Muscle hypertrophy: Science behind Muscle Growth
Swarna Anchal* Chirag Sethi**
*Technical writer,Classic Fitness Academy, New Delhi, India.
*** Director at Classic Fitness Academy, New Delhi, India.
Introduction
Adaptive increase in size of skeletal muscle called as muscle hypertrophy. Muscular hypertrophy plays an important role in competitive?bodybuilding?and strength sports like?power lifting,?football?and?Olympic weightlifting through a growth in size of muscle component cells. Two factors contribute to hypertrophy:?sarcoplasmic?hypertrophy, which focuses more on increased?sarcoplasmic?fluid in the muscle cell with no accompanying increase in muscular strength; whereas during myofibrillar hypertrophy,??contractile proteins (actin and myosin) increase in number and add to muscular strength as well as a small increase in the size of the muscle (Baechle and Earle, 2008).Sarcoplasmic hypertrophy is greater in the muscles of?bodybuilders?because studies suggest sarcoplasmic hypertrophy shows a greater increase in muscle size while myofibrillar hypertrophy proves to increase overall muscular strength making it more dominant in?Olympic weightlifters (Kraemer WJ, Zatsiorsky, 2006). These two forms of adaptations occurs with one another as one can experience a large increase in sarcoplasmic fluid with a slight increase in proteins, on the other hand a large increase in proteins with a small increase in fluid can be experienced, or a relatively balanced combination of the two.
Skeletal muscle helps in (1) providing stability for body posture and (2) in movements by contractions so each skeletal muscle must be able to contract with different levels of tension to perform various functions. Progressive overload/stress makes the muscles able to adapt hence increase in size and amount of contractile proteins occurs (Russell et al, 2000). As people having different goals such as gaining strength, power, or endurance so the best approach to specifically achieve muscle growth remains controversial but it was observed that consistent anaerobic strength training will produce hypertrophy over the long term, in addition to muscular strength and endurance also (Soares, 1992; Prior et al, 2004).
Strength and anaerobic training cause microtrauma, which is tiny damage to the fibers, may play a significant role in muscle growth (Charge and Rudnicki, 2004).?When microtrauma occurs, it works as the stimulus for protein synthesis and for neuro-muscular adaptations. Body respond by overcompensating, replacing the damaged tissue by adding more?sarcomeres?(actin and myosin) and increasing non-contractile elements like?sarcoplasmic?fluid (Schoenfeld, 2016) hence enhancing the voluntary muscular contraction to exert greater force. However, the precise mechanisms are not clearly understood.
Hypertrophy is also affected by a number of factors such as biological factors based on DNA?and sex, nutrition taken, training variables and individual differences in genetics account for a substantial portion of the variance in existing muscle mass. A study estimated that about 53% of the variance in lean body mass is heritable (Arden and Spector, 1997) along with about 45% of the variance in muscle fiber proportion (Simoneau and Bouchard, 1995). ?In this article, a brief but relevant review of the literature is presented to better understand the multifaceted phenomenon of skeletal muscle hypertrophy.
Types of skeletal muscles
Skeletal muscle fibers are classified into two major categories; slow-twitch (Type I) and fast-twitch fibers (Type II). The difference between the two fibers can be distinguished (Table 1.0) by metabolism, contractile velocity, neuromuscular differences, glycogen stores, capillary density of the muscle, and the actual response to hypertrophy.
Table 1.0: different types of muscle fibers and their properties
|
Properties |
Type I |
Type II | |
|
Type IIa |
Type IIb | ||
|
Contractions |
Very Slow |
Contracts rapidly but slower than type IIb |
Contracts very fast |
|
Activity Duration |
Sustain activities more than 3 minutes |
Sustain activities upto 3 minutes |
Only for a few seconds ?(8-10) |
|
No. of mitochondria and myoglobin |
High number of mitochondria and myoglobin |
Have mixed conc. of both |
Have high conc. of glycolytic enzymes |
|
Colour |
Red due to myoglobin |
Lighter in colour |
White in colour |
|
Resistance to Fatigue |
Fatigue resistance |
Less quickly as compared to type IIb |
Fatigue rapidly |
|
Capillary Density |
high |
intermediate |
low |
|
Energy Pathway |
aerobic |
anaerobic |
Creatine phosphate |
|
Involvement in Activities |
Marathon, long distance cycling |
400 meters running, moderate intensity weight training |
100 meters sprinting, power lifting |
Type I Fibers: they are also known as slow twitch oxidative muscle fibers, are responsible for maintenance of body posture and skeletal support. They are more involved in endurance activities due to increase in capillary density. These fibres are able to generate tension for longer periods of time but also generate less force. They utilize fats and carbohydrates better because of the increased reliance on oxidative metabolism (Robergs and Roberts, 1997). Type I fibers have been shown to hypertrophy considerably due to progressive overload (Kraemer et al, 1996,Hakkinen et al, 2001) as well as to some degree with aerobic exercise (Carter et al, 2001).
Type II Fibers: These fibers generate greater amounts of force but for shorter periods of time. Type II fibers can be further classified as Type IIa and Type IIb muscle fibers.
Type IIa Fibers also known as fast twitch oxidative glycolytic fibers (FOG), are hybrids between Type I and IIb fibers and carry characteristics of both Type I and IIb fibers. They rely on both anaerobic and oxidative metabolism to support contraction (Robergs et al, 1997). As per the requirements, Type IIb fibers convert into Type IIa fibers, causing an increase in the percentage of Type IIa fibers within a muscle (Kraemer et al, 1996).
Type IIb Fibers are fast-twitch glycolytic fibers (FG) and rely solely on anaerobic metabolism for energy that?s why they have high amounts of glycolytic enzymes. These fibers generate the greatest amount of force due to an increase in the size of the nerve body, axon and muscle fiber but maintain tension for a shortest period of time of all the muscle fiber types (Robergs et al, 1997). Type IIb fibers convert into Type IIa fibers with resistance exercise. It is believed that resistance training causes an increase in the oxidative capacity of the strength-trained muscle. Because Type IIa fibers have a greater oxidative capacity than Type IIb fibers, the change is a positive adaptation to the demands of exercise (Kraemer et al, 1996).
Resistance Training (RT) and Hypertrophy
Muscle hypertrophy occurs when a message of damaged muscle fiber (due to strength or resistance training) filters down to cells and additional contractile proteins added to muscle fiber which mean hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells. However, there is also some limit to how large a myofibril can become but?skeletal muscle?cells are unique in having multiple nuclei and their number can increase (Bruusgaard et al, 2010). Researches has shown that muscle hypertrophy that occurs at initial stages of strength training are mostly attributable to muscle damage induced cell swelling with the majority of strength gains resulting from neural adaptations. Schoenfeld et al in 2019 shows that foundations for individuals seeking to maximize muscle growth should be hypertrophy-oriented RT consisting of multiple sets (3?6) of six to 12 repetitions with short rest intervals (60 s) and moderate intensity of effort (60?80% 1RM) with subsequent increases in training volume (12?28 sets/muscle/week).
Training variables, in the context of strength training, such as frequency, intensity, and total volume also directly affect the increase of muscle hypertrophy (Wernbom, 2007). Progressive overload?(a strategy of progressively increasing resistance or repetitions over successive bouts of exercise in order to maintain a high?level of effort) can induce Muscular hypertrophy (Seynnes et al, 2007). By maintaining the different specific plans such as short-duration, high-intensity?anaerobic exercises, Muscular hypertrophy can be increased however Lower-intensity, longer-duration?aerobic exercise?generally does not result in very effective tissue hypertrophy; instead, endurance athletes enhance storage of fats and?carbohydrates?within the muscles (Saures, 1992; Prior et al, 2004). Increased blood flow to metabolically active muscle areas causes muscles to temporarily increase in size or being "pumped up" or getting "a pump" (Eitel, 2017) but after 2 hours of a workout and typically for seven to eleven days, muscles swell due to an inflammation response as tissue damage is repaired.
Furthermore, persistence in training and diet is essential. Recently, research has shown that muscle hypertrophy that occurs at initial stages of RT (~4 sessions) is mostly attributable to muscle damage induced cell swelling with the majority of strength gains resulting from neural adaptations (8?12 sessions). Within the latter phase of RT (6?10 weeks), muscle growth begins to become the dominant factor (Damas et al, 2018). Longer-term hypertrophy occurs due to more permanent changes in muscle structure.
Protein diet and hypertrophy
For?anabolism, more calories are consumed rather than burned called as positive energy balance hence results in muscle hypertrophy. That?s why protein intake is must especially?branched-chain amino acids?(BCAAs). To elevate the protein synthesis, an increased requirement for protein BCAAs make up a high proportion of the amino acids in muscle. They are unique because they are the only amino acids burned by muscles as fuel; thus, both blood and muscle levels of BCAAs decrease after exercise (Phillips, 2004).
In a series of experiments it is shown that protein synthesis is directly related to diet containing proteins (Tang et al, 2008). It was seen that when a complete protein (one that contains all the amino acids) was consumed, protein synthesis increased. When just essential amino acids were consumed without non-essential amino acids, the same increase was noted indicating non-essential amino acids are not required to stimulate protein synthesis. When only BCAAs were consumed, there was again the same increase in protein synthesis. Finally when just leucine was consumed, protein synthesis still increased to the same magnitude. A study carried out by American College of Sports Medicine (2002) put the recommended daily protein intake for athletes at 1.2?1.8?g/kg of body weight (Tarnopolsky et al, 1992; Rankin, 2002; Lemon, 1991). A review of the scientific literature (Schoenfeld and Aragon, 2018) concluded that for the purpose of building lean muscle tissue, a minimum of 1.6 g protein/kg of body weight is required, which can be divided over 4 meals over the day. In another report by?Di Pasquale?(2008), a minimum protein intake of 2.2 g/kg is recommended for anyone involved in competitive or intense sports and wants to maximize lean body mass but does not wish to gain weight. In those attempting to minimize body fat and thus maximize body composition, for example in sports with weight classes and in bodybuilding, it's possible that protein may well make up over 50% of their daily caloric intake (Di Pasquale, 2008).
A small study performed on young and elderly found that ingestion of 340 grams of lean?beef?(90?g protein) did not increase muscle protein synthesis any more than ingestion of 113 grams of lean?beef?(30?g protein). In both groups, muscle protein synthesis increased by 50%. The study concluded that more than 30?g protein in a single meal did not further enhance the stimulation of muscle protein synthesis in young and elderly (Symons et al 2009). It is not uncommon for bodybuilders to advise a protein intake as high as 2?4g/kg of bodyweight per day.?However, scientific literature has suggested this is higher than necessary, as protein intakes greater than 1.8?g per kilogram of body weight showed to have no greater effect on muscle hypertrophy (Tarnopolsky et al, 1992).?
Physiology
The physiology of skeletal muscle hypertrophy will explore the role and interaction of satellite cells, immune system reactions, growth factor proteins and hormones (Table 2.0).
(1)?? Satellite Cells: These cells are termed satellite cells because they are located on the outer surface of the muscle fiber, in between the sarcolemma and basal lamina (uppermost layer of the basement membrane) of the muscle fiber. Satellite cells have one nucleus, with constitutes most of the cell volume. Usually these cells are dormant, but they become activated when the muscle fiber receives any form of trauma, damage or injury, such as from resistance training overload and function to facilitate growth, maintenance and repair of damaged skeletal muscle tissue (Hawke et al, 2001). The daughter satellite cells are drawn to the damaged muscle site where they then fuse to the existing muscle fiber, donating their nuclei to the fiber, which helps to regenerate the muscle fiber. This satellite cell activation and proliferation period lasts up to 48 hours after the trauma or shock from the resistance training session stimulus (Hawke et al, 2001). The amount of satellite cells present within in a muscle depends on the type of muscle. Type I or slow-twitch oxidative fibers, tend to have a five to six times greater satellite cell content than Type II (fast-twitch fibers), due to an increased blood and capillary supply (Hawke et al, 2001). This may be due to the fact that Type 1 muscle fibers are used with greatest frequency, and thus, more satellite cells may be required for ongoing minor injuries to muscle.
Table 2.0 showing various factors responsible for hypertrophy
|
Satellite Cells ? Function to facilitate growth, maintenance and repair of damaged skeletal muscle tissue |
Immunology ? Leading to inflammation to response the damage, repair the damage by producing cytokines ? Cytokines: Stimulate the arrival of lymphocytes, neutrophils, monocytes to the injury site to repair the injured tissue. Types:
|
Growth Factors ? Insulin-Like Growth Factor: Regulates insulin metabolism and stimulates protein synthesis Types:
? Fibroblast Growth Factor: Stored in skeletal muscle and cause proliferation and differentiation of satellite cells ? Hepatocyte Growth Factor: Responsible for satellite cells to migrate to the injured area |
|
Hormones Growth Hormone: Satellite cell activation, proliferation and differentiation ? Cortisol: Breaks down muscle proteins, inhibiting hypertrophy ? Testosterone: Increases protein synthesis |
(2)?????????????? Immunology: The immune system responds with a complex sequence of immune reactions leading to inflammation to response the damage, repair the damage, and clean up the waste products (Shephard and Shek, 1998). The immune system causes a sequence of events in response to the injury such as Macrophages involved in phagocytosis and to secrete cytokines, growth factors and other substances.
Cytokines: are proteins which are responsible for cell-to-cell communication. Cytokines stimulate the arrival of lymphocytes, neutrophils, monocytes, and other healer cells to the injury site to repair the injured tissue (Pedersen, 1997). The three important cytokines relevant to exercise are Interleukin-1 (IL-1), Interleukin-6 (IL-6), and tumor necrosis factor (TNF) which produces most of the inflammatory response (Pedersen and Hoffman-Goetz, 2000). They are responsible for protein breakdown, removal of damaged muscle cells, and an increased production of prostaglandins (hormone-like substances that help to control the inflammation).
(3)?????????????? Growth Factors: Growth factors are highly specific proteins and in regard with hypertrophy, growth factors of particular interest include insulin-like growth factor (IGF), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF). These growth factors work in conjunction with each other to cause hypertrophy (Adams and Haddad, 1996).
Insulin-Like Growth Factor: IGF is a hormone that is secreted by skeletal muscle. It regulates insulin metabolism and stimulates protein synthesis. There are two forms, IGF-I, which causes proliferation and differentiation of satellite cells and is elevated during progressive overload resistance excerise. IGF-II, which is responsible for proliferation of satellite cells (Singh et al, 1999).
Fibroblast Growth Factor: FGF is stored in skeletal muscle. FGF has nine forms, five of which cause proliferation and differentiation of satellite cells, leading to skeletal muscle hypertrophy. The amount of FGF released by the skeletal muscle is proportional to the degree of muscle trauma or injury (Yamada et al, 1989).
Hepatocyte Growth Factor: HGF is a cytokine with various different cellular functions. Specific to skeletal muscle hypertrophy, HGF activates satellite cells and may be responsible for causing satellite cells to migrate to the injured area (Hawke and Garry, 2001).
(4)?????????????? Hormones: The following hormones are of special interest in skeletal muscle hypertrophy.
Growth Hormone: Growth hormone (GH) is a peptide hormone that stimulates insulin like growth factor (IGF) in skeletal muscle, promoting satellite cell activation, proliferation and differentiation (Frisch, 1999). However, the observed hypertrophic effects from the additional administration of GH, investigated in GH-treated groups doing resistance exercise, may be less credited with contractile protein increase and more attributable to fluid retention and accumulation of connective tissue (Frisch, 1999).
Cortisol: Cortisol is a steroid hormone which is produced in the adrenal cortex of the kidney. It is a stress hormone, which stimulates gluconeogenesis, which is the formation of glucose from sources other than glucose, such as amino acids and free fatty acids. Cortisol also inhibits the use of glucose by most body cells. This can initiate protein catabolism (break down), thus freeing amino acids to be used to make different proteins, which may be necessary and critical in times of stress. In terms of hypertrophy, an increase in cortisol is related to an increased rate of protein catabolism. Therefore, cortisol breaks down muscle proteins, reduce amino acid uptake by muscle tissue, and inhibits protein synthesis thus inhibiting skeletal muscle hypertrophy (Izquierdo et al, 2001, Manchester, 1970).
Testosterone: Testosterone is an androgen, or a male sex hormone. The primary physiological role of androgens is to promote the growth and development of male organs and characteristics. Testosterone affects the nervous system, skeletal muscle, bone marrow, skin, hair and the sex organs. During puberty in males, hypertrophy occurs at an increased rate due to?testosterone as it?is one of the body's major growth hormones. This contributes to the gender differences observed in body weight and composition between men and women. Testosterone increases protein synthesis, which induces hypertrophy (Vermeulen et al, 1999). With skeletal muscle, testosterone, which is produced in significantly greater amounts in males, has an anabolic (muscle building) effect. On average, males find hypertrophy much easier to achieve than females and on average, have about 60% more muscle mass than women (Miller et al, 1993). Taking additional testosterone, as in?anabolic steroids, will increase results further but high doses can have side effects such as?testicular atrophy, cardiac arrest (Fineschi et al, 2007)?and?gynecomastia (Basaria, 2010). It is also considered a?performance-enhancing drug and can cause competitors to be suspended or banned from competitions. Testosterone is also a medically regulated substance and one can not possess it without a?medical prescription.
Conclusion
Muscular hypertrophy is a multidimensional process, with numerous factors involved. Researches has shown that muscle hypertrophy that occurs at initial stages of strength training are mostly attributable to muscle damage induced cell swelling with the majority of strength gains resulting from neural adaptations. It involves a complex interaction of satellite cells, the immune system, growth factors, and hormones with the individual muscle fibers of each muscle. Although our goals as fitness professionals and personal trainers motivates us to learn new and more effective ways of training the human body, the basic understanding of how a muscle fiber adapts to an acute and chronic training stimulus is an important educational foundation of our profession.
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