the muscle that requires the cells to increase in size and strength to prevent future trauma. Causing the muscle to work under a progressive overload is essential for hypertrophy. The workload placed on the muscle can be increased in two ways. As the muscle adapts to the current workload, the amount of weight can be increased or the number of repetitions or sets can increase. The former is a good option, however, proper form must be the focus because heavy weights tend to cause a loss of form. The latter works until the repetitions reach 15, after that point it is believed that the slow-twitch type I muscle fibers begin to taker over. These fibers do not have the same propensity for size gains as the fast-twitch type II fibers.
The best overload system for hypertrophy is a combination of the two increases. Increasing the repetitions with a weight until 12 can be performed in good form and then increasing the weight will cause muscular hypertrophy with a continuous overload on the muscles. The key to growth is increasing the workload as soon as the muscle adapts to the current workout. Keep the muscle under trauma during workouts and allow enough rest time for the body to cause muscle hypertrophy during repair.