Neural adaptions to resistance training

An introduction to the neural adaptions to resistance training

A spike in protein synthesis occurs straight away after your very first workout but changes in muscle size due to muscle protein accretion are not seen until several weeks after training, however, there is a significant increase in strength when you begin training and this is therefore believed to be due to neural adaptations. It in fact may be those neural adaptations which allow the body to achieve an increase in muscle growth, as the body develops the neural adaptions needed to correctly perform certain exercises.
 

Neural drive

One neural adaption from resistance training is an increased neural drive. A study using the twitch interpolation technique, which is when an electrical stimulus is delivered to a muscle during it’s maximal voluntary contraction and tested using EMG to make sure it is 100% activated, and then compared to measurements of muscle activation using EMG before training and after a period of training, this study found that quadriceps femoris activation is only at 95% before any training regime and after 6 weeks of training it went up to 97%. It is impossible for an individual to voluntarily maximally activate a muscle but with consistent resistance training over a long time they can get close. The increase in muscle activation is believed to be due to an increase in the number of muscle fibres recruited for a muscle action and an increase in rate coding, which is the frequency at which the motor units are stimulated. Muscle fibre recruitment follows the size principle, meaning that smaller, low-threshold, slower motor units are recruited initially followed by progressively larger, higher-threshold, fast motor units as the force demands for a particular task increase, this allows for small fine movements to be conducted and a gradual increase in force production. Lifting heavy weights or even a lightweight quickly (especially in muscles which have a high quantity of slow twitch muscle fibres) will ensure a large portion of high and low-threshold motor units are recruited. Regular resistance training will drop the recruitment threshold for the motor units. As fatigue accumulates across a set the recruitment threshold for motor units will decrease, so even if you are using a low weight and performing the reps slowly once you reach a certain level of fatigue the high-threshold motor units will be recruited. The limit for motor unit recruitment is about 85% during maximal isometric force, and this is even less during dynamic movements, this shows limitations to how much adaptions in motor unit recruitment can affect strength. It’s not just resistance training which can recruit high-threshold motor units, in fact even cardiovascular exercises will if an individual pushes themselves hard enough.
 

Motor unit synchronisation

Another possible neural adaption is motor unit synchronisation, which is when two or more motor units send an action potential to muscles simultaneously resulting in more muscle fibres being activated at once resulting in a more forceful muscular contraction. There is research to support and contradict this theory, but if motor unit synchronisation does occur as a result of resistance training then the effects are likely minimal.
 

Antagonist coactivation

One adaption that appears to be certain is antagonist coactivation, this is where the activation of the antagonist muscle of an exercise decreases so the agonist muscle can perform a more forceful contraction. Carolan and Cafarelli found that hamstring coactivation decreased by 20% after one week of maximal voluntary isometric knee exercises. The extent to which antagonist coactivation has an effect on strength is unclear.
 

Doublets

Resistance training has been shown to increase the number of doublets fired by motor neurons. Doublets are two-action potentials sent in quick succession, less than 5 ms apart. Van Cutsem and colleagues found that the percentage of motor units firing doublets increased from 5.2% to 32.7% after 12 weeks of dynamic resisted dorsiflexion exercise with a load of 30-40% 1RM. It has been proven that this increase in the rate of doublets increased the speed of muscle activation, and it therefore likely plays an important role in strength development.
 

Practical implications of neural adaptions to resistance training

It is plausible that muscle growth only occurs when the body’s neurological system has first adapted by increasing neural drive, motor unit synchronisation, antagonist coactivation and the rate of doublet firing, which allows for a greater forceful contraction and the stimulation of target muscles. When a new lifter starts training using machines and performing exercises heavy and close to failure, it likely allows for the quickest neural adaptions which are needed before muscle growth occurs. Machines are likely better at stimulating neural adaptions over free weights because they require less technique and coordination, the lifter can also push harder with a reduced risk of injury due to poor form. However, free weight movements, and particularly compound movements, should still be performed alongside machine exercises because these will likely provide the best avenue for muscle growth in the long run.
 

Protein synthesis

An introduction to protein synthesis

Hypertrophy occurs when the rate of muscle protein synthesis exceeds the rate of muscle protein breakdown, creating a positive protein turnover. In the fasted state muscle protein breakdown will exceed muscle protein synthesis, but following a meal protein synthesis will exceed muscle protein breakdown. During resistance training protein synthesis is suppressed and muscle protein breakdown increases, creating a negative protein turnover. The muscle protein breakdown which occurs as a result of resistance training is important for promoting hypertrophy because it helps to support amino acid relocation and prevents the build-up of misfolded and non-functional proteins. Following resistance training muscle protein synthesis increases by 2-5 times its baseline level, as well as nutrient delivery which lasts at least 48 hours following exercise. 

The process of protein synthesis

Understanding the process of protein synthesis will help you understand hypertrophy. We have 23 pairs of chromosomes located in the nucleus of cells, 1 of these pairs is known as the sex chromosomes and the other 22 are known as autosomes, these are responsible for creating the proteins essential to allow the body to function. Chromosomes are made up of DNA which has 2 strands which curl around each other in a formation known as a double helix. Each strand has nucleotides which connect to the other nucleotides on the other strand via hydrogen bonds. There are 4 types of nucleotides to concern ourselves with for now, these are termed A, T, G and C, and these are always bonded to another particular nucleotide. An A nucleotide is bonded with a T nucleotide via 2 hydrogen bonds and vice versa, and a G nucleotide is boned to a C nucleotide via 3 hydrogen bonds and vice versa. The first step of protein synthesis is known as transcription, this is when the enzyme RNA polymerase comes along and breaks the hydrogen bonds between 10-20 pairs of nucleotides and separates the 2 strands, one of these strands is called the coding strand, as it codes for amino acids and the other is known as the template strand, RNA polymerase reads the template strands and produces the apposing nucleotide, but this time instead of producing a T nucleotide to oppose an A nucleotide, it will instead produce a U nucleotide when it reads an A nucleotide. When RNA polymerase has finished reading the strand it would have created mRNA, which moves away from the DNA and the DNA closes back up with help from the enzyme DNA ligase. Parts of the mRNA can also be removed, these parts are known as introns, and the rest is recombined, this part is known as an exon. The process of transcription is regulated by various proteins, such as transcription factors and coactivators, which work to make sure that the correct gene is transcribed in response to the appropriate signals. The mRNA concentration for a particular protein is regulated by the mitochondrial density and the transcription factors required for the expression of the gene. After transcription, the next step of protein synthesis is known as translation. The mRNA leaves the nucleus and enters the sarcoplasm (the cytoplasm of a muscle fibre), it then enters a ribosome. tRNA moves over to the ribosome, carrying with it an amino acid, the tRNA has 3 nucleotides which match up with the corresponding nucleotides of the mRNA. The 3 nucleotides of the mRNA is known as the codon, whilst its complementary nucleotides on the tRNA is known as the anticodon. For every 3 nucleotides of the mRNA, there is 1 amino acid which it matches. It doesn’t matter which order the nucleotides are in for coding for particular amino acids, for example, the U, G, C nucleotides will code for the same amino acid as the C, U, G nucleotides, also different nucleotides can code for the same amino acid. The tRNA will read 3 nucleotides of the mRNA, then it will move on and read the next 3, and so on, until the whole mRNA strand has been read, and for every 3 nucleotides read of the mRNA the tRNA will make its amino acid combine with the next one, until it eventually creates a chain of amino acids, known as a protein. Because translation relies on ribosomes the rate of translation is highly dependent on the quantity of ribosomes in the sarcoplasm, ribosomal biogenesis is therefore very important to support long-term muscle growth, luckily ribosomal biogenesis does occur with resistance training. Like transcription, translation is also regulated by various proteins, these are initiation factors (eIF), elongation factors (eEF) and release factors (eRF), the availability and activation of these factors is what determines the rate of translation. Translation appears to be the limiting step in the process of protein synthesis, certain hormones and growth factors, therefore, affect the process of translation, rather than transcription, in order to regulate protein synthesis.

