Mechanisms of Hypertrophy
Individuals might be training for the improvement of sports performance measures or for the benefits of their own health but in both situations, resistance training should play an important part in achieving these aims.
Resistance training can result in increased functional strength, the stabilisation of joints and improved posture, increases in power output and increases in bone mineral density all of which, to some degree, rely on an increase in muscle mass, a process known as hypertrophy. While mechanisms other than hypertrophy also contribute to increases in strength, the muscle's ability to produce force clearly increases as it's cross-sectional area becomes greater (Ikai and Fukunaga,1968).
In order to be effective in programming activities that induce muscular hypertrophy, it’s key to first understand the mechanisms by which hypertrophy occurs. It’s been suggested that they fall broadly into 3 categories; mechanical tension, exercise induced muscular damage and metabolic stress.
This page gives a brief overview of these mechanisms for those that are keen exercisers or early career fitness professionals but also contains links to further, more detailed sources of information for those that are interested.
Once you've read this page you might like to read the accompanying page on neuromuscular factors.
1. Mechanical Tension
Mechanical tension has been suggested as the primary driver for training induced muscular hypertrophy and results from performing a movement that requires some form of muscular contraction.
Increased mechanical resistance disturbs the structure of skeletal muscle, and causes signals to be sent to the muscle fibres and cells to repair themselves and become stronger by favouring synthesis instead of breakdown. A series of events then follow this signalling process with, for example, increased levels of growth hormone being released. The mechanosensors responsible for initiating the signalling process are sensitive to both magnitude and duration of loading which has implications for the optimisation of training protocols.
Although mechanical tension alone can result in hypertrophy, some training protocols that predominantly employ large amounts of mechanical tension have been shown to induce neural adaptations rather than hypertrophic ones i.e. the participants became stronger without necessarily developing more muscle fibres. While this situation may be desirable in some cases, it places a limit on the potential for development as strength is largely related to the cross-sectional area of the muscles.
For more on the biology of hypertrophy including cellular and molecular mechanisms see Spiering, et al. (2008), Schiaffino et al. (2020).
2. Muscle Damage
Intense exercise, particularly if novel or at an unaccustomed intensity, can cause damage to skeletal muscle. Anyone that’s performed a vigorous exercise for the first time (e.g. ridden a horse, completed a hard plyometric session or a long run) will have experienced the sensation of Delayed Onset Muscle Soreness (DOMS). DOMS along with muscular stiffness, swelling and reduced force-producing capacity are all symptoms of excessive Exercise Induced Muscle Damage (EIMD) which can in turn affect the ability to train at a high level. It’s worth noting at this point though that DOMS is a complex phenomenon and while linked to EIMD and micro-trauma, it has also been linked to inflammation, enzyme efflux, connective tissue damage, muscle spasm and lactic acid accumulation.
For hypertrophy to occur however, EIMD does not have to be so severe that it’s accompanied by DOMS. Although levels of EIMD have been shown to be a key factor in optimising hypertrophy, research has shown that excessive EIMD can be detrimental to hypertrophy for a number of reasons.
EIMD is influenced by the type of muscular action that is being executed and although concentric and isometric exercise can cause EIMD, eccentric exercise will be most likely to cause EIMD.
With regular exposure to a particular activity or exercise the likelihood of muscular microtrauma (EIMD) and particularly the symptoms of more severe damage like DOMS, are decreased. This is known as the repeated bout effect and knowledge of its existence can be useful in supporting those new to exercise but also help us appreciate the need for appropriately periodised and progressive programmes for more experienced trainers.
See Schoenfeld (2012), Schoenfeld and Contreras (2013) and Damas et al (2018) for more on EIMD.
3. Metabolic Stress
While it's important to lift heavy weights (at least in relation to an individual’s capabilities) so that appropriate levels of mechanical tension and exercise-induced muscle damage are created, it's also important that the environment in which the muscle operates and recovers is optimised for muscle growth. A resistance training protocol that utilises moderate to high loads, moderate to high reps and short recovery periods will create an exercise-induced accumulation of metabolites such as lactate, inorganic phosphates and H+ and this is known as metabolic stress.
Hypertrophic adaptations from exercise-induced metabolic stress include mechanisms related to increased fibre recruitment, elevated systemic hormonal production, alterations in local myokines, heightened production of reactive oxygen species (ROS) and cellular swelling.
The process of metabolic stress is that which underpins Blood Flow Restriction (BFR) training and hypoxic resistance training, both of which seek to create heightened levels of ischemia in the working muscle as they contract. For more on BFR see Loenneke et al (2014) and for more on hypoxic training see Scott et al (2015).
