How Muscle Develops

Introduction
Personal trainers and fitness professionals often spend countless hours reading articles and research on new training programs and exercise ideas for developing muscular fitness. However, largely because of its physiological complexity, few fitness professionals are as well informed in how muscles actually adapt and grow to the progressively increasing overload demands of exercise. In fact, skeletal muscle is the most adaptable tissue in the human body and muscle hypertrophy (increase in size) is a vastly researched topic, yet still considered a fertile area of research. This column will provide a brief update on some of the intriguing cellular changes that occur leading to muscle growth, referred to as the satellite cell theory of hypertrophy. 


Trauma to the Muscle: Activating The Satellite Cells
When muscles undergo intense exercise, as from a resistance training bout, there is trauma to the muscle fibers that is referred to as muscle injury or damage in scientific investigations. This disruption to muscle cell organelles activates satellite cells, which are located on the outside of the muscle fibers between the basal lamina (basement membrane) and the plasma membrane (sarcolemma) of muscles fibers to proliferate to the injury site. In essence, a biological effort to repair or replace damaged muscle fibers begins with the satellite cells fusing together and to the muscles 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 where as the majority differentiate (the process cells undergo as they mature into normal cells) and fuse to muscle fibers to form new muscle protein stands (or myofibrils) and/or repair damaged fibers. Thus, the muscle cells’ myofibrils will increase in thickness and number. After fusion with the muscle fiber, some satellite cells serve as a source of new nuclei to supplement the growing muscle fiber. With these additional nuclei, the muscle fiber can synthesize more proteins and create more contractile myofilaments, known as actin and myosin, in skeletal muscle cells. It is interesting to note that high numbers of satellite cells are found associated within slow-twitch muscle fibers as compared to fast-twitch muscle fibers within the same muscle, as they are regularly going through cell maintenance repair from daily activities.