Muscular adaptions to resistance training

An introduction to the muscular adaptions to resistance training

Resistance training causes a disruption in the intracellular and extracellular components of a muscle cell which creates a signalling cascade leading to muscle growth. The cause of hypertrophy can be due to an increase in sarcomeres in series or in parallel, an increase in noncontractile elements and sarcoplasmic fluid, or an increase in satellite cell activity which all leads to an increased total muscle cross-sectional area.

Myofibrillar hypertrophy

The biggest cause of muscle growth as a result of traditional resistance training is due to an increase in sarcomeres in parallel, termed myofibrillar hypertrophy. An increase in sarcomeres in series is believed to be due to the muscle adapting to a new functional length. Studies have shown that when muscles are immobilised for long periods of time in the stretched position it leads to an increase in sarcomeres in series whilst it leads to a reduction of sarcomeres in series when a muscle is immobilised for long periods of time at a short length. When it comes to resistance training, it appears that it is the eccentric portion of an exercise which leads to an increase of sarcomeres in series, whilst if a resistance training regime only includes concentric training it leads to a decrease of sarcomeres in series, however, it is believed that if concentric movements are performed at a fast speed than it can too cause an increase in sarcomeres in series. 

Sarcoplasmic hypertrophy

The increase in noncontractile elements and sarcoplasmic fluid is termed sarcoplasmic hypertrophy, this includes an increase in sarcoplasm, T-tubules, sarcoplasmic reticulum, mitochondria, glycogen stores, fibrous endomysial connective tissue and sarcolemma. Because sarcoplasmic hypertrophy causes no increase in the contractile elements of muscle it does not cause any increases in strength. Sarcoplasmic hypertrophy is believed to be primarily induced by high-repetition exercise, evidence for this is in the different composition of tissue found in bodybuilders and powerlifters, bodybuilders tend to have more glycogen stores and fibrous endomysial connective tissue as compared to powerlifters, which is believed to be due to differences in their training regimes. After just 5 months of resistance training, you can likely expect to increase glycogen stores by 66%. Bodybuilders have been found to have double the glycogen stores of untrained individuals, this is likely due to enzymatic alterations and a greater storage capability of muscles simply due to them being larger, however, it is not the glycogen itself which is significantly increasing muscle size, it is the water it brings with it, as 1g of glycogen attracts about 3-4g of water, this is because glycogen has an osmotic effect in order for the ratio of fluid to proteins to remain the same and to allow for cellular signalling to occur. The reason why bodybuilding type training and not powerlifting type training is believed to create greater glycogen stores in muscles is due to the higher rep range which typically occurs with such training. Glucose is relied on for energy during high-rep training rather than stored ATP, so it is reasonable to believe that this will create a stimulus for the muscles to be able to store more glucose as glycogen. Bud and colleagues reported that training at 90% 1RM increased sarcoplasmic protein synthesis more than training at 30% 1RM directly after exercise, but training at 30% 1RM did increase sarcoplasmic protein synthesis more 24 hours after exercise than training at 90% 1RM did, this reveals that both high weight, low repetition exercise and low weight, high repetition exercise can both stimulate an increase in muscle glycogen stores.

Satellite cells

Skeletal muscle is postmitotic, meaning that it does not have the ability to divide anymore following differentiation, because of this skeletal muscle does not undergo any significant cell replacement, therefore, to maintain healthy tissue and avoid cell death another method for regenerating muscle fibres must occur, luckily this is where satellite cells come into play. Satellite cells are actually a form of myogenic stem cells which reside between the sarcolemma (the membrane of muscle cells) and the basal lamina, which is a layer of extracellular matrix on top of the cell membrane. Satellite cells can be activated via the Notch signalling pathway or through the transcription factor, serum response factor. Once activated satellite cells produce 2 daughter cells, one of these will self-renew in order to preserve the satellite cell pool whilst the other will differentiate into a myoblast which multiplies and fuses to existing muscle fibres, they then provide the necessary agents which are needed for the repair and remodelling of the muscle cell. Once satellite cells are activated they are often coupled with the co-expression of myogenic regulatory factors, such as MyoD, Myf5, MRF4 and myogenin, these work to express muscle genes forming the proteins to help build different parts of the muscle. Following resistance training, satellite cell activation increases until it peaks 72-96 hours post-workout. Type 1 muscle fibres appear to have a greater pool of satellite cells at rest when compared to type 2 muscle fibres, however, the population of satellite cells increase to a greater extent in type 2 muscle fibres, when compared to type 1 muscle fibres following resistance training. The biggest benefit of satellite cells is that they have a nucleus so when they become part of the muscle fibre the nucleus can be used to create the proteins to help build different components in the muscle and therefore grow the muscle, due to the concept of myonuclei domain which states that the mRNA production from a nucleus can only support the protein production needed for a finite volume of sarcoplasm, therefore, for muscles to grow the size of the myonuclei domain must increase, or the amount of myonuclei domains in the muscle cell must increase due to a greater number of nuclei, luckily satellite cells contribute to both. Satellite cells also regulate the remodelling of extracellular matrix components, which is another benefit of an increase in satellite cell activity. Initially, satellite cells are not required to increase the number of myonuclei domains because the muscle is not big enough and the current nuclei of the muscle can just expand their current myonuclei domain, however, when the muscle does get larger satellite cells are required to increase the amount of myonuclei domains there are in the cell to allow for the continuous growth of the muscle. Kadi and colleagues found that there was only myonuclear addition required when myofiber hypertrophy reached 26% due to reaching the limit of the current muscle's myonuclear domains, 26% is just the average in the study and the exact number appears to vary greatly between individuals. Once a satellite cell has differentiated and fused to the muscle cell it is believed that it can not revert back to a satellite cell, this means a muscle cell's myonuclei number can only ever increase, even if training is completely stopped for long periods of time, this is not to say atrophy doesn’t occur when training is stopped. Because the myonuclei number in muscle cells can only increase it means that if an individual initially builds a good amount of muscle, then stops training until their muscles return to the size they were at before training but then starts training again their muscles will grow significantly faster than what it would have when they first started training, but only if their muscles got to a size where they needed to recruit more myonuclei to allow for further muscle growth.

Hyperplasia

Hyperplasia is an increase in the number of muscle fibres in a muscle, there is limited evidence that this occurs in humans, but evidence of hyperplasia certainly exists in animals, as is evident in multiple studies used on quals which showed significant hyperplasia when their wings were forced to bare a load for a prolonged period of time. The reason for hyperplasia is believed to be due to muscle fibres simply getting so large that they need to split in order to allow for additional hypertrophy. In humans, I believe it is unlikely that hyperplasia occurs apart from possibly in those who use large amounts of anabolic steroids, have elite-level genetics and have been training hard for a long time meaning that muscles have grown to a very large size.

Practical implications of muscular adaptions to resistance training

An increase in the number of sarcomeres in parallel is the primary cause of muscle growth and it likely occurs with all forms of resistance training, controlling the eccentric portion of exercises, and temporarily pausing the exercise when the muscle is in the most stretched position before completing the concentric portion of a lift will likely help to increase sarcomeres in series and help to add muscle mass. sarcoplasmic hypertrophy on the other hand typically occurs in the high rep range, which I usually consider to be between 12-30 reps per set of an exercise, sarcoplasmic hypertrophy is a large reason why bodybuilding type training typically stimulates a greater increase in muscle mass over powerlifting type training, so I would highly recommend that anyone who is seeking muscle growth regularly trains in the high rep range. 
 

Thankfully due to satellite cell's ability to increase the number of myonuclei it means that muscle loss due to injury, sickness or inactivity can be regained once returning to training, quicker than what it took to originally build.

