How do muscles grow?

Charge, S.B.P., & Rudnicki, M.A. 2004. Cellular and molecular regulation of muscle regeneration. Physiological Reviews, 84, 209-38.

Fitness professionals spend countless hours reading articles and research on new training programs and exercise ideas for developing muscular fitness. However, largely because of the topic's physiological complexity, few personal trainers or instructors are thoroughly informed about how muscles actually adapt to, and grow to meet, the progressively increasing overload demands of exercise. In fact, skeletal muscle is the most adaptable tissue in the human body. Already a widely studied topic, muscle hypertrophy is still considered a fertile research area. This column will provide a brief update on some of the intriguing cellular changes that lead to muscle growth, also known as the satellite cell theory of hypertrophy.

Muscle Trauma: Activating Satellite Cells

When muscles undergo intense exercise, as from a resistance training bout, the muscle fibers undergo trauma that is referred to scientifically as "muscle injury" or "muscle damage." This disruption of the fibers also causes damage to the muscle cell proteins within the muscle fibers, thus activating satellite cells. These cells, located on the outside of the muscle fibers between the basal lamina (basement membrane) and the plasma membrane (sarcolemma) of muscle fibers, proliferate to the injury site (Charge & Rudnicki 2004). In essence, a biological effort to repair or replace dam aged muscle fibers begins with the satellite cells fusing together and to the muscle fibers, often leading to increases in muscle fiber cross-sectional area or hypertrophy. The satellite cells have only one nucleus and can replicate by dividing. As the satellite cells multiply, some remain as organelles on the muscle fiber, whereas the majority differentiate (the process that cells undergo as they mature into normal cells) and fuse to muscle fibers to form new muscle protein strands (or myofibrils) and/or to repair damaged fibers. Thus, the muscle cells' myofibrils increase in thickness and number. See Figure 1.

After fusing with muscle fibers, some satellite cells serve as a source of new nuclei to supplement the growing fibers. With these additional nuclei, muscle fibers can synthesize more proteins and create more contractile myofilaments, known as actin and myosin, in skeletal muscle cells. Higher numbers of satellite cells are found in slow-twitch muscle...