Muscle Composition & Genetics

How it works

Our muscles are made of two main types of fibers: slow-twitch and fast-twitch. We move our bodies by contracting and releasing our muscles, and fiber types play a role how a muscle contracts. While slow-twitch fibers contract slowly, they tire less easily. Fast twitch fibers contract quickly, offering short bursts of powerful energy but tire quickly in comparison to slow-twitch muscles.

Training for a marathon? Your slow-twitch fibers help you go the distance. Always bringing your ‘A game’ to sports like sprinting, throwing, and jumping? Your fast-twitch fibers help you bring maximum power right out of the gate.

The genetic link

While training and physical fitness influence athletic performance, scientific evidence suggests that genetics also influence prowess. Studies have found that most elite power athletes have a specific genetic variant in a gene related to muscle composition called the ACTN3 gene. This variant causes muscle cells to produce alpha-actinin-3, a protein found in fast-twitch muscle fibers.

Did you know?

Almost all elite power athletes who have participated in genetic studies produce the alpha-actinin-3 protein. However, you don’t need this protein to be physically fit. This genetic variant only explains about 2-3% of the difference in muscle performance between different people, and for most athletes that margin is too small to even notice. So, if you don’t have this genetic variant, don’t sweat it.

Explore more

Itching and twitching to learn about your muscle fibers? 23andMe’s Health + Ancestry Service can tell you more. Pick up a kit to see what your genes have to say about your muscle composition.

References

Alfred T et al. (2011). “ACTN3 genotype, athletic status, and life course physical capability: meta-analysis of the published literature and findings from nine studies.” Hum Mutat. 32(9):1008-18.

Berman Y and North KN. (2010). “A gene for speed: the emerging role of alpha-actinin-3 in muscle metabolism.” Physiology (Bethesda). 25(4):250-9.

Druzhevskaya AM et al. (2008). “Association of the ACTN3 R577X polymorphism with power athlete status in Russians.” Eur J Appl Physiol. 103(6):631-4.

Eynon N et al. (2013). “Genes for elite power and sprint performance: ACTN3 leads the way.” Sports Med. 43(9):803-17.

Guth LM and Roth SM. (2013). “Genetic influence on athletic performance.” Curr Opin Pediatr. 25(6):653-8.

Lee FX et al. (2016). “How does α-actinin-3 deficiency alter muscle function? Mechanistic insights into ACTN3, the ‘gene for speed’.” Biochim Biophys Acta. 1863(4):686-93.

Ma F et al. (2013). “The association of sport performance with ACE and ACTN3 genetic polymorphisms: a systematic review and meta-analysis.” PLoS One. 8(1):e54685.

MacArthur DG et al. (2007). “Loss of ACTN3 gene function alters mouse muscle metabolism and shows evidence of positive selection in humans.” Nat Genet. 39(10):1261-5.

MacArthur DG et al. (2008). “An Actn3 knockout mouse provides mechanistic insights into the association between alpha-actinin-3 deficiency and human athletic performance.” Hum Mol Genet. 17(8):1076-86.

Moran CN et al. (2007). “Association analysis of the ACTN3 R577X polymorphism and complex quantitative body composition and performance phenotypes in adolescent Greeks.” Eur J Hum Genet. 15(1):88-93.