Hormones and myokines

An introduction to hormones and myokines

Endocrine hormones are produced within glands and they are then released into the bloodstream where they can travel to muscle cells where they can bind to receptors on the sarcolemma or in the sarcoplasm and carry out their effects. The primary anabolic hormones include insulin-like growth factor-1 (IGF-1), growth hormone (GH), testosterone and insulin. Baseline levels of anabolic hormones are typically not affected by regular resistance training but most anabolic hormones are usually temporarily elevated following resistance training, this response may have a larger influence on muscle growth than what elevations in baseline levels of anabolic hormones would because following resistance training proteolysis increases (protein breakdown) and muscles are likely more responsive to anabolic hormones, so anabolic hormones likely have a super compensatory response, however, some evidence suggests that the spike in anabolic hormones following resistance training only serves for the purpose of controlling fuel stores or it may even be simply a response to the unfamiliar circumstances and the muscle damage which occurs from exercise, this means that acute spikes in anabolic hormones post-workout may not have any effect at all on hypertrophy, but if they do it is likely minimal. Myokines are cytokines which are expressed and secreted by skeletal muscle due to mechanical stimulation and they carry out their effects directly on muscle. They work in a paracrine fashion, meaning between adjacent cells, or in an autocrine fashion, meaning within the cell itself. Myokines bind to cellular receptors and regulate signal transduction via intracellular messengers and transcription factors to promote anabolic or catabolic processes. In this section, I talk about some of the best-studied myokines, these include mechano growth factor (MGF), interleukins (ILs), myostatin, hepatocyte growth factor (HGF), leukaemia inhibitory factor (LIF), although, there are other myokines including follistatin, platelet-derived growth factor-BB, vascular endothelial growth factor and chitinase 3-like protein 1 and more.

IGF-1

IGF-1 gets its name from its structural similarities to insulin. In vitro studies, it is clear that IGF-1 initiates protein synthesis, inhibits protein breakdown, and increases satellite cell differentiation, the fusion of myoblasts to muscle fibres, myotube diameter and the number of nuclei per myotube, which helps satellite cells perform their functions, this is likely the primary reason for IGF-1s hypertrophic effects. Evidence suggests that IGF-1 does not have to use a receptor for it to exert its effects. There have been three different IGF-1 isoforms identified in humans, these being IGF-1 Ea, IGF-1 Eb and IGF-1 Ec. IGF-1 Ea and IGF-1 Eb are produced in the liver and then secreted into the bloodstream, but other tissues also produce and secrete IGF-1 Ea and IGF-1 Eb and their production and secretion increase in response to physical activity, contracting muscles produce the majority of these isoforms during intense physical activity and much of them travel to muscle cells, serum concentrations of IGF-1 increases significantly following exercise, this may be due to the increase in growth hormone which potentiates its release or whether exercise itself increases IGF-1 levels directly. IGF-1 Ec is expressed as a result of mechanical tension, and it carries out its effects in a paracrine fashion or in an autocrine fashion because it performs its functions locally it is actually a myokine, whilst the other two IGF-1 isoforms are hormones. IGF-1 Ec is also known as mechano growth factor (MGF) because it is expressed in response to mechanical tension and its chemical structure differs from the other 2 isoforms, I discuss this myokine more in its own separate section. Serum concentrations of IGF-1 decrease with age which leads to muscle atrophy. IGF-1 activates the PI3K/ Akt pathway through signal transduction which initiates protein translation. Serum concentrations of IGF-1 have not been correlated with an increase in protein synthesis following exercise, and it is, therefore, unclear the extent to which serum IGF-1 stimulates hypertrophy through an increase in muscle protein synthesis. Normal serum IGF-1 levels in healthy young men is 182-780 ng/mL, and women typically have slightly lower levels. 

Growth hormone

Growth hormone is a polypeptide which gets released from the anterior pituitary gland. The anterior pituitary gland will secrete GH in a pulsatile manner. GH nonexercise secretion is at its highest during sleep. GH stimulates the breakdown of lipids in a process known as lipolysis, it also promotes the uptake of amino acids into cells and their transformation into proteins. At physiological levels it appears that growth hormones primary anabolic effects are due to it stimulating an increase in collagen synthesis. GH has been shown to have a significant impact on hypertrophy due to it increasing serum concentrations of IGF-1 Ea, IGF-1 Eb and MGF, although it has no effect on the hepatic secretion of IGF-1. Animal studies have shown that GH does not stimulate hypertrophy unless IGF-1 receptors are present. It is clear though that GH only potentiates the effects of IGF-1, and IGF-1 is not reliant on it. mRNA levels of MGF appear to greatly increase when elderly men performed resistance training and performed recombinant GH treatment, but similar results were not found in young men, it is therefore hypothesised that exogenous GH administration is only needed when natural GH levels drop due to ageing, which is necessary to reach the threshold to stimulate a large release of MGF. There is a reduction in muscle fibre size in individuals who are deficient in GH receptors even in young adults, this shows the importance of growth hormone towards hypertrophy. Growth hormone has also been shown to regulate myoblasts fusion with muscle cells which helps satellite cells perform their function. Growth hormone also appears to have an effect on testosterone-mediated muscle protein synthesis, but how and to what extent is unclear. 
 

Normal serum GH levels are 0.4-10 ng/mL in men and 1-14 ng/mL in females, and levels generally spike up to 20 ng/mL during sleep. Exercise will increase GH levels to a significant degree, Fujita and colleagues reported an increase in GH levels following blood flow restriction training of 10-fold over its baseline level, whilst, Takarada and colleagues reported a 290-fold increase in GH levels over baseline, either way the increase in GH levels over baseline due to resistance training is significant, this is believed to be due to metabolite accumulation and also H+ accumulation due to the breakdown of glycogen for energy which causes the stimulation of intramuscular metaboreceptors and group III and IV afferents. Even though GH levels are significantly spiked post-workout the evidence is not clear as to whether this leads to muscle growth. Recombinant GH administration consists of one isoform, whilst over 100 isoforms of GH have been found to be produced endogenously, for this reason, recombinant GH administration likely does not enhance muscle growth, in fact, it has even been shown to reduce the natural GH production post-workout, which is necessary to get the other GH isoforms, this may diminish the rate of muscle growth.

Testosterone

Testosterone is a steroid hormone derived from cholesterol and produced and secreted in the Leydig cells of the testis via the hypothalamic-pituitary-gonadal axis, and in the adrenals and ovaries but only in small amounts. Postpubescent men have about 10 times more testosterone than women, this is likely the reason for the significant differences in muscle mass between men and women. Unbound testosterone in the bloodstream is biologically active and ready to be taken up by bodily tissues, however, this is only about 2% of testosterone in the bloodstream, and about 60% of testosterone is bound to sex hormone-binding globulin and about 38% of testosterone is bound to albumin these can’t be taken up by bodily tissues, however, the testosterone which is bound to albumin is weak and they can rapidly disassociate to create biologically active testosterone. Once unbound testosterone enters cells it can bind to androgen receptors in the cytoplasm of cells, where the testosterone-androgen complex can then travel to the nucleus of the cell where it regulates gene transcription. The anabolic effects of testosterone are drastic, testosterone is believed to increase protein synthesis directly and diminish proteolysis, testosterone is also believed to increase the release of other anabolic agents, such as growth hormone and IGF-1, and inhibit the release of IGFBP-4 which inhibits the actions of IGF-1. Myoblasts have been shown to contain androgen receptors, and this likely means that testosterone is necessary to allow myoblasts to fuse to muscle fibres, increased testosterone levels have also been shown to increase satellite cell proliferation and differentiation, and also increase the number of active satellite cells, basically the evidence strongly suggests that testosterone is either very important for increasing the actions of satellite cells or is essential for satellite cells to perform their functions. However, there is evidence that when testosterone is suppressed anabolic agents do not decrease, but hypertrophy is still severely inhibited so testosterone may stimulate hypertrophy through another process which doesn’t rely on anabolic agents or an anabolic agent which we are not yet aware of. 

