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  • Writer's pictureJuan-Pierre Pieri

Physiology of Hypertrophy

Hypertrophy can appear complex for some individuals while straightforward for others. In recent years, the widely accepted belief that muscle must be broken down in order to be built up has been thoroughly disproven.

So, if muscle breakdown is not the cause of hypertrophy, what is? Is it metabolites, satellite cell proliferation, or the stimulation of anabolic hormones? Well, studies have refuted these factors when examined in isolation. However, when these phenomena are observed in conjunction with two specific prerequisites, researchers have observed muscle growth and gained a better understanding of the hypertrophy process.

Fast twitch vs slow twitch

These are the only two mechanisms for growth that MUST occur together;

  1. Recruitment of high-threshold motor units (HTMUs). HTMUs are responsible for innervating and controlling fast-twitch muscle fibers, which play a crucial role in generating significant force for moving heavy objects and have the highest growth potential. There are thousands of fast twitch fibers and a handful of slow twitch fibers.

  2. Muscle fibers need to experience mechanical tension (MT). MT is the force produced by the interaction between actin and myosin, which counteracts an external load. This mechanical signal is detected and converted into a biological signal for growth, known as mechanotransduction.

HTMUs can be recruited either by load or fatigue, in accordance with Henneman's Size Principle.

Load accelerates this process (in terms of reps) as low-threshold motor units (LTMUs) are unable to overcome the weight, leading to the recruitment of intermediate motor units and eventually HTMUs. This occurs within milliseconds.

Fatigue also triggers the recruitment of HTMUs (e.g., high repetitions) as LTMUs initially activate the muscles but eventually become fatigued. Intermediate motor units and then HTMUs are subsequently recruited to sustain the contractions. Although less efficient, this method is beneficial for individuals with tendon and ligament issues as it minimizes the force exerted on them.

Once HTMUs are recruited, the fast-twitch fibers responsible for the contraction must experience MT. This occurs when there is an INVOLUNTARY reduction in rep speed. As muscle contraction velocity decreases, the forces experienced by the fibers increase. This relationship is known as the force-velocity relationship. However, intentional slowing of the rep (tempo training) due to suboptimal loads results in little voluntary effort and prevents HTMU recruitment which consequently means that the thousands of fast twitch fibers cannot experience MT.

Tempos can generally still be used for eccentric contractions in order to standardize each repition however long, drawn out tempos beyond 3 seconds don't capitalise on more passive tension and only serve to increase fatigue via stretch activated ion channels which would impair the active tension element of the lift which is the concentric contraction.

Training to or close to failure is the only way to simultaneously recruit HTMUs and generate high forces, thereby promoting the necessary mechanotransduction for muscle growth.

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