ACTN