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Science of muscle function and growth

Posted by: Paul

Human skeletal muscle is composed of several fiber types that vary in function and physiological properties. Their proportion is influenced mainly by genetics, the action of the muscle and adaptation to physical activity and exercise. Skeletal muscle is defined based on characteristics of function. These include excitability, contractility, extensibility and elasticity.

Muscle fibers are generally classified according to twitch-speed and color. Type I fibers, or slow-twitch/fatigue resistant red fibers are associated with greater endurance and stamina. They contain high concentrations of myoglobin and mitochondria and have high numbers of blood capillaries. Their cross-sectional growth potential is limited by genes to a maximum of 15%. Type II fibers or fast-twitch/white fibers have less endurance but are faster, stronger and appear white under a microscope. These fibers have the capacity to increase in diameter by up to 50%.

A single skeletal muscle is an organ of the muscular system, just like the heart is an organ of the circulatory system. Each muscle consists of hundreds and even thousands of independent muscle fibers. Individual muscle fibers are relatively long, cylindrical and threadlike. Encased by a thin film of connective tissue skeletal muscle fibers are controlled by the voluntary or central nervous system. First we think, then we move.

“Muscles respond to our will, and a will of iron is required to lift the same.”

A muscle fiber is actually a single muscle cell called a myocyte. Each myocyte is internally packed with additional bundles of finer myofibrils. Myofibrils are the contractile elements of skeletal muscle. Electron micrographs of myofibrils reveal even finer column-like structures called myofilaments.

In a nutshell, controlled, repetitive, strenuous exercise induces a “cell disruption” effect causing filament microtrauma and cell damage. This leads to an immediate adaptive response as a consequence of biological necessity. Adaptation is defined as a change in structure, function or form. Ability to adapt is what improves the chance of survival for an organism contained within a specific environment.

Inflammation, delayed onset muscle soreness (DOMS), changes in reduction/oxidation pathways, immune alteration and tissue repair are just some of the reactions and biological processes associated with the effects of training. The final desired outcome of improved twitch response and hypertrophy (increased size) is collectively known as fiber remodeling. In addition to the stimulation (or demand) caused by the actual training, the process of muscle growth as a function of biological adaptation is absolutely dependent on biological supply. Without sufficient rest, sleep and optimum nutrition, individual fibers cannot grow, joints become susceptible to injury from exposure to heavy loading and recovery is incomplete at best.

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