Muscle building is the work and process leading to muscular hypertrophy. Muscular hypertrophy is an increase in the muscle mass with an increased cross sectional area.  Since no new muscle fibers are formed, the increase in size is usually in the preexisting muscle fibers.Muscles are made up of two types of fibers.  These are individually responsible for hypertrophy and collectively more muscle mass. Hypertrophy of the sarcoplasm is mostly due to increased storage of glycogen, excess carbohydrates. On the other hand, myofibril hypertrophy is normally due to sustained protein deposition with increasing myofibril size.

Skeletal muscles and cardiac muscles when exposed to regular and increasing loads that have an exceeding force than preexisting capacity, have two ways to react. They can either break, or commonly, they adapt by increasing their mass in order to be able to handle the new work load. With a constantly increasing work load, the muscular size is also forced to increase. The net effect is increased muscle mass, which was the desired effect all along.

The main mechanism behind muscle building is that applying varying, increasing and intermittent levels of tension on a certain muscle group creates a counteracting stress in the muscle group leading to adaptation in order to overcome the initial tension.  The underlying science has been stipulated to be that with overloading of a muscle, more proteins are deposited in this muscle for growth of more myofibrils eventually causing increased muscle mass.

It then makes sense for factors such as testosterone, diet and exposure to work load to contribute to muscle bulking. A high protein diet, exercise and testosterone all increase the adaptability of muscle to tension and size. Low testosterone levels, old age and a low protein diet, make adaptation by muscle to loads minimal.

To provide the work load in order to start off the adaptation process, one has to be ready to consume enough protein containing foods. High intensity and regular activities such as weight training are invaluable. Aerobic exercises are also needed for the adaptation of cardiac muscle.  The overall effect is increased blood flow and well being.