264-916 ng/dL is the normal testosterone level range for healthy young men, fluctuations within this range does not appear to affect hypertrophy, except for possibly individuals at the top end of this range, which may have an increased ability to grow muscle. Hypogandolism is a condition when the testosterone levels of a male drop below 264 ng/dL, individuals with this condition have been shown to have a reduced ability to grow muscle, and individuals with hypergandolism, which is when the testosterone levels of a males increase above 916 ng/dL, certainly appear to have an increased ability to grow muscle. Serum testosterone levels in healthy women typically fall in the range of 15-70 ng/dL, and women with testosterone levels above this range, like men, certainly appear to have an increased ability to increase muscle mass. The amount of androgen receptors in the cytoplasm of cells will also have a significant impact on the effects of testosterone. Androgen receptor concentrations drop after resistance training, but they then significantly rise over the hours after, there appears to be an association between increased androgen receptor concentrations and increased hypertrophy, so the androgen receptor count clearly has a clear effect on hypertrophy. Testosterone levels rise immediately after exercise but then have a rapid decline within 1 hour, this is the time that androgen receptor concentrations rise, also protein synthesis 45-150 minutes after exercise begins to rapidly increase and remain elevated for up to 4 hours, so it has been hypothesised that the drop in testosterone is due to androgen receptors taking up the testosterone and using it for muscle protein synthesis. However, It is not clear as to whether resistance training can increase basal levels of testosterone, but testosterone levels do seem to increase directly after exercise by about 15%. Some evidence suggests that bodybuilding-type training increases testosterone levels more than strength-type training. After the age of 30 testosterone levels appear to drop by about 1% every year in males, this may be partly responsible for the muscle loss which occurs with age.

Insulin

When blood glucose is high the pancreas beta cells are stimulated to release insulin into the bloodstream in order to regulate blood glucose levels, however, insulin also plays a role in hypertrophy. Insulin regulates various eIFs and eEFs which helps the translation process of protein synthesis. Insulin also activates the mammalian target of rapamycin (mTOR), increasing muscle mass. Even though insulin does appear to have anabolic properties its main effect on hypertrophy appears to be due to its ability to inhibit protein breakdown, however, the exact mechanism, which insulin achieves this is unknown. Unlike many of the other anabolic hormones and myokines, exercise has no effect on insulin levels for nondiabetic individuals, in fact, exercise has even been shown to blunt insulin release, depending on its intensity, duration and the pre-exercise nutritional consumption, so it is the diet which has the largest influence on insulin release. Insulin is secreted into the bloodstream with the consumption of macronutrients, particularly carbohydrates. In between meals blood glucose levels usually fall into the 80-120 ng/dL range, and in healthy individuals consuming a healthy diet blood sugar levels never stray too far out of this range, generally not exceeding 200 ng/dL, normal insulin levels range from 60-100 mg/dl and can spike up to 140 mg/dl following a high carbohydrate, fast-digesting meal. 

MGF

MGF mRNA expression increases acutely following resistance training and it is believed to repair muscles following myotrauma. Evidence suggests that those who have the highest peak in MGF following resistance training (126%) see the greatest increases in muscle mass over those that have only a small peak or no peak in MGF following resistance training, this displays how important MGF is for hypertrophy. MGF is believed to directly stimulate muscle protein synthesis through phosphorylation of p70S6 kinase or through the PI3K/ Akt pathway. There is also evidence that MGF suppresses FOXO nuclear localisation and transcriptional activities which inhibits protein breakdown. MGF has also been shown to be involved in primarily the initial phases of the satellite cell response, these being activation and proliferation, over the later phases, whilst  IGF-1 Ea and IGF-1 Eb are more involved in the later stages of the satellite cell response over the initial phases, this is consistent with the findings that MGF is expressed before IGF-1 from the liver following exercise, however, the evidence for MGFs effects on satellite cells is contradictory. MGF has been shown to regulate extracellular signal-regulated kinase 1 and 2 (ERK1/2) which may be the reason for its effects on the satellite cell response. ERK1/2 increases due to resistance and aerobic training, and the extent of its increase depends on the intensity of the exercise, this may suggest that MGF has the greatest increases during very intense exercise.
 

Interleukins

Interleukins are a form of cytokines released by various body tissues in order to coordinate the inflammatory response. There have been 6 interleukins isoform which has been found to be involved in hypertrophy, these being IL-4, IL-6, IL-7, IL-8, IL-10 and IL-15, of these IL-6 has been the most studied, which is an early stage myokine found to stimulate collagen synthesis in healthy tendons, to keep them strong and healthy to withstand tension. IL-6 is believed to primarily carry out its effects on hypertrophy by inducing satellite cell proliferation and influencing satellite cell-mediated nuclear accretion, but it also has been shown to stimulate muscle protein synthesis directly through the stimulation of Janus kinase/signal transducer and activator of transcription (JAK/STAT), ERK1/2 and PI3K/ Akt signal transduction pathways. Resistance training can create a 100 times increase in IL-6 over baseline, metabolic stress may increase its production to an even greater degree. IL-6 is synthesised by contracting skeletal muscle, connective tissue, adipocytes and the brain. IL-6 in the bloodstream is elevated before other cytokines and its release is much larger than the others. It was once believed that IL-6 increases due to muscle damage, but it has since been proven that this is not the case and muscle damage may instead lead to a decreased peak and a delayed spike in IL-6. IL-15 is the second interleukine which I want to discuss. IL-15 is primarily synthesised by muscles, especially during exercise, particularly resistance training due to its release from microtears due to inflammation, oxidative stress or both. Interestingly, type 2 muscle fibres have been shown to have a greater increase in IL-15 mRNA levels over type 1 muscle fibres. There is some evidence that IL-15 works to increase muscle protein synthesis and decrease protein breakdown in differentiated myotubes, however, it is possible that IL-15 serves for the purpose of regulating oxidative and fatigue properties of skeletal muscle, rather than hypertrophy or in preventing atrophy. The research on IL-4, IL-7, IL-8 and IL-10 is very limited. IL-4 is believed to play a role in myogenic differentiation. IL-7 is believed to play a role in hypertrophy and the fusion of myoblasts to the muscle fibre. IL-8 has been shown to have anticatabolic effects on skeletal muscle. IL-10 is believed to mediate some of the processes that lead to myoblast proliferation and myofibre growth.
 

Myokines are released as part of the inflammatory response so chronically elevated levels of myokines indicate chronic inflammation which has been linked to a reduction in muscle mass with age possibly due to inflammation causing damage to healthy tissue and an increase in the activation of catabolic processes, in addition to this, those with chronic inflammation have been shown to have a reduced ability to put on muscle due to resistance training. Interestingly, IL-6 has another property, an acute increase in the myokine following resistance training likely helps to aid hypertrophy, however, when this myokine is chronically elevated it leads to the suppression of muscle protein synthesis. Thankfully those suffering from chronic inflammation can likely deal with this problem with nonsteroidal anti-inflammatory drugs (NSAIDs), which have been shown to increase muscle protein synthesis and decrease proteolysis in ageing rats.
 

Delayed onset muscle soreness (DOMS) is caused by the inflammation in muscles created by the muscle damage which occurs as a result of resistance training. NSAIDs taken after training may be able to reduce the accumulation of inflammation which causes delayed onset muscle soreness, this means that individuals can train more frequently or more intensely during a session without the fear of DOMS. NSAIDs are believed to reduce inflammation due to their inhibition of cyclooxygenase (COX), which is a group of enzymes which catalyse the conversion of arachidonic acid to pro-inflammatory prostanoids. Prostanoids are believed to lead to an increase in muscle protein synthesis due to an increase in the activation of the PI3K/ Akt pathway and ERK1/2, it is also believed to enhance satellite cell proliferation, differentiation and fusion. This, therefore, indicates that by NSAIDs inhibiting COX it may lead to a reduced ability to grow muscle, evidence suggests that this is more likely to happen due to impaired satellite cell function over an inhibition of muscle protein synthesis which means that NSAIDs may be potentially seriously hindering gains for advanced young and middle-aged trainees who have reached the ceiling of the myonuclear domains in their muscle and require satellite cells to allow for myonuclear accretion. It has been hypothesised that even though NSAIDs reduce the rate of muscle growth they also may reduce the rate of muscle breakdown so their effects are not as large as you initially may have thought. It has been found that elderly individuals aged between 60 and 85 actually benefited from NSAIDs due to the suppression of chronic inflammation which is common with individuals in this age range. NSAIDs reducing inflammation may suppress the release of certain myokines further inhibiting growth, studies have shown that IL-6 is inhibited when NSAIDs are administered. In conclusion, irregular use of NSAIDs likely has no noticeable impact on hypertrophy, but for young and middle-aged people who have likely reached the ceiling for the myonuclear domains in muscles, I would not recommend their use. NSAIDs likely help hypertrophy only in those who have chronic inflammation. DOMS is very common, particularly in large, advanced, resistance-trained individuals, so NSAIDs may be a useful method for reducing DOMS so the individual can train more intensely and more frequently, there is a possibility that this leads to more muscle growth in the long run.

