Muscle fiber type (power vs endurance)

ACTN3 builds a protein in fast-twitch muscle fibers used for explosive power. A common variant switches it off, and people without it are less represented among elite sprinters. The popular idea that the same variant boosts endurance is not well supported.

What the ACTN3 R577X variant does

ACTN3 makes alpha-actinin-3, a structural protein found only in fast-twitch muscle fibres, the ones used for short explosive efforts. This variant introduces a premature stop codon, so the X version makes no protein at all.

The striking thing is that this changes nothing obvious. Roughly a fifth of people worldwide make no alpha-actinin-3 and have entirely normal muscles, because the related protein alpha-actinin-2 covers the structural role. What the loss appears to shift is a fine-grained property of fast-twitch fibres rather than muscle function as such.

How large the athletic association actually is

The association is real at the elite end and small everywhere else. Meta-analysis of elite power athletes found the working R allele associated at an odds ratio of about 1.21 overall, and a separate comparison found endurance athletes roughly 1.9 times more likely than power athletes to carry two non-working copies rather than two working ones.

Those are differences between groups of Olympians. At the level of ordinary training, the site's own evidence file records a single genome-wide association for this variant, on body composition in 33,588 people, and ClinVar classifies it as benign. Nothing here predicts how strong or fast a given person can become.

The endurance half of the story is weaker than the sprint half

The popular framing is that working copies make you a sprinter and non-working copies make you an endurance athlete. The first half has reasonable support. The second is much thinner: the evidence for a positive endurance advantage from the non-working version is inconsistent, and the honest reading is under-representation among sprinters rather than over-representation among marathoners.

This site's catalog says the same thing, and it is worth repeating on a page people arrive at hoping for a training prescription.

How common it is

In 1000 Genomes reference data, about 19% of European-ancestry individuals carry two non-working copies, along with roughly 24% of East Asian and 33% of South Asian-ancestry individuals, but only about 3% of African-ancestry individuals.

A variant carried in double dose by a fifth of Europe cannot be what separates athletes from everyone else. There are Olympic sprinters with two non-working copies.

What this variant does not tell you

Athletic performance is polygenic and overwhelmingly environmental: training history, coaching, injury, motivation, body proportions, and starting young. Commercial tests that assign children to sports on ACTN3 genotype have been criticised by sports science bodies for exactly this reason.

What each rs1815739 genotype means

rs1815739 has three possible genotypes: CC, CT and TT.

Power and sprint type (rs1815739 CC). You have two working copies of ACTN3, which builds a protein in the fast-twitch muscle fibers used for short, explosive efforts. This power version is more common among sprinters and strength athletes, though everyday fitness depends far more on training than on this gene.

Mixed power and endurance (rs1815739 CT). You have one working and one switched-off copy of ACTN3, so you sit between power and endurance. In practice this gene nudges your muscle make-up only slightly, and training matters much more.

Less explosive-power tendency (rs1815739 TT). Both your copies of ACTN3 are switched off, so your fast-twitch fibers don't make this particular protein. This version is under-represented among elite sprinters and strength athletes, so it leans slightly away from explosive power. The common claim that it gives an endurance advantage is not well supported, and training matters far more either way.

Evidence & sources

Across global populations, about 20% of people carry two copies of this variant, and about 32% carry none.

This variant appears in 1 published genetic study: PubMed 36402844.

Common questions

Should I train differently because of my ACTN3 result? No. The association shows up when comparing elite sprinters with elite endurance athletes, not when predicting how an individual responds to training. Training design should follow your goals, your response and your injury history.

Does two non-working copies mean I cannot sprint? No. Roughly a fifth of people of European ancestry carry two non-working copies, including some elite sprinters. It shifts the odds at the extreme tail of performance and says almost nothing about an individual.

Related

Endurance / aerobic capacity (PPARGC1A) · Eye color (blue, green/hazel, brown) · Freckling / sun sensitivity · Hair thickness / shovel-shaped incisors · Height · Red hair / fair skin (MC1R gene) · Skin pigmentation (light vs dark)