Physiological Adaptations to Hypertrophic Strength Training

Increases in muscle cross sectional area and motor unit recruitment have long been identified with heavy resistance training (Hakkinen, 1989). However further investigations into different training protocols have revealed differences in the resulting adaptations to muscle cell structure and fibre composition (Tesch, 1988). Furthermore, different exercise protocols have been shown to activate specific intracellular signalling pathways, resulting in specific adaptations induced by different training regimes (Nader & Esser, 2001). This discussion will present the main physiological processes that lead to muscular hypertrophy with an understanding of the different muscular structure and composition observed between weightlifters and bodybuilders.

Hypertrophy typically arises from an increased number or size of contractile elements within the muscle fibres in a response to a heavy training stimulus, causing a growth in cross sectional area of the muscle fibres and consequently entire muscle structure (Vierck et al., 2000). When skeletal muscle sustains significant tension that disrupts the integrity of muscle fibres, a chain of myogenic events take place that results in an increased number of contractile proteins actin and myosin within the myofibril. During this process, sarcomeres are usually added in parallel (Paul & Rosenthal, 2002; Tesch & Larsson, 1982) which forces the muscle fibres to expand thus causing muscle cross sectional area (CSA) to increase (Toigo & Boutellier, 2006). This should also cause an increase in the angle of pennation and the muscle’s ability to generate force (Aagaard et al., 2001). Hypertrophy of the sarcomeres can occur in series, however this has not been shown to take place under normal training conditions and is not likely to result in any form of strength gains (Toigo & Boutellier, 2006).

Hypertrophy amongst bodybuilders has also been proposed to consist of a significant increase in the non-contractile elements such as fluid within the sarcoplasmic reticulum (MacDougall, Sale, Alway, & Sutton, 1984; Zatsiorsky, 1995). Bodybuilders have specifically been shown to have a greater proliferation of the fibrous endomysium and larger glycogen stores than powerlifters (MacDougall, Sale, Elder, & Sutton, 1982; Tesch, 1988). These differences in the cell structure of hypertrophied muscle fibres between bodybuilders, weightlifters, powerlifters and control subjects suggests that different training protocols result in different physiological adaptations (Tesch & Larsson, 1982). Said “non-functional” or “sarcoplasmic hypertrophy” may result in considerable gains in muscle mass without corresponding gains in strength (Siff, 2004). Some evidence to this was provided by Sale, Martin and Moroz (1992) who demonstrated a significant increase in the CSA of knee extensor muscles after 19 weeks of weight training with no increase in isometric strength or motor unit recruitment. It has further been suggested that an increased sarcoplasmic reticulum in the early stages of adaptation may later facilitate greater “functional” growth through increased synthesis of contractile proteins (Schoenfeld, 2010).

Further to the differences in cell structure between weightlifters, powerlifters and bodybuilders, evidence shows considerable differences in the composition of different muscle fibre types (MacDougall et al., 1982). Elite bodybuilders show greater percentages of type I than type II muscle fibres (Tesch & Larsson, 1982) which contrasts with weightlifters and powerlifters who exhibit significantly greater numbers of type II than type I fibres (Fry, 2003a; Fry, 2003b). Alongside this difference in fibre type is the associated CSA to such fibres where bodybuilders exhibit greater CSA of type I fibres than type II, whereas weight/powerlifters exhibit greater CSA of type II fibres. Reviewing the findings of several studies, Fry (2004) revealed that long term bodybuilding type training induces considerable hypertrophy of both type I and type II fibres whereas weight/powerlifters selectively hypertrophy type II fibres only. He went on to suggest that greater hypertrophy of type II fibres in bodybuilders by training at higher intensities may contribute to greater overall increases in muscle CSA.

Schoenfeld (2010) identified three main mechanisms that lead to muscle hypertrophy; suggesting that a hypertrophic training protocol should aim to optimize an increase in mechanical tension, muscular damage and metabolic stress. These mechanisms consequently lead to a cascade of chemical interactions that result in the growth and repair of attenuated muscle fibres.

