Hypertrophy is an increase in cross-sectional area of muscle fibres​. This is not to be confused with hyperplasia, an increase in the number of muscle fibres.

What forms of muscular hypertrophy are there?

There are two primary forms of hypertrophy:

Sarcoplasmic hypertrophy​ (also known as 'non-functional’ hypertrophy) is an increase in the non-contractile elements and fluid​​ within the muscle cell. This isn’t associated with increases in strength, and is often associated with bodybuilding training modalities. This typically occurs in response to higher rep ranges and lower mechanical tension.

Myofibrillar hypertrophy (also known as ‘functional' hypertrophy)​ is an increase in the number of myofibrils within the muscle cell and is associated with increases in force production capabilities of the muscle. This can happen in series, resulting in a shorter sarcomere length, and in parallel whereby there is an increase in fibre diameter. This typically occurs in response to heavier training loads and higher mechanical tension.

Why does hypertrophy occur?

In essence, hypertrophy occurs as an adaptive response to stress placed on the muscle tissue. The process by which this occurs is called mechanotransduction, where mechanical loading leads to a chemical process cascade and ultimately adaptation within the cell (Zou et al., 2011)​​. In order for hypertrophy to occur, however, this stress must be unaccustomed (following the overload principle).

What are the important training principles to consider with hypertrophy?

There are a number of key principles which don’t change in training, which we need to be mindful of when designing a training programme. These are as follows:

Overload - any training stress must be unaccustomed in order for hypertrophy to occur. If we don’t provide the body with sufficient overload, then we won’t hit the threshold stimulus for adaptation.

Specificity - adaptations in the body are specific to the training stress experienced and in proportion to the magnitude of the stress. This comes down to understanding what the intended outcome of the training programme is, and aligning the training stimulus with this outcome. If our training goal is myofibrillar hypertrophy but we are completing 1-3 reps of explosive concentric strength work at light loads, we are not as likely to initiate sufficient eccentric training stress to initiate this, for example.

Reversibility - any adaptations we stimulate will be lost if we stop training or cease to provide the body with a sufficient stimulus to drive adaptation. We must continue to provide overload if we are to continue causing positive adaptive responses in the body.

What are the key factors that drive hypertrophy adaptations?

There are three primary factors that drive this adaptive response in the body. These are mechanical tension, metabolic stress and muscle damage.

Mechanical tension - mechanically induced tension initiated during strength training (and therefore force production), along with stretch is vital for muscle growth (Schoenfeld, 2010). Eccentric training can be an effective way of achieving this, as we can provide high mechanical overload.

Metabolic stress - this occurs when there is a build-up of metabolites in response to muscular contraction. It is associated with cell swelling and elevations to hormones linked to metabolic stress. Shorter rest periods can be a way to initiate higher levels of metabolic stress, for example as longer rest periods can dissipate this stress.

Muscle damage - inflammation and sarcomere disruption are characteristics of muscle damage and are associated with the commonly used term ‘DOMS’ (delayed onset muscle soreness). In order for this to occur, the loading must be eccentrically focussed, high volume, and meets the overload principle. It has been suggested that hypertrophy can occur without the need for muscle damage, which is positive if DOMS may be disruptive for our overall training week.

Training variables to consider

There are a number of considerations with regards to a training programme to consider if we are to initiate hypertrophy and the specific form of hypertrophy that we want. These include volume, frequency, tempo, load, and rest periods. These are the means by which we meet the principles of overload and specificity necessary to initiate hypertrophic adaptive responses.

Volume - the greater the volume (up to a stimulatory threshold), the greater the likelihood of inducing hypertrophy. This is also a way that we can ensure overload of the tissue, but must be aligned with the training age of the athlete and their adaptive capabilities. Typically 3-4 training sessions per week is appropriate.

Frequency - the more frequently that we train, the more regularly we can provide a stimulus to the tissue to initiate the chemical cascade that leads to hypertrophy. However, we need to ensure that this isn’t so frequent that we inhibit the ability of the tissue to adapt and recover between training sessions. Typically 48-72 hours between sessions of the same muscle group is appropriate.

Tempo - this variable can be used to place a greater eccentric training stress on the tissue, which can be a key factor in stimulating hypertrophy responses. It is also a way to initiate greater levels of muscle damage and metabolic stress, owing to the increased time under tension that the tissue experiences. Increasing the eccentric portion of the lift (at a heavy enough load to stimulate adaptation) can be a useful way to achieve hypertrophy and overload.

Load - heavier loads (5-10RM) are generally superior to lighter loads for inducing hypertrophy unless taken to failure (Burd et al., 2010). Lighter loads may not be sufficient for fatiguing high threshold motor units, however, owing to the size principle. This is a key consideration for sports performance versus that of bodybuilding goals.

Rest - this can be manipulated to initiate higher or lower metabolic stress in the tissue. For example, if we took an intra-set rest period between reps we would reduce the metabolic stress placed on the tissue, and conversely, reductions in rest would increase the total metabolic stress experienced by the tissue.

Summary

In summary, there are some key principles that are unchanging when training for hypertrophy. It is important that we know what it is we are hoping to achieve, what adaptation this looks like, and how our programme will drive these adaptive responses. The overload and specificity principles underpin all training programmes, as they direct whether, and to what extent, the body will adapt. The principles of mechanical tension, metabolic stress and muscle damage will then determine whether we achieve our goal of hypertrophy.

If you have enjoyed this article, then subscribe to the mailing list for regular content around training and performance!

References

Burd NA, West DW, Staples AW, Atherton PJ, Baker JM, Moore DR, Holwerda AM, Parise G, Rennie MJ, Baker SK, Phillips SM. Low-load high volume resistance exercise stimulates muscle protein synthesis more than high-load low volume resistance exercise in young men. PLoS One. 2010 Aug 9;5(8):e12033.

Schoenfeld BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res. 2010 Oct;24(10):2857-72.

Zou, Kai & Meador, Bejamin & Johnson, Brian & Huntsman, Heather & Valero, Maria Carmen & Boppart, Marni. (2011). The α7B1 Integrin Promotes Skeletal Muscle Hypertrophy Following Eccentric Exercise. Medicine & Science in Sports & Exercise. 43. 412.