Seventy to eighty percent of how your body responds to a training stimulus is determined by your genetics. This means that your muscle fibre predominance can be accentuated and even optimised through a proper training stimulus. In short, this means if your training reflects your muscle fibre genotype you’re more likely to get the maximum benefits out of exercise. The following provides a summary of what science tell us about genetics and strength training.
There are two distinct muscle fibre types with an additional sub-type which are summarised below:
- Slow twitch type 1 fibres: ST 1 produces low tension, fatigue resistance, small α motor neurone
- Fast twitch type 2A fibres: FTa/ FOG produces high tension, fatigue resistance to a point, intermediate size α motor neurone
- Fast twitch type 2B: FTb produces very high tension, fast fatiguing, large size α motor neurone
Obvious signs of genetics are seen in our body shape (See figure 1). We call this somatotyping. First
coined by Carter and Heath (2000), somatotyping is a classification system to describe the variation of
human body shape across a broad continuum.
There are 3 distinct somatotypes:
1. Ectomorphs tend to be thin with long limbs with higher proportion of slow type 1 muscle fibres.
2. Mesomorphs tend to be squarer and lean looking with higher proportion of fast type 2a and 2b muscle
3. Endomorphs tend to be rounded or apple shaped with a higher proportion of fast type 2b muscle fibre.
That being said, if we were to put these categories on a continuum, some of us may find ourselves at one
extreme or the other, perhaps slap bang in the middle, or merging somewhere in between the middle of
one of the extremes. Regardless, we all pose a mixture of slow type 1 and fast type 2a and 2b muscle
Other genetics factors account for inter-participant variability in how muscle development is expressed in
strength and size which are summarised:
- Myostatin: an anti-growth genotype inhibiting muscular growth (Kim et al. 2007)
- Interleukin-15: IL-15 is a moderator of muscle mass in response to strength training (Reichmann et al 2004)
- Ciliary neurotrophic factor: CNTF G/G and G/A genotypes show greater muscular hypertrophy compared to A/A homozygotes (Roth et al 2001)
- Alpha-actinin-3: ACTN-3 R577X genotype is highly represented in strength/ power based athletes (Roth et al 2008; Norman et al 2009)
- Myosin light chain kinase: an enzyme involved in the coupling of myosin and actin. More of this protein, the more cross-bridge connection, therefore the more strength can be expressed. The downside is, your rate of fatigue is comparably greater (McGuff and Little 2009)
- Angiotensin converting enzyme: an enzyme that determines vascular tone. You either have an insertion gene “I” of deletion gene “d”. Individuals with “ii” tend to have high proportion of ST muscle fibres, those with “dd” have predominantly FT muscle fibres, and individuals with “id” fall in the middle (McGuff and Little 2009). ACE d-allele positively affects muscle development (Folland et al 2000)
What does this all mean to you?
In general terms regarding what can be expected from the long term effects of strength training are:
- Ectomorphs can expect to get stronger, but not necessarily bigger
- Mesomorphs can expect to get bigger, stronger and enhanced athleticism.
- Endomorphs, well they tend to be strong anyway, so they’re going to get even bigger and even stronger.
Determining your respective muscle fibre type
- Determine your one repetition maximum (1RM) on an exercise
- Rest for 15 minutes
- Perform as many repetitions as possible with 80% of your 1RM
- Less than 7 repetitions – fast twitch (FT) dominant
- 7 or 8 repetitions – fast oxidative gylocolytic (FOG)
- more than 8 repetitions – slow twitch (ST) dominant
Those with a FT or FOG predominance will benefit from one set to muscular failure per body part. FT
trainees have a higher propensity to inroad (intense fatigue), so multiple sets and high training frequency
is not recommended as this will delay recovery, and it’s the recovery that’s the most important part of
training in order to maximize results. Those with ST predominance may benefit from multiple sets, and
though they may not see the hypertrophy gains compared to FT and FOG trainees, they will increase their
respective strength. Figure 2 provides a guide to training volume, frequency and intensity.
Time under load
Regardless of somatotyping, it’s important that we choose a training stimulus that recruits all types of
muscle fibres. Too short (i.e.: <60 sec), then we don’t fully recruit all the slow type 1 muscle fibres. Too
long (i.e.:180> sec) then we allow the slow type 1 fibres to recover and so will be continued to be used
and therefore will offset the recruitment of type 2a and 2b muscle fibres which are important for
What this means in practical terms is finding a training stimulus that fatigues a muscle or muscle group
within 180 seconds. This will ensure the recruitment of all muscle fibres, which will promote strength
development and positively impact global health.
Carter JEL, Heath BH. Somatotyping: development and applications. Cambridge, UK: Cambridge University
Folland J, Leach B, Little T, et al. Angiotensin-converting enzyme genotype affects the response of human
skeletal muscle to functional overload. Exp Physiol 2000; 85: 575-9.
McGuff D, Little J. Body by Science. McGraw and Hill. 2009: pp 166-174.
Kim JS, Petrella JK, Cross JM, et al. Load-mediated downregulation of myostatin mRNA is not sufficient to
promote myofiber hypertrophy in humans: a cluster analysis. J Appl Physiol 2007; 103: 1488-95.
Norman B, Esbjörnsson M, Rundqvist H, et al. Strength, power, fiber-types and mRNA expression in
trained men and women with different ACTN3 R577X genotypes. J Appl Physiol 2009; 106: 959-65.
Roth SM, Walsh S, Liu D, et al. The ACTN3 R577X nonsense allele is under represented in elite-level
strength athletes. Eur J Hum Genet 2008; 16: 391-4.
Roth SM, Schrager MA, Ferrell RE, et al. CNTF genotype is associated with muscular strength and quality in
humans across the adult age span. J Appl Physiol 2001; 90: 1205-10.
Riechman SE, Balasekaran G, Roth SM, et al. Association of interleukin-15 protein and interleukin-15
receptor genetic variation with resistance exercise training responses. J Appl Physiol 2004; 97: 2214-9.
Stewart CEH, Rittweger J. Adaptive processes in skeletal muscle: Molecular regulators and genetic
influences. J Musculoskelet Neuronal Interact 2006; 6: 73-86.
Chris Allen is the owner of Vis-Therapeia, which specializes in
preventative strength training and remedial strength therapy.
Chris has a bachelor’s degree and master’s degree in applied sports
and exercise science. He has a wealth of experience working as a
fitness professional and has lectured on a number of sport programmes
in colleges in London.