Insulin-like growth factor (IGF-1) is considered the primary anabolic response to increased mechanical tension and metabolic stress (Hameed et al., 2004). During resistance exercise IGF-1 is released from surrounding muscle and fat tissues (Kraemer, 2000) in the isoform often referred to as mechano growth factor (MGF), which then interacts with receptors found in the myofibres and satellite cells (Barton-Davis, Shoturma, & Sweeney, 1999). The release of MGF therefore initiates the hypertrophy process; it then splices towards other IGF-1 isoforms whose myogenic effects can be observed for up to 72 hours post exercise (McKay, O’Reilly, Phillips, Tarnopolsky, & Parise, 2008). IGF-1 directly initiates chemical signalling pathways that lead to protein synthesis and muscle fibre adaptation including the Akt – mTOR (Hameed et al., 2004) and calcineurin (Cn) pathways (Musarò, McCullagh, Naya, Olson, & Rosenthal, 1999). IGF-1 also plays a significant role in activating satellite cells, mediating their proliferation and differentiation (Yang & Goldspink, 2002); and facilitating their interaction with myofibres (Toigo & Boutellier, 2006).

Once released from surrounding tissue, MGF binds to specific receptors at the cell membrane of the target tissues where insulin receptor signal-1 (IRS1) then activates the PI-3k – Akt – mTOR signalling pathway which ultimately results in protein synthesis. There is considerable research in this area that supports the role of the Akt – mTOR signalling pathway in muscle fibre hypertrophy by initiating protein synthesis (Bodine et al., 2001; Nader, McLoughlin, & Esser, 2005; Rommel et al., 2001). Akt’s main role in muscle hypertrophy is activating mammalian target of rapamycin (mTOR), whilst also signalling the transport of glycogen and inhibiting forkhead box O (FoxO) which leads to protein degradation (Coffey & Hawley, 2007). Akt also interferes with AMPK activity by direct phosphorilation of TSC2 and GSK3β and thus reduces their effects on inhibiting protein synthesis (Hahn-Windgassen et al., 2005; Inoki, Li, Zhu, Wu, & Guan, 2002). mTOR primarily up regulates p70S6K which in turn initiates the translation of protein synthesis leading to proliferation of satellite cells and an increase in myofibre cell size. mTOR (raptor) also suppresses 4EBP1 from inhibiting protein synthesis (Bodine et al., 2001.

Satellite cells situated between the basal lamina and sarcolemma facilitate hypertrophy (Hawke & Garry, 2001; Rosenblatt, Yong, & Parry, 1994). Their importance has been observed through trials of inactivation, whereby most of the hypertrophy response to appropriate resistance training has been prevented (Adams, Caiozzo, Haddad, & Baldwin, 2002; Rosenblatt et al., 1994). Usually inactive, satellite cells are regulated by transcription factors myogenic differentiation (MyoD) and myogenin (MyoG). Significant loading of skeletal muscle leads to the expression of MyoD and MyoG thus activating satellite cells (Bickel et al., 2005). Once stimulated, they grow and fuse with either existing myofibre cells or other satellite cells to form new myofibres (Toigo & Boutellier, 2006). During hypertrophy, the ratio of myonuclei to fibre mass must remain constant; so satellite cells lend their myonuclei to the myofibres to facilitate the synthesis of contractile proteins (Moss & Leblond, 1971). It has further been stipulated that muscle fibre hypertrophy is limited to the supply of additional myonuclei from surrounding satellite cells (Adams, 2006).

A complete understanding of calcium dependant pathways is yet to be established, though current research suggests that as calcineurin (Cn) is involved with the hypertrophy of both fast and slow muscle fibres (Dunn, Burns, & Michel, 1999), it may play a significant role in adaptations to a bodybuilding type training programme (Coffey & Hawley, 2007). Calcineurin contributes to the proliferation and differentiation of satellite cells by activating transcription factors MyoD and MEF2 (Friday, Mitchell, Kegley & Pavlath, 2003). Cn-dependent signalling is also associated with the transformation of muscle fibre phenotype from fast to slow (Talmadge et al., 2004) which corresponds to an adaptive process that bodybuilders may undergo (Tesch & Larsson, 1982). Furthermore different patterns of neural activity promote specific changes in gene expression that establish the oxidative and contractile characteristics of both slow and fast myofibres through the calcineurin pathway (Chin et al., 1998). Depending on the intensity and duration of exercise, it has been proposed that this may result in a more metabolically efficient phenotype (Coffey & Hawley, 2007). This coincides with evidence of increased capillarisation among bodybuilders (Tesch, 1988) in an adaptive response to deal with fatigue.