Myostatin

Large amounts of muscle mass cost a lot of energy and cause large amounts of fatigue during exercise, evolutionarily there is, therefore, no good reason to have a very large amount of muscle and certain myokines exist to stop this from happening, one of these is myostatin (MSTN). A mutation in the MSTN gene which leads to its reduced expression has been shown to lead to increased hypertrophy. There has been shown to be extreme muscularity in infants who do not have the MSTN gene, the same goes for other animals, these have been used for certain purposes for humans, the Belgian Blue is a breed of cow which do not have the MSTN gene, these cows are extremely muscular and provide a lot of beef for human consumption, a lot of successful race dogs have been shown to not have the MSTN gene which makes them more powerful and faster, although, there is research that in mice low levels of MSTN does lead to an increased muscle mass but it does also lead to impaired calcium release from the sarcoplasmic reticulum in muscles so it does not lead to a proportional increase in strength and power as much as you may expect. Low MSTN has been shown to reduce the rate of muscle growth in muscle groups made up of primarily slow-twitch muscle fibres, this means that those with low MSTN levels often have poor muscular endurance.
 

MSTN signals the transcription factors SMAD2 and SMAD3 which leads to the increased expression of the catabolic enzyme muscle ring finger protein-1 (MuRF-1). MSTN carries out its effects through its ability to decrease muscle protein synthesis and possibly satellite cell proliferation and differentiation but to a less significant degree. Not only does MSTN increase catabolic processes it also decreases the effects of anabolic processes, MSTN has been shown to inhibit the Akt/ mTOR pathway, downregulate calcineurin signalling and the transcription factors MyoD and myogenin. Interestingly, The downregulation of the mTOR pathway has been shown to cause an increase in MSTN. When an individual begins resistance training regulatory factors, such as MSTN, slowly begin to get downregulated, with consistent resistance training MSTN can be reduced to 3 times greater than what it once was, this may lead to greater muscle growth. Reductions in MSTN appear to be greater for larger muscle groups, such as the quadriceps.
 

Hepatocyte growth factor (HGF)

HGF is a myokine which has mitogenic actions on numerous bodily cell types, including muscle cells, meaning that it induces cell division. It has been suggested that HGF is essential for the activation of satellite cells, and it is the only myokine which has been proven to stimulate dormant satellite cells. Muscular contractions activate the dystrophin-associated protein complex, which leads to nitric oxide synthase activation which stimulates the release of HGF from the extracellular matrix of muscle cells and facilitates its interaction with receptors on satellite cells. Chronically elevated HGF levels have been shown to increase MSTN mRNA production, which may inhibit the functions of satellite cells and return them to quiescence.

Leukaemia inhibitory factor (LIF)

Exercise leads to the increased expression of LIF in skeletal muscle, likely due to intracellular calcium concentration fluctuations. LIF may exert anabolic effects by acting in a paracrine fashion with satellite cells to induce their proliferation and prevent premature differentiation. 

The three mechanisms of hypertrophy

Mechanical tension

Mechanical tension is the amount of force placed on a certain square area. Mechanical tension has been proven to stimulate mTOR, possibly due to the activation of the ERK/tuberous sclerosis complex 2 (TSC2) pathway and/or through the activation of the p70S6K pathway, it is possible that these pathways are mediated by the synthesis of the lipid second messenger phosphatidic acid produced by phospholipase D, activation of any of these pathways leads to an increase in muscle protein synthesis. There are more pathways which are believed to be activated through mechanical tension, these include N-terminal kinase (JNK), mitogen-activated protein kinase (MAPK), PI3K/ Akt, calcium dependant pathways and the phosphatidic acid pathway, these pathways overlap in their processes and may interact with one another. Fascinatingly, different muscle actions and forms of tension appear to be responsible for the activation of different pathways. Eccentric contractions of the muscle lead to the greatest increase in JNK and ERK1/2, followed by concentric contractions and then isometric contractions. A high peak tension appears to host the greatest increases in MAPK, whilst, a high time under tension appears to host the greatest increases in JNK and p70S6K. Muscular contractions concentrically appear to cause an increase in the number of sarcomeres in parallel whilst muscular contractions eccentrically appear to cause an increase in the number of sarcomeres in series, this proves that mechanosensors in muscles can detect the different forms of muscular contractions and it stimulates the muscle to grow in different ways, through a process known as mechanotransduction, as is proven by evidence of different gene expression due to different forms of muscular contractions. 
 

Mechanosensors are located all throughout the muscle, including in the tendons. There are many components which work to help carry out mechanotransduction, these include stretch-activated ion channels, caveolae, myosin motors, cytoskeletal proteins, nuclei, titin, G protein-coupled receptors and the extracellular matrix, these components work with structural components in the muscle, such as the cytoskeleton to sense tension in the muscle and signal for an increase in muscle protein synthesis. One of the most important mechanosensors is integrin, which resides in the cell membrane and interacts with ECM proteins in the extracellular matrix, to sense mechanical and chemical information outside of the cell, this is an example of a focal adhesion complex, which is a sarcolemmal protein which connects components in the extracellular matrix with the cytoskeleton to sense muscle tension and activate muscle protein synthesis. Titin is an important mechanosensor, and the stiffer it is the more it will initiate a greater anabolic response. Myonuclei gets stretched and flattened during resistance training and this even may lead to the signalling of various growth-related proteins which enter the nucleus from the cytoplasm of the cell and initiate the process of protein synthesis. Once mechanosensors sense tension, enzymatic cascades are activated which leads to a nuclear response and gene expression, ultimately leading to a greater number of muscular proteins. Focal adhesion kinase (FAK) is an enzyme which is activated due to mechanical tension and it appears to play a key role in activating muscle protein synthesis. As well as anabolic pathways there are also catabolic pathways, these being autophagy-lysosomal, cysteine protease caspase enzymes, the ubiquitin-proteasome system and calcium-dependent calpain. The MSTN-SMAD pathway and the 5’-AMP-activated protein kinase (AMK) pathway are believed to control the other catabolic pathways and suppress anabolic pathways, they do this during times when the body is in need of energy, such as when in the fasted state or during exercise, it is a possibility that if these factors are combined by performing exercise in a fasted state the ability to grow muscle may be reduced.
 

The PI3K/ Akt pathway increases anabolic signalling and reduces catabolic signalling. There are three isoforms of Akt, these being Akt1, Akt2 and Akt3, of these, Akt1 appears to have the greatest increase due to mechanical tension. Akt activates mTOR which leads to an increase in various anabolic agents which will lead to elevated muscle protein synthesis further downstream, although, it is important to bare in mind that this is not the only pathway which activates mTOR. mTOR has two different signalling complexes, these being mTORC1 and mTORC2. mTORC1 appears to be inhabited by the pharmaceutical agent rapamycin. Interestingly, mTOR is also activated during a positive energy balance and inhibited during a negative balance, this shows how a hypercaloric diet can have a positive effect on muscle growth even without resistance training. mTOR carries out its effects via activating p70S6k which has a role in initiating mRNA translation, it also inhibits eIF4EB1 which is a negative regulator of the eIF4E protein which is important for mRNA translation. p70S6k can also be activated during resistance training, independent of the Akt pathway. Prolonged elevated mTOR levels have been shown to impair fast-twitch muscle fibre growth in mice and lead to anabolic resistance amongst the elderly population, so even though mTOR hosts great benefits, when it is elevated for too long it can cause problems, in fact, it may even reduce life span. As well as activating mTOR the Akt pathway can also activate the translocation of FOXO proteins so they away move from the nucleus and into the cytoplasm, this means they can’t be transcribed and create MuRF-1 and atrogin-1 proteins which are catabolic. The Akt pathway also inhibits GSK3β activation, this protein blocks the eIF2B protein which is believed to have a role in initiating translation in every cell of the body, not just in muscle. On top of all of this, the Akt pathway has also been proven to have a role in promoting satellite cell differentiation, this may be the primary anabolic effect of Akt1 due to resistance training.