If you only read one article
Schoenfeld, B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. The Journal of Strength & Conditioning Research, 24(10), 2857-2872.
References and Further Reading
Balshaw, T. G., Massey, G. J., Maden-Wilkinson, T. M., Morales-Artacho, A. J., McKeown, A., Appleby, C. L., & Folland, J. P. (2017). Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training. European journal of applied physiology, 117(4), 631-640. https://doi.org/10.1007/s00421-017-3560-x
Beardsley, C. (n.d.) Hypertrophy Mechanisms www.strengthandconditioningresearch.com/hypertrophy/hypertrophy-mechanisms/
Brentano, M. A., & Martins Kruel, L. F. (2011). A review on strength exercise-induced muscle damage: applications, adaptation mechanisms and limitations. J Sports Med Phys Fitness, 51(1), 1-10. [abstract]
Buckner, S. L., Jessee, M. B., Mouser, J. G., Dankel, S. J., Mattocks, K. T., Bell, Z. W., ... & Loenneke, J. P. (2019). The Basics of Training for Muscle Size and Strength: A Brief Review on the Theory. Medicine and Science in Sports and Exercise. https://doi.org/10.1249/MSS.0000000000002171
Bush-Joseph, C. A. (2012). Muscle Soreness and Delayed-Onset Muscle Soreness. Clin Sports Med, 31, 255-262.[abstract]
Chapman, D., Newton, M., Sacco, P., & Nosaka, K. (2006). Greater muscle damage induced by fast versus slow velocity eccentric exercise. International Journal of Sports Medicine, 27(08), 591-598. https://doi.org/10.1055/s-2005-865920
Clarkson, P. M., & Hubal, M. J. (2002). Exercise-induced muscle damage in humans. American Journal of Physical Medicine & Rehabilitation, 81(11), S52-S69.[abstract]
Damas, F., Libardi, C. A., & Ugrinowitsch, C. (2018). The development of skeletal muscle hypertrophy through resistance training: the role of muscle damage and muscle protein synthesis. European journal of applied physiology, 118(3), 485-500. https://doi.org/10.1007/s00421-017-3792-9
Dankel, S. J., Mattocks, K. T., Jessee, M. B., Buckner, S. L., Mouser, J. G., Counts, B. R., ... & Loenneke, J. P. (2017). Frequency: the overlooked resistance training variable for inducing muscle hypertrophy?. Sports Medicine, 47(5), 799-805. https://doi.org/10.1007/s40279-016-0640-8
Davies, T., Halaki, M., Orr, R., Mitchell, L., Helms, E., Clarke, J., Hackett, D. (2021) Effect of Set-Structure on Upper-Body Muscular Hypertrophy and Performance in Recreationally-Trained Male and Female, Journal of Strength and Conditioning Research doi: 0.1519/JSC.00000000000039
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Hornberger, T. A. (2011). Mechanotransduction and the regulation of mTORC1 signaling in skeletal muscle. The international journal of biochemistry & cell biology, 43(9), 1267-1276. https://doi.org/10.1016/j.biocel.2011.05.007
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Ikai, M., & Fukunaga, T. (1968). Calculation of muscle strength per unit cross-sectional area of human muscle by means of ultrasonic measurement. Internationale Zeitschrift für Angewandte Physiologie Einschliesslich Arbeitsphysiologie, 26(1), 26-32.
Kraemer, W. J., & Ratamess, N. A. (2005). Hormonal responses and adaptations to resistance exercise and training. Sports Medicine, 35(4), 339-361.
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Lewis, P. B., Ruby, D., & Bush-Joseph, C. A. (2012). Muscle soreness and delayed-onset muscle soreness. Clinics in Sports Medicine, 31(2), 255-262. [abstract]
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Loenneke, J. P., Dankel, S. J., Bell, Z. W., Buckner, S. L., Mattocks, K. T., Jessee, M. B., & Abe, T. (2019). Is muscle growth a mechanism for increasing strength?. Medical hypotheses, 125, 51-56. https://doi.org/10.1016/j.mehy.2019.02.030
Lopez, P., Radaelli, R., Taaffe, D. R., Newton, R. U., Galvão, D. A., Trajano, G. S., Teodoro, J., Kraemer, W. J., Häkkinen, K., & Pinto, R. S. (2020). Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis. Medicine and science in sports and exercise, https://doi.org/10.1249/MSS.0000000000002585
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