Different types and stimuli of hypertrophy

There are 3 different types of hypertrophy, each of these can occur, intensify, induce and co-vary another, myofibrillar hypertrophy is an increase in contractile proteins and in sarcomeres in series or in parallel. Connective tissue hypertrophy is an increase in connective tissue, which takes up up to 20% of the muscle. Sarcoplasmic hypertrophy is an increase in the size or amount of organelles such as mitochondria (for energy), sarcoplasmic reticulum, titin, nebulin (stabilises actin), T-tubules or metabolic products. Up to 3% of the muscle is glycogen and 5% triglycerides and you can increase the quantity of these by training in the larger rep range (8-12 reps) in particular. Bodybuilders tend to have more connective tissue and sarcoplasmic hypertrophy because they usually have more stores of glycogen and more connective tissue products as compared to powerlifters. 

There are 3 stimuli of hypertrophy, mechanical tension is the most important, followed by metabolic stress and muscle damage. It is unsure if muscle damage causes muscle growth or if they both occur due to the same or similar things. 

70% of proteins in the muscle are myofibrillar proteins, 20% sarcoplasmic proteins and 10% mitochondrial proteins.

Mechanical tension

Mechanical tension is the force generated by the muscle fibres, components within the muscle fibre known as mechanoreceptors detect the forces produced by the muscle fibre and transduce these forces into a signalling cascade which results in muscle protein synthesis. The more tension detected the more muscle growth stimulated. The duration of the signal output is proportional to how much muscular growth it occurs, to stimulate this you can use a larger ROM (range of motion) or do more reps, sets or both, but this doesn't necessarily promote going to failure (when you no longer can perform another rep of an exercise with good form) because at failure there is no extra motor unit recruitment as compared to 2 or 3 RIR (reps in reserve) so it will likely not cause any noticeable extra muscle gain, as backed by studies. But you must go close to failure as this is where the larger, faster twitch fibres grow the most, this is because the motor neurons that innervate these muscles are stimulated the best here.

If there is stress on connective tissue it can grow, due to collagen synthesis increasing 24 hours after the stress occurs, which over time causes an increase in connective tissue.

Metabolic stress

Not as great of a stimulus for muscle growth as mechanical tension is metabolic stress, when working anaerobically byproducts particularly lactate quickly accumulate which stimulates muscle growth. This is directly proportional to how many and for how long metabolites are present in the muscle. Since the most metabolites accumulate closer to failure, it is promoted that you train close to but not to failure, for the reasons discussed earlier.

Cell swelling particularly for faster twitch muscle fibres triggers muscle growth, not to say you need a ‘crazy pump’ to promote muscle growth, but it will probably help. The swelling occurs as the contractions cause an increase in blood flow to the muscle, muscular contractions not allowing blood out of the muscle and metabolite accumulation which also triggers swelling as this causes water to be pulled into the cell, this is because when we need more ATP than we have oxygen available we get energy from the phosphoketolase pathway (not important to know for the purpose of hypertrophy), which produces lactate as a byproduct, this has a strong osmotic gradient and it pulls water into the muscle cell causing it to swell. The swelling puts pressure on the cell membrane which stimulates amino acid transport, anabolism (protein synthesis), growth hormone and testosterone. This is why high reps (8-12 reps) tend to be better for hypertrophy.

Hypoxia is the state of oxygen not being sufficiently available to tissue, in this case, we‘re talking about muscle, this state can be induced by partaking in blood flow restriction training, which makes the muscle work more anaerobically (without oxygen) for energy, increasing lactate accumulation. In a low oxygen environment, relative oxygen species accumulate, such as nitric oxide which tells blood vessels to dilate, so when you are out of the hypoxic state you get hyperemia (more blood flow to that area) carrying with them all the metabolites necessary for growth, therefore increasing hypertrophy.

Muscle damage

As the muscle lengthens damage occurs to the myofibrillar units, this creates an inflammatory response in order to promote repair of the muscle, it also stimulates satellite cells to fuse with the muscle cell and donate their nuclei so more amino acids and therefore proteins can be produced. The nuclei can also transcribe more growth pathways for muscle growth such as MTOR (an enzyme that can be signalled to increase protein synthesis), MAPKs (an enzyme that regulates cell proliferation, differentiation, motility and survival) and calcium signalling pathway (helps cell growth and proliferation). When damage is done to the muscle in the eccentric portion of an exercise (when the muscle is contracting and lengthening), by controlling the part of the exercise when the muscle is getting stretched, there will be an increase in the number of sarcomeres in series. Whilst, when damage is done to the muscle in the concentric portion of an exercise (when the muscle is contracting and shortening) sarcomeres will increase in parallel.

Conclusion

Using the information given, we now know that to optimally stimulate hypertrophy it is best to exercise within a high rep range (8-12reps) with about 2 RIR (reps in reserve) by the end of the set if you are looking to increase the rate of hypertrophy, it appears it will be best to add sets, reps or ROM (range of motion) as opposed to intensity each set, providing you were already training at a high enough intensity (of about 2 RIR), as to stimulate mechanoreceptors the best and therefore muscle protein synthesis. Each rep should be slow and controlled with special attention to the eccentric portion where it is challenging to keep the reps slow due to the ease of just dropping the weights, this will cause the most muscle growth. However, don’t make the reps too slow as you want to try and activate the fast-twitch muscle fibres which are the largest and most prone to growth, do this by keeping each rep at about an approximate 3-second eccentric portion and a 1.5-second concentric portion, this may get difficult to perform as the exercise continues and fatigue sets in, training in this way will cause large limitations for an increase in muscular strength, especially for more advanced lifters and for very advanced lifters they may even experience a decrease in strength as this training style fails to cause major adaptions to the neuromuscular system or maintain adaptions already created through specialised strength training, as the neuromuscular system is very important in the case of strength gains.

Hopefully, this gave you a decent understanding of how hypertrophy occurs, however, by only using information from this you will not have all the necessary knowledge to train optimally, as this wasn’t the goal of the previous information given, as the goal was purely to give you information on what is going on inside the muscle to cause hypertrophy.