Hypertrophy refers to the enlargement of an organ or tissue by increasing the cell size. Not to be confused with Hyperplasia, the process of increasing the number of cells, hypertrophy is the process of increasing the size of the cells that are already there!
When you exercise, especially during resistance/weight training, your muscle fibers are across resistance and muscle contraction happens, which facilitates micro-tears inside the muscle fibers (myofibrils). Once these myofibrils are broken, the body responds to it by repairing it, but while repairing, it creates them much thicker – hence better quantity and quality. This kind of muscle hypertrophy is called Myofibril Hypertrophy. This occurs through a physiologic process in which the amount of contractile proteins (actin and myosin) in each muscle fiber increase. With the right training regimen, you can catalyze this process
However, the pace of hypertrophy varies among individual even with the exact same workout routine as it is also highly dependent on genetic factors. Even the shape of these muscles, which is dependent on the length of the tendons of muscle are also dependent on genetics!
The second type of muscle hypertrophy is Sarcoplasmic Hypertrophy, wherein the volume of the muscle increases because of the increase in the volume of sarcoplasmic fluid in the muscle cell, with no actual increase in muscular strength. Sarcoplasm is the fluid that surrounds myofibrils, to provide energy to the muscles (myofibrils). It is basically ATP, glycogen, creatine phosphate and water. Sarcoplasmic hypertrophy is a kind of artificial muscle growth wherein muscles increase it’s size visually by increasing the amount of fluid it can hold, but is not increasing the density of the muscles fibers.
In short, Myofirbil hypertrophy increases the strength and speed of the muscles, while sarcoplasmic hypertrophy increases the energy fluid (sarcoplasm) around muscles and hence higher endurance.
For sarcoplasmic hypertrophy, one should do workouts that are high volume, with rest periods at 60 seconds and below. Reps in the range of 8-15 with 3 to 4 sets. For Myofibril, rep ranges is around 1-5 with longer rest intervals of 3-5 minutes. but more sets – say, 5 to 10