The Physiology of Muscle Hypertrophy: Mechanisms and Types

The Physiology of Muscle Hypertrophy: Mechanisms and Types

The complexities of the human body are beyond comprehension. We are capable of millions of different actions, all performed synergistically in order to complete the most simplest of tasks complex of tasks. These internal complexities increase ten-fold when you consider that our bodies have an amazing ability to adapt to our external environment– either  acutely, or chronically dependent on the stimuli- temperature, body position, blood pressure are just a few examples.

When muscles are presented with a stimulus, and consequently challenged they too go through an adaptive change, particularly if the stimulus continues to be regular and progressive. If so, then increases in cross sectional area of the muscle are likely to occur. However, the rate or magnitude at which they adapt is dependent on a number of factors which will be discussed throughout this series of articles.

Current research (Schoenfeld, 2010) suggests that adaptation occurs via three primary mechanisms:

  1. Muscle tension. Mechanically induced tension through the muscle during weight training is seen as an essential component of the adaptive response. Simply, providing the muscle with mechanical overload produces muscle mass. However, muscle tension is not thought to be the sole reason for hypertrophy adaptations, and to a point, extremely high loads may promote neural adaptations not necessarily structural ones, therefore higher loads don’t always equate to higher hypertrophy levels.
  2. Muscle damage. The feeling of soreness in the days after training due to localised damage and disturbance is a typical response to weight training, and has been likened (on a cellular level) to that of the acute phase inflammatory response. This process has been linked to a number of growth responses that play a role in adaptation.
  3. Metabolic stress. Weight training assists in the production of a number of metabolic by-products that have been theorised to signal a process of cell swelling as a    method of self-preservation and protection. The cell   therefore adapts by becoming stronger and increasing its integrity– again, a method of adaptation.

Types of muscle hypertrophy:

The process of muscle hypertrophy is a multidimensional process with numerous factors involved. As previously seen, muscular adaptation can be achieved through mechanical load, by damage and metabolite accumulation, or of course a combination of them. The adaptive process is therefore itself reliant on a number of variables.

Myofibrillar hypertrophy occurs when the diameter of the individual muscle fiber (made up of actin and myosin myofilaments housed with a contractile sarcomere) increases. Thus the ‘contractile machinery’ of the muscle increases in cross sectional area.  Hypertrophy of this type may occur through either an ‘in-series’ effect or an ‘in-parallel’ effect*

Sarcoplasmic hypertrophy occurs when there is a volume increase in the non-contractile part of the muscle and also an increase in fluid within its compartment. Hypertrophy of this kind is thought to be training specific and is often referred to as ‘non-functional’ in that it does not assist in strength. However, chronic hypertrophy of this kind may well augment processes that assist in the development of contractile synthesis and growth.

Lastly, it is worth noting that the process of hypertrophy does not occur during exercise– in fact quite the opposite. Whilst exercising you are breaking down your muscle as tension and damage occurs, and protein breakdown increases. However, in the period after exercise , protein synthesis increases and a process of super-compensation occurs, allowing hypertrophy to take place. It goes without saying then that it is important to get the balance right   between protein suppression through training, and synthesis through recovery. This is a theme that will be   discussed elsewhere in this resource.

The myogenesis (birth of muscle) of sarcomeres can occur two ways- either IN-SERIES or IN-PARALLEL. Both result in a net increase in muscle size. This works a little bit like the build-up of traffic.

See each lane of the motorway as a muscle fiber. IN-SERIES hypertrophy is when cars start to build up within existing lanes- each lane therefore gets longer. With regards to hypertrophy, in-series hypertrophy may be more likely to occur in those new to weight training

IN-PARALLEL hypertrophy is where the body senses a build-up in traffic and therefore opens up additional lanes. This is the primary mechanism of contractile hypertrophy in those that lift weights regularly. Essentially the body will only let the lanes build up with traffic so far before it opens up new lanes

Either way though, traffic will build up (in this case though we want it to as it’ll mean more cars in total and AN INCREASE IN SARCOMERES- MUSCLE)! If we can maximize the amount of traffic in each lane, but also ensure that we are opening new lanes we can maximize the hypertrophy response


  1. Schoenfeld, BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res 24(10): 2857–2872, 2010