Athletes 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 Trainees or 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.
Trauma & Satellite Cells
When muscles are forced to undergo intense exercise, as from a resistance training bout, there is trauma to the muscle fibers. This disruption to muscle cell organelles activates satellite cells, which are located on the outside of the muscle fibers between the basal lamina and the plasma membrane 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. 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.
Growth factors are hormones or hormone-like compounds that stimulate satellite cells to produce the gains in the muscle fiber size. These growth factors have been shown to affect muscle growth by regulating satellite cell activity.
• Hepatocyte growth factor (HGF) is a key regulator of satellite cell activity. It has been shown to be the active factor in damaged muscle and may also be responsible for causing satellite cells to migrate to the damaged muscle area.
• Fibroblast growth factor (FGF) is another important growth factor in muscle repair following exercise. The role of FGF may be in the revascularization during muscle regeneration.
• A great deal of research has been focused on the role of insulin-like growth factor-I and –II (IGFs) in recent times. The IGFs play a primary role in regulating the amount of muscle mass growth, promoting changes occurring in the DNA for protein synthesis, and promoting muscle cell repair.
• Insulin also stimulates muscle growth by enhancing protein synthesis and facilitating the entry of glucose into cells. The satellite cells use glucose as a fuel substrate, thus enabling their cell growth activities. And, glucose is also used for intramuscular energy needs.
• Growth hormone is also highly recognized for its role in muscle growth. Resistance exercise stimulates the release of growth hormone from the anterior pituitary gland, with released levels being very dependent on exercise intensity. Growth hormone helps to trigger fat metabolism for energy use in the muscle growth process. As well, growth hormone stimulates the uptake and incorporation of amino acids into protein in skeletal muscle.
• Testosterone also affects muscle hypertrophy. This hormone can stimulate growth hormone responses in the pituitary, which enhances cellular amino acid uptake and protein synthesis in skeletal muscle. In addition, testosterone can increase the presence of neurotransmitters at the fiber site, which can help to activate tissue growth. As a steroid hormone, testosterone can interact with nuclear receptors on the DNA, resulting in protein synthesis. Testosterone may also have some type of regulatory effect on satellite cells.
The Big Picture
The above discussion should clearly illustrate that muscle growth is a complex molecular biological process involving the interplay of numerous cellular organelles and growth factors. Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. Both, the synthesis and breakdown of proteins are controlled by complimentary cellular mechanisms.
Resistance exercise can profoundly stimulate muscle cell hypertrophy and the resultant gain in strength. However, the time course for this hypertrophy is relatively slow, generally taking several weeks or months to be apparent. Interestingly, a single bout of exercise stimulates protein synthesis within 2-4 hours after the workout which may remain elevated for up to 48 hours from that point.
Studies show that men and women respond to a resistance training stimulus very similarly. Due to differences in body size, body composition and hormone levels, gender will have a varying effect on the extent of hypertrophy one may possibly attain. As well, greater changes in muscle mass will occur in individuals with more muscle mass at the start of a training program.
Aging also mediates cellular changes in muscle decreasing the actual muscle mass. This loss of muscle mass is referred to as sarcopenia. Happily, the detrimental effects of aging on muscle have been shown be restrained or even reversed with regular resistance exercise.
Heredity differentiates the percentage and amount of the two markedly different fiber types, Type I slow-twitch (ST), or slow-oxidative (SO) fibers and Type II fast-twitch (FT), or fast-glycolytic (FG) fibers. The proportions and types of muscle fibers vary greatly between adults and appears to have vast ramifications on athletic potential and response to exercise.
Muscle Hypertrophy Summary
Resistance training leads to trauma or injury of the cellular proteins in muscle. This prompts cell-signaling messages to activate satellite cells to begin a cascade of events leading to muscle repair and growth. Several growth factors are involved that regulate the mechanisms of change in protein number and size within the muscle. The adaptation of muscle to the overload stress of resistance exercise begins immediately after each exercise bout, but often takes weeks or months for it to physically manifest itself at appreciable levels. Skeletal muscle is remodeled after continuous, and carefully designed, resistance exercise training programs in conjunction with favorable nutrition and hormonal support.