Written by Stefan Ianev (Clean Health Research & Development Specialist)
An increase in muscle cross sectional area occurs primarily as a result of hypertrophy. Hypertrophy is simply the increase in the diameter of a muscle fiber. On the other hand, hyperplasia, which is the increase in the number of muscle fibers, may also contribute to increasing muscle cross sectional area, although this is controversial.
There is limited evidence in humans to suggest that hyperplasia occurs in humans, and if it does occur, it probably accounts for less than 5% of the total increase in muscle size (1). For that reason, we are going to focus on the different types of hypertrophy here.
There are two primary types of hypertrophy including:
- Myofibrillar Hypertrophy
- Sarcoplasmic Hypertrophy
Myofibrillar hypertrophy refers to the increase in the contractile elements of a muscle fiber, while sarcoplasmic hypertrophy refers to the increase in non-contractile elements such as mitochondria and the sarcoplasmic fluid surrounding the myofibrils. The sarcoplasm is a water solution containing ATP and phosphagens, as well as the enzymes and intermediate product molecules involved in many metabolic reactions. Similar to the cytoplasm in other cells, the sarcoplasm is critical for maintaining ion and pH balance within muscle fibers (2).
Conventional bro wisdom has it that training with heavy loads and longer rest periods causes primarily myofibrillar hypertrophy, while high volume training with shorter rest periods causes primarily sarcoplasmic hypertrophy.
This claim is often accompanied by a reference to “The Science and Practice of Strength Training” by Zatsiorsky and Kraemer, which is regarded as one of the top books on the topic of strength training, by two industry giants (3).
However, the evidence-based community has largely rejected this claim for many years now because there has been limited evidence to suggest that one type of hypertrophy occurs in the absence of the other. That is to say, the myofibrils and sarcoplasm will typically increase in proportion when hypertrophy takes place. Typically, the myofibrils make up about 85% of muscle volume, while the sarcoplasm makes up about 15%, and that ratio tends to remain fairly constant when muscle size increases (2).
A few recent studies have suggested that disproportionate sarcoplasmic hypertrophy may indeed occur in response to high volume training, however, at this point in time it is still not clear if that is a transitory response from localized edema (2). In addition, if disproportionate sarcoplasmic does indeed occur, it likely does only in well trained individuals that reached a myofibrillar protein accretion threshold (2).
As with hyperplasia, it is unlikely that disproportionate sarcoplasmic hypertrophy contributes in any meaningful way to overall hypertrophy, except maybe in those lifers approaching the ceiling of their genetic potential. Most lifters probably do not need to worry too much about training specifically for sarcoplasmic hypertrophy.
What about fiber specific hypertrophy?
Some authors have suggested that high-load and low-load resistance training may cause specific fiber type adaptations. The theory behind this hypothesis is that high loads may be needed to fully stimulate the high threshold motor units associated with type IIx fibers, while with low-load training, the lower threshold motor units will be under load for a longer period, which in turn, might increase the hypertrophic response of type I muscle fibers.
While some evidence indicates that low-load resistance training, when performed to muscle failure, may induce a greater hypertrophic response in type I muscle fibers and that high-load resistance training may induce preferential growth of type II muscle fibers, the body of literature remains equivocal on this topic as the majority of studies do not support this hypothesis (4).
It is likely that in any set approaching failure, in those last few reps all the motor units are being recruited and fatigued, irrespective of the load. This is why most studies show similar hypertrophy between high and low load resistance training when sets are in close proximity to failure (4).
Therefore, as with sarcoplasmic and myofibrillar hypertrophy, you probably don’t need to worry too much about training for fiber specific hypertrophy. Variation in rep ranges is more important for preventing boredom than anything else, which can lead to central fatigue, since central nervous system drive is largely influenced by your level of motivation. Diminished central drive will lead to an impaired ability to recruit the high threshold motor units which have the greatest growth potential.
However, there is research to support that certain types of muscle contractions can lead to the addition of sarcomeres either in series or parallel. Sarcomeres are the basic contractile units of myofibrils.
For example, eccentric contractions and training a muscle in the lengthened position, have been shown to favour the addition of new sarcomeres in series, while training a muscle in the middle or shortened position has been shown to increase sarcomeres in parallel (6,7).
When sarcomeres are added in series, a muscle fiber will increase in volume mainly by increasing in length. This will cause greater hypertrophy in the distal portion of a muscle. When sarcomeres are added in parallel, a muscle fiber will increase in volume mainly by increasing in diameter. This will cause greater hypertrophy in the belly of a muscle.
For that reason, different modes of contraction, and exercises targeting different points of the strength curve should be used for each muscle group, in order maximize hypertrophy in the different regions of the muscle.
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- Kraemer WJ, Duncan ND, Volek JS. Resistance training and elite athletes: adaptations and program considerations. Journal of Orthopaedic & Sports Physical Therapy. 1998; 28(2), 110-119.
- Roberts MD, Haun CT, Vann CG, Osburn SC, Young KC. Sarcoplasmic Hypertrophy in Skeletal Muscle: A Scientific “Unicorn” or Resistance Training Adaptation?. Front Physiol. 2020;11:816. Published 2020 Jul 14. doi:10.3389/fphys.2020.00816
- Zatsiorsky VM, Kraemer WJ. (2006). Science and practice of strength training (6th ed). Champaign, IL: Human Kinetics.
- Grgic J, Schoenfeld BJ. Are the Hypertrophic Adaptations to High and Low-Load Resistance Training Muscle Fiber Type Specific?. Front Physiol. 2018;9:402. Published 2018 Apr 18. doi:10.3389/fphys.2018.00402
- Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. Strength and Hypertrophy Adaptations Between Low- vs. High-Load Resistance Training: A Systematic Review and Meta-analysis. J Strength Cond Res. 2017 Dec;31(12):3508-3523. doi: 10.1519/JSC.0000000000002200. PMID: 28834797.
- Franchi MV et al. Architectural, functional and molecular responses to concentric and eccentric loading in human skeletal muscle. Acta physiologica (Oxford, England). 2014;210. 10.1111/apha.12225.
- Valamatos MJ, Tavares F, Santos RM, Veloso AP, Mil-Homens P. Influence of full range of motion vs. equalized partial range of motion training on muscle architecture and mechanical properties. Eur J Appl Physiol. 2018;118(9):1969-1983. doi:10.1007/s00421-018-3932-x