High volume bodybuilding style training interventions with short rest periods utilise the aforementioned pathways by placing a high metabolic stress on the body via increases in lactate, hydrogen ions, creatine and inorganic phosphate resulting from glycolitic ATP production whilst providing significant mechanical tension by working at a moderate load of 10-12RM (Siff, 2004). Accumulation of lactate from glycolisis within the muscle cell results in cell swelling through increased water up take (Sjogaard, Adams, & Saltin, 1985) and has been shown to stimulate an increase in protein synthesis and a decrease in protein degradation (Millar, Barber, Lomax, Travers, & Shennan, 1997). Such training will also promote an increase in glycogen storage which may further augment cell swelling as glycogen attracts three times its weight in water (Chan, Johnson, Moore, Kapadia, & Dudley, 1982). It has been suggested that cell swelling causing an increased pressure on the cell membrane may disturb its structural integrity, consequently leading to processes that reinforce its extracellular structure (Schoenfeld, 2010). This theory is somewhat supported by the evidence of thickened endomysial connective tissue and increased glycogen stores amongst bodybuilders though not evident in weightlifters who typically train at lower rep ranges avoiding considerable lactate build up (MacDougall et al., 1982; Tesch, 1988). Cell swelling also facilitates an enhanced transport of amino acids such as glutamine into the cell which are essential to protein synthesis and thus plays a significant role in the response to bodybuilding type hypertrophy (Low, Rennie, & Taylor, 1997).

Global increases in metabolites, especially lactate concentrations in the blood further result in increased growth hormone (GH) production from the pituitary gland; which in turn stimulates a greater release of insulin like growth factor (IGF-1) from the liver (Kraemer et al., 1991). This process can take between 8 and 29 hours, continuing the interactions between IGF-1 and its binding receptors at the target tissue to activate the Akt – mTOR signalling pathway and up regulation of protein synthesis as part muscle fibre remodelling post exercise (Kraemer, 2000).

More recently, the inclusion of eccentric contractions in a hypertrophic training programme known as German Volume Training (GVT) which is also founded on high volume, moderate loads and short rest periods, has been made famous by Charles Poliquin (2006). It is well established that eccentric contractions induce larger increases in muscle mass than concentric contractions alone (Farthing & Chilibeck, 2003; Higbie, Cureton, Warren, & Prior, 1996) and it has been postulated that eccentric training is essential in optimizing muscle hypertrophy (Hather, Tesch, Buchanan, & Dudley, 1991). Eccentric exercise has also been shown to recruit fast twitch over slow twitch fibres (Shepstone et al., 2005) whilst recruiting an increased number of motor units to manage larger amounts of tension (McHugh, Connolly, Eston, & Gleim, 2000), thus providing an increased capacity for muscle hypertrophy. Firstly heavy eccentric contractions elicit greater muscle damage through Z-line streaming which is a prerequisite for myofibrillar growth (Crameri et al., 2004), as well as greater damage to structural components (Macpherson, Dennis, & Faulkner, 1997). Secondly it has been shown to elicit greater metabolic stress through an increased lactate build up resulting in increased production of anabolic hormones (Ojasto & Häkkinen, 2009). As previously discussed, these mechanisms have been shown to initiate the releases of MGF from surrounding tissues which initiates the Akt – mTOR signalling pathway that leads to protein synthesis and muscle remodelling. Additionally, eccentric exercise has been shown to induce the MAPK signalling pathway which also leads to the activation of transcription factors for cellular growth and repair (Boppart et al., 1999) as well as being associated with increased IGF-1 mRNA expression (Shepstone et al., 2005) and increased levels of activated p70S6K (Eliasson et al., 2006). In light of this evidence, an eccentric tempo of 2 to 4 seconds has been suggested as an essential condition forming part of a structured training programme towards optimizing all forms of muscle hypertrophy (Schoenfeld, 2010; Poliquin, 2006).

Empirically evident from bodybuilders’ physiques, it can be acknowledged that high volume, moderate load strength training results in significant hypertrophy. However the inclusion of eccentric contractions and/or higher loads are viewed as essential criteria in maximising mechanical tension and muscular damage alongside metabolic stress; and should therefore attenuate higher levels of hypertrophy of type II muscle fibres, especially to the contractile element, than compared with programmes reliant on metabolic stress and moderate loads alone. Furthermore the literature presents a theoretical argument that an increase in non contractile elements of the muscle structure in early stages of hypertrophic strength training may later enhance the production of contractile proteins, which should be taken into consideration when periodising hypertrophic strength programmes.

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