Skeletal muscles are made up of thousands of cylindrical muscle fibers. The fibers are bound together by connective tissue that has blood vessels that feed the muscle. The number of fibers is set early in life. You can not increase the number of fibers only the fiber size. Increased strength and muscle mass comes about through an increase in the thickness of each fiber and increases in the quantity of connective tissue. Many factors contribute to muscle hypertrophy and/or atrophy. Acute exercise, lack of exercise, diseases, hormones, drugs, age and genetics all affect the muscle cells by growth or shrinkage. Each of these things affects the muscle fibers, that is, muscles at the cellular level. While some of these factors are controllable like exercise, hormone levels and drug use the others are not so easily managed. Many can leave the muscle cells irreversibly damaged.
             Weight resistance training is probably the most commonly known way to build muscle. This happens by breaking down the muscle and rebuilding it bigger and usually stronger. Actin/myosin filaments become damaged during high-tension contractions. Plus, small breaks in plasma membrane let calcium leak into the muscle cells after training. This intracellular increase in calcium levels activates enzymes called calpains. They break off pieces of the damaged contractile filaments. Then a protein called ubiquitin binds to the broken filaments marking them for destructive purposes. Neutrophils are chemically attracted to the area and quickly increase in number. They release toxins, including oxygen radicals, which increase membrane permeability and phagocytize the tissue debris. Once the phagocytic stage is complete, the damaged fibers are rapidly broken down. The muscle is now in a weakened state(4). When the sarcolemma is damaged by tension like in weight training, growth factors like IGF-I are produced and released in the cell(5,6). Macrophages have an important role in b...