The MAPK pathway is activated by ERK1/2, p38 MAPK and JNK during resistance training. The MAPK pathway is likely essential for maximal muscle growth through its stimulation downstream increase in protein synthesis, it also has been shown to increase ribosomal biogenesis, and of course, ribosomes are essential for the process of protein synthesis, so it is conceivable that an increase in ribosomes will lead to an increase in protein synthesis in muscles, which has been proven in studies. As previously mentioned p38 MAPK is responsible for activating MAPK. p38 MAPK is primarily activated due to aerobic exercise. There are believed to be four p38 MAPK isoforms, these being p38α, p38β, p38γ and p38δ. P38δ is not found within muscle tissue, p38γ is only found in muscle tissue but it is mostly upregulated in slow-twitch muscle fibres over fast-twitch muscle fibres. p38α and p38β is found all throughout the muscle. The p38 isotopes don’t directly bind to DNA to stimulate protein synthesis, instead, it activates certain transcription factors which initiate the process of protein synthesis. There is also some evidence that p38 stimulates notch signalling which is believed to be essential for satellite cell activation, proliferation and progression. JNK is also responsible for activating the MAPK pathway, and it is very sensitive to mechanical tension, especially during eccentric contractions during resistance training. The rise in JNK due to resistance training is responsible for a rapid rise in transcription factors that are responsible for cell proliferation and DNA repair, it also appears to lead to an increased ability to forcefully contract muscles. 
 

Calcium is contained within the sarcoplasmic reticulum inside of muscle cells and when it is released it binds to the troponin on the actin filament which makes the troponin fall away, this means that actin and myosin can now bind to allow for a muscular contraction. The increase in calcium inside the muscle cells is the reason for the calcium-dependent pathway. The increase in intracellular calcium levels activates TORC1 which increases protein synthesis. The enzyme calcineurin is a calcium-regulated phosphate which is stimulated by an increase in calcium levels inside of the muscle. Once activated calcineurin acts on downstream anabolic effectors, these being MEF2, GATA transcription factors and NEAT. Calcineurin and PI3k/ Akt activation appears to be essential for IGF-1’s anabolic effects. Calcineurin does not appear to be necessary for muscle growth, but it likely expedites it, particularly in slow-twitch muscle fibres. The calcium-calmodulin-dependent kinases are CaMKII and CaMKIV, these both have many isoforms, some of which detect and respond to calcium in the muscle via downstream signals. The calcium-calmodulin-dependent kinases have been shown to have a role in muscle plasticity. CaMKII is activated by short and long-duration exercise so it is therefore believed to induce muscle growth and mitochondrial biogenesis. One CaMKII isoform has been shown to increase due to muscle atrophy, it is therefore believed to work to reduce the extent of muscle loss.
 

The phosphatidic acid pathway is activated by several different classes of enzymes, particularly phospholipase D1 (PLD1), which hydrolyzes phosphatidylcholine into phosphatidic acid and choline. When activated phosphatidic acid increases protein synthesis, this is because it activates p70S6k, by binding to and activating mTOR or by activating it in an mTOR-independent manner. Very high levels of PLD1 have been shown to cause a decrease in catabolic factors, including FOXO3, atrogin-1 and MuRF-1, this is believed to be due to PLD1 activation of Akt resulting in the activation of mTORC2. There is evidence that PLD1 is essential to activating mTOR due to mechanical tension. The effects of the other enzymes which are part of the phosphatidic acid pathway is relatively unknown. Phosphatidic acid appears to increase exponentially due to mechanical tension, especially during the eccentric portion of exercises.
 

The AMPK pathway is activated by an increase in the AMP/ATP ratio, which occurs when ATP is broken down for energy at a faster rate than usual, such as during intense exercise. When activated AMPK suppresses energy-consuming anabolic processes and increases catabolic processes so that muscle can be broken down and used for energy, this displays AMPKs importance for regulating cellular energy levels. It is plausible that by consuming a high calorie, fast-digesting meal before or during exercise ATP in the cell will be replenished quicker than if no meal was consumed, this means the AMP/ATP ratio may not increase as much meaning that AMPK is activated to a lesser degree, this may lead to a slightly greater amount of muscle growth over time. AMPKs catabolic effects appear to be at least in part due to its associated upregulation in atrogin-1, and possibly through an increase in FOXO transcription factors, autophagy and through suppression of satellite cell differentiation. Not only does AMPK appear to have a catabolic response it also inhibits the rate of protein synthesis, it is theorised to do this through many pathways, one is by reducing the anabolic effect of mTOR by first inhibiting ras homolog enriched in brain (RHEB), it does this either via phosphorylating mTOR directly or by indirectly phosphorylating the tuberous sclerosis complex (TSC), it could possibly perform its effects through both. Another way AMPK is believed to suppress protein synthesis is through the inhibition of translation elongation and through the indirect suppression of the anabolic effector eIF3F. Suppression of the AMPK pathway appears to lead to a noticeable increase in muscle mass, especially with the addition of resistance training, whilst an increase in the AMPK pathway appears to lead to a noticeable decrease in muscle mass. AMPK is activated post-workout but so is other pathways, such as mTOR, which are shown to be anabolic, this likely overwhelms the AMPK activation leading to muscle hypertrophy.

Metabolic stress

Metabolic stress is believed to be another cause of hypertrophy, although it can be argued that its effects on hypertrophy are redundant without mechanical tension, some evidence even suggests that it may lead to more hypertrophy than mechanical tension, although I believe this is unlikely. Metabolic stress is a build-up of metabolites in the muscle due to exercise. The most common metabolites that build up in muscles are hydrogen ions, lactate and inorganic phosphate. Fatigue has been contributed to the depletion of the ATP-PCr system and the accumulation of hydrogen ions. In particular, the accumulation of hydrogen ions is believed to be responsible for the fatigue which is associated with multiple sets. Hydrogen ion accumulation in the muscle has been shown to denature the enzymes which are needed as part of the metabolism to produce the energy required to perform an exercise, contributing as a cause for the failure of an exercise. Lactate is produced during anaerobic metabolism, and the build-up of inorganic phosphate in the muscle has been shown to bind to the myosin binding sites and stop calcium from going back to the sarcoplasmic reticulum to allow for further muscular contractions, troponin then binds to these actin filaments further reducing the possibility of cross bridging between the actin and myosin filaments, this also contributes as a cause for failure of an exercise. Lactate, hydrogen ions and inorganic phosphate are not the only 3 metabolites as there have been about 4,000 discovered in the bloodstream, some of these are likely to also have an anabolic effect. The accumulation of metabolites primarily occurs during resistance training lasting 15-120 seconds, this is because this is the time frame in which the glycolytic system is primarily utilised, any shorter and the ATP-PCr system will be primarily utilised, where stored ATP is used for muscular contractions and creatine phosphate is used to combine ADP and a phosphate to replenish ATP, any longer than 120 seconds and oxygen will likely be able to enter the muscle sufficiently so ATP can be created in the mitochondria as part of the oxidative system, during resistance training the contractions of muscles compress blood vessels reducing the ability of oxygen getting to muscles and creating acute hypoxia further decreasing the possibility of the body relying on the oxidative system during resistance training. The oxidative and ATP-PCr systems do not create any of the byproducts which have been associated with hypertrophy, and that is why using the glycolytic system for energy during resistance training is necessary for metabolite accumulation. 
 

Lactate is believed to activate calcium-dependant pathways, therefore inducing hypertrophy and inhibit histone deacetylase, which negatively regulates muscle growth. The accumulation of hydrogen ions decreases the pH inside of muscle cells, type 2 muscle cells are particularly sensitive to this and it reduces the ability of calcium binding to actin to allow for a muscular contraction as less ATP can be produced, this means that type 1 fibres are relied on to produce the necessary force output for an exercise this leads to the greater development of these muscle fibres. Metabolite accumulation begins when the oxygen supply to muscles is insufficient for the oxidative system, this also produces reactive oxygen species (ROS) which has been shown to induce hypertrophy, possibly due to stimulating myogenesis through activation of HIF-1α.
 

Metabolite accumulation has been shown to decrease the muscle fibre recruitment  threshold which is likely the most prominent cause of hypertrophy due to metabolic stress, allowing for the activation of high threshold motor units and fast twitch muscle fibres through low weight. Metabolite accumulation is believed to carry out its effects on muscle fibre recruitment through the depletion of glycogen stores and organic phosphate splitting, also the accumulation of hydrogen ions in working muscle fibres will necessitate the recruitment of more high threshold motor units to produce the necessary force output. The good news about this is that fast-twitch muscle fibres can be activated and stimulated to grow without the need for high weights, this decreases the risk of injury and fatigue, however, muscle fibre recruitment does still appear to be greater with heavy weight over lightweight, although, it has not shown to have a larger anabolic effect. 

There is a possibility that metabolite accumulation also stimulates hypertrophy through myokine production, or it may inhibit atrophy through the suppression of certain catabolic myokines. IL-6 and MSTN have contradictory research surrounding them relating to their relationship with metabolic stress, it is possible that metabolite accumulation leads to an increased expression of the anabolic myokine IL-6 and a decrease in the catabolic myokine MSTN, although other research contradicts this. It is likely that metabolite accumulation leads to the downregulation of the catabolic myokines FOXO3A, atrogin-1 and MuRF-1 which may lead to greater hypertrophy.
 

Cell swelling occurs when intracellular hydration increases, and it is speculated to have a role in muscle hypertrophy. The primary cause of cell swelling in muscles is from muscular contractions causing an increase in blood supply to the muscle whilst inhibiting venous outflow through the compression of the veins which take blood away from the muscle. Lactate which is produced in muscles during resistance training may also increase cell swelling, this is because it has an osmotic effect drawing water from the bloodstream and into the muscle, lactate accumulation is also believed to activate volume regulatory mechanisms, and its effects are amplified with an increase in hydrogen ions, the reason for this is to protect it from excessive swelling. An increase in intracellular hydration levels has been shown to increase protein synthesis and reduced protein breakdown in many cell types, including muscle cells, the reason for this is believed to be due to the pressure which is created on the cytoskeleton and cell membrane which is perceived as a threat by the cell so anabolic signalling is stimulated in order to reinforce the structure of the cell. Integrin-associated volume osmosensors within cells detect the increase in intracellular hydration and turns on anabolic-kinase transduction pathways, such as PI3K, mTOR and MAPK. There is also evidence that cell swelling triggers satellite cell proliferation and fusion, which is another possible explanation for cell swelling's anabolic effects. Fast-twitch muscle fibres are particularly sensitive to cell swelling, this is believed to be because they contain a greater quantity of aquaporin-4 (AQP4) water-transporting channels as compared to slow-twitch muscle fibres, this may even be partly the reason why fast-twitch fibres are shown to grow at a significantly greater rate than slow-twitch muscle fibres.
 

Anabolic hormones appear to be significantly elevated due to the metabolite accumulation from resistance training, particularly growth hormone, which levels post-training were found to be 10 times greater in those performing blood flow restriction training, which has been shown to cause significant metabolite accumulation, over resistance training of a similar intensity. The metabolites lactate and hydrogen ions are believed to be responsible for the spike in growth hormone. Because GH leads to the upregulation of IGF-1, metabolite accumulation also likely leads to an increase in IGF-1, the evidence towards this isn’t clear, and the studies which have shown this to be the case only have tested for IGF-1 levels in the bloodstream. 

Blood flow restriction training is when a pressure cuff is tightened around a limb to reduce blood flow to a particular muscle which is getting worked through resistance training, for example by tightening a pressure cuff around the upper arm just below the delt and performing bicep curls will create a significant metabolite accumulation in the biceps and stimulate muscle growth, this allows for a greater metabolite accumulation and makes the exercise significantly more challenging with significantly less weight, this means a stimulus for muscle growth can be created when trying to limit strain put on muscles due too extreme fatigue, such as at the end of a contest diet, or when recovering from an injury. Blood flow restriction training has been shown to be effective, displaying how metabolite accumulation can still create a stimulus for muscle growth even with minimal tension. 

Muscle damage

Intense, high-volume exercise or exercise which the participant is not used to leads to the most exercise-induced muscle damage (EIMD), other forms of exercise do too but they don’t typically lead to muscle growth, this casts doubt as to whether muscle damage which occurs from resistance training leads to muscle growth. The damage in muscles can just be just a few macromolecules, or it can cause large tears in the structure of the muscle, through the sarcolemma, basal lamina, sarcomeres, cytoskeleton and the surrounding connective tissue. Muscle damage typically is correlated with an increase in local inflammation, disturbed calcium ion regulation in muscles, the activation of protein breakdown in muscles and secretion of certain substances from damaged muscles into the bloodstream, such as creatine kinase. Severe myocellular damage can lead to tissue necrosis (the death of tissue) and myocellular disruptions, which is far too much damage out of the optimal range. Fast-twitch muscle fibres have been shown to develop greater muscle damage than slow-twitch muscle fibres due to resistance training, this may be due to fast-twitch muscle fibre's reduced oxidative abilities, their increased ability to generate a force and the differences in the muscle fibre’s structure. Eccentric muscular actions have been shown to lead to significantly greater muscle damage than isometric or concentric muscular actions. Eccentric muscular actions create muscle damage through disruption of the actomyosin bonds, rather than ATP-dependant detachment, which appears to be the primary cause of muscle damage during isometric or concentric muscle actions, this is likely the reason why eccentric muscular actions create more muscle damage. It has also been speculated that because different-strength sarcomeres reside within different parts of the muscle, this means as the muscle lengthens it begins to break, which leads to the deformation of T-tubules and the disruption of calcium homeostasis, which leads to the secretion of calcium-activated neutral proteases which leads to more muscle breakdown. Symptoms of muscle damage include a decreased ability to produce force in the muscles, stiffness, swelling, DOMS and a larger physiological stress response to exercise which includes an elevated heart rate and an increased lactate production due to exercise, beyond its normal range. Fascinatingly upper-body muscles are more likely to experience muscle damage than lower-body muscles, this is believed to be an evolutionary adaption so that muscles which are more commonly used are not as poorly affected by DOMS.
 

The repeated bout effect, is when muscle damage, and its associated symptoms, decrease as an athlete becomes more accustomed to a particular training regime. The repeated bout effect is strong, there is evidence that when performing a new exercise the associated muscle damage is reduced by ⅔ between the first time and the second time performing it, other research has shown that 10 weeks of performing a new training regime to failure will bring markers of muscle damage down to undetectable levels. It has been hypothesised that the reason for this is due to the development of stronger connective tissue, more efficient motor unit recruitment, more synchronised motor unit recruitment, a greater distribution of load among muscle fibres and an increased contribution to perform an exercise from synergist muscles. Α7β1 integrin expression is increased due to muscle damage, this initiates the transcription of genes which are involved in the protection of the muscle from mechanical stress and they may also promote the expression of genes which are involved with hypertrophy, because muscle damage may lead to the expression of genes which inhibit protein breakdown this may lead to greater muscle growth. 
 

Interestingly, It has been hypothesised that a reason why muscle growth typically only occurs a few weeks after beginning training although protein synthesis is at its highest when training begins in those who are untrained is because muscle damage is so high to begin with that the elevated protein synthesis levels is only enough to counteract the extent of muscle breakdown. Too much muscle breakdown leads to muscle loss, interference with growth-related processes and a reduction in a muscle's force-producing capabilities, which makes stimulating training difficult to achieve. Muscle breakdown increases with each set of an exercise performed, so there is certainly a ceiling as to where too much volume leads to diminishing returns.
 

The damage to muscle fibres creates an inflammatory response. Neutrophils enter the damaged area as part of the inflammatory response and they release proteases which help breakdown the proteins which are creating debris, they also secrete cytolytic and cytotoxic substances which cause further damage to the muscle and cause damage to other tissue in the area, this is part of the immune response to destroy cells which may oppose a threat to health, this is likely the primary role of neutrophils in muscle, however, neutrophils also release a variety of reactive oxygen species (ROS) which are believed to lead to hypertrophy by signalling to other inflammatory cells which have an anabolic effect. ROS has been shown to activate the MAPK pathway, to further support this there is evidence that MAPK activation is at its greatest following eccentric over concentric or isometric exercise, this may be due to the increased muscle damage, leading to greater ROS production which activates MAPK. As well as activation of MAPK, ROS may also increase levels of IGF-1, displaying another avenue of potential growth due to muscle damage. However, there is evidence that ROS inhibits the signalling of many serine/threonine phosphates, one of which is calcineurin, ROS does this by blocking its calmodulin-binding domain which doesn’t allow it to carry out its anabolic effects. It is not guaranteed that ROS are activated due to muscle damage, and if they even have an anabolic or catabolic effect, if they do, it is likely reliant on the specie of ROS produced and the exercise type. 
 

Macrophages produce myokines which have an anabolic effect, however, there has not been any concrete evidence that an increase in macrophages due to exercise-induced muscle damage leads to a greater amount of myokines, rather muscle contractions may be the primary cause of an increase in myokines, this at least appears to be the case for IL-6 and fibroblast growth factors (FGF) which is a powerful proliferative agent involved in hypertrophy. Muscle contractions are believed to lead to a greater amount of myokines because muscle contractions are believed to lead to the movement of substrate from fuel depots so that glucose levels do not drop too low during intense exercise. It is possible that muscle contractions and the inflammatory process favours the increase in different myokines which when combined will lead to the greatest amount of muscle growth.

When a cell gets damaged it must gain more myonuclei to repair otherwise it will die, inflammatory cells appear to be critical for this process. Satellite cells reside under the myoneural junction, so it has been hypothesised that the activation of motor neurons innervating damaged muscle fibres leads to their stimulation. Initial signalling of satellite cells due to muscle damage however is believed to be due to the release of muscle-derived nitric oxide and possibly HGF. Muscle damage leading to the activation of muscle growth is believed to be controlled in some manner by COX-2. COX-2 is believed to induce the synthesis of prostaglandins which are responsible for satellite cell proliferation, differentiation and fusion. Non-steroidal anti-inflammatory drugs have been shown to reduce the satellite cell response post-exercise, this may be because it causes a reduction in COX-2, although mechanical tension still has other avenues which it can use to stimulate satellite cells.

IGF-1 has been shown to be elevated by muscle damage. The MGF isoform has been shown to be elevated 24 hours post-exercise, suggesting that this isoform is essential for muscle recovery. The other two IGF-1 isoforms have only been shown to be elevated until 72 hours following training. Bamman and colleagues demonstrated how eccentric muscular actions lead to a 62% increase in IGF-1 mRNA and a 57% reduction in IGFBP-4 mRNA, which is a strong inhibitor of IGF-1, whilst concentric and isometric muscular actions only lead to a slight decrease in IGFBP-4 mRNA concentrations, because eccentric muscular actions lead to the greatest muscle damage this study provides strong evidence that IGF-1 plays a role in muscle repair, but this will still need to be further studied. 

As previously discussed cell swelling occurs during exercise but it also can occur as a result of inflammation, from muscle damage, which exceeds the rate of lymphatic drainage leading to cell swelling in the muscle, in addition to this the damage to capillaries during resistance training may lead to even more edema. Upper arm circumference can increase by up to 9% and remain elevated for up to 9 days in untrained lifters who perform a short training regime of eccentric elbow flexion exercises, cell swelling is likely not as large in trained lifters but it can still persist for at least 48 hours following exercise. Cell swelling has been shown to reduce muscle breakdown and increase protein synthesis, so it is possible that muscle damage has an anabolic effect through cell swelling.

Practical implications of the three mechanisms of hypertrophy

Mechanical tension likely plays the most significant role in stimulating muscle growth. Peak tension is likely the determining factor for the size of the activation of certain growth pathways, as appears to be the case for MAPK, and time under tension is likely the determining factor for the size of the activation of certain other growth pathways, as appears to be the case for JNK and p70S6K. Training in a variety of rep ranges will ensure that peak tension and total time under tension is prioritised which will lead to the greatest increase in the activation of all the different growth pathways, perhaps leading to a greater gain in muscle mass over time, although there is limited evidence to back this up. JNK, ERK1/2 and phosphatidic acid all appear to increase the greatest during eccentric muscular actions, over concentric or isometric muscular actions, so it is essential that any lifter who is seeking the largest gain in muscle size possible controls the eccentric part of the lift for roughly 1-4 seconds. Active tension occurs when the muscle is moving, passive tension is when a muscle is held isometrically at a stretched length, this has been shown to activate mechanoreceptors in the muscle and stimulate muscle growth, it, therefore, may have an additional hypertrophic effect if before beginning the concentric portion of a lift, following the eccentric portion, you pause the lift for a very brief period when the muscle is in the targeted stretched position, although as you near failure as you progress through the set pausing may mean you lose momentum and can’t squeeze out another rep or two which would likely be more beneficial towards muscle growth.
 

Metabolite accumulation likely plays the second most significant role in stimulating muscle growth. Metabolite accumulation occurs to the greatest extent typically in the high rep range, which I consider to be between 12-30 reps, this is because this is where the glycolytic system is utilised which produces many byproducts. The byproducts associated with hypertrophy in muscles include lactate, hydrogen ions and ROS but there are likely more. Metabolite accumulation is believed to carry out its effects through the activation of calcium-dependant pathways and HIF-1α, it has also been shown to increase growth hormone and downregulate histone deacetylase, FOXO3A, atrogin-1 and MuRF-1 which have all been shown to be catabolic. Cell swelling has also been associated with metabolite accumulation and exercise in the high rep range, the increase in cellular hydration levels is believed to have an anabolic effect through the activation of anabolic-kinase transduction pathways, such as PI3K, mTOR and MAPK, in addition to this cell swelling has been shown to stimulate satellite cell proliferation and fusion. Metabolite accumulation has been shown to reduce the recruitment threshold of high-threshold motor units and it also may call on the help of these high-threshold motor units as the current working muscle fibres get fatigued, this means that fast twitch muscle fibres can still be stimulated to grow, which are the largest and add the greatest amount of size to the muscle, this means that you shouldn’t fear working in the high rep range because fast twitch muscle fibres can still be stimulated in this rep range, although this may not be as much as the low rep, so occasionally working in the low rep range, which I consider to be between 8-12 reps, is recommended. Unlike mechanical tension, it is commonly believed that metabolite accumulation doesn’t primarily stimulate myofibrillar hypertrophy, instead, it primarily stimulates sarcoplasmic hypertrophy which still can lead to a significant increase in muscle mass but without the associated increase in strength, these adaptions are particularly useful in certain sports which rely on anaerobic endurance, such as boxing, tennis and hockey, due to the increase in cellular components such as mitochondria and glycogen.

Muscle damage occurs through disruption of the actomyosin bonds or through ATP-dependant detachment, It is controversial as to whether muscle damage has any anabolic effect at all, but one thing for certain is that too much muscle damage through overtraining leads to muscle loss, interference with growth-related processes and a reduction in a muscle's force-producing capabilities, which makes stimulating training difficult to achieve. If muscle damage does have an anabolic effect it is likely due to its ability to stimulate satellite cells through the activation of motor neurons innervating damaged muscle fibres leading to their stimulation, and through the increase in IGF-1, COX-2, Α7β1 as well as ROS and macrophages which both occur as part of the inflammatory process stimulated by the damaged muscle tissue. The inflammation associated with muscle damage leads to DOMS, which can make it difficult to train the affected muscle for up to five days, this may make it difficult to regularly stimulate the muscle to grow, in order to achieve the desired increase in muscle mass, but thankfully due to the repeated bout effect every time a new exercise is completed the extent of the muscle damage it causes drops, as long as the exercise is being performed regularly, this means the exercise can be trained more frequently without causing excessive muscle damage.