Muscle is a specialized tissue which does all kinds of movement in the body. There are three types of muscle tissues. They are cardiac muscle, smooth muscle, and skeletal muscles. Apart from the two muscles types, Skeletal muscles are voluntary muscles and they are the one responsible for bodybuilding and strength sports.
Skeletal muscles are in the form of fibers and are a basic unit of contraction. Muscles contract when you get signals from the stimuli. The better you become at responding to this signals the stronger you get.
Growth of muscles and bones are an anabolic process. They are classified into hypertrophy and hyperplasia. Stress is a major component involved in the growth of muscles. In hypertrophy, additional tension on the muscles helps to increase its capacity by increasing its cell organelles like mitochondria concentration. Thus, the volume of cells increases. Hypertrophy does not lead to the formation of new muscle fibers.
There is another type of muscle growth without necessarily increasing the size of muscle cells. This is caused by the addition of muscle glycogen helping the muscle to swell along with connective tissues. People can get the appearance of larger muscles without increasing strength. This type of growth is Sarcoplasmic hypertrophy.
During physical stress, some muscle fibers get damaged. It sends the stress signal and activates hyperplasia. In this process, the stem cells from the bone marrow migrate to injured part and undergo cell division. It causes the formation of new muscle fibers and increases muscle mass density.
Muscle growth occurs when the rate of protein synthesis is greater than the rate of protein breakdown.