Muscles growth require three basic ingredients:
1. Stimulation – Motions at a specific intensity and for specific durations stimulates the anabolic activities of the body. The consequence of these anabolic activities is muscle growth.
2. Nutrition – All the stimulation in the world amounts to nothings without providing the building blocks for the body to build muscle. What you eat provides these building blocks.
3. Recuperation – Rest time between exercise sessions allows the body to use the building blocks you provide through nutrition and build muscles. Without this rest and healing time, the necessary repairs and building do not happen.
Size of the muscle is the cross section of the muscle fibers which is a hypertrophic adaptation to movements at loads beyond what muscle fibers are used to. Exercise intensity which is a function of load influence increase the size of fast twitch type II fibers. With the increased size of the fast twitch muscles, strength increases as well.
During the process energy stored in the body is depleted at various rates. It is during recovery phase between sets and sessions body replenishes the primary muscle food which is glycogen from the carbs stored in the liver and fat cells.
The catabolic activity of exercise stimulates the anabolic activity during rest which requires amino acids coming from protein intake.
Progression is the process that allows you to continue working at higher intensities with appropriate rest and nutrition so that you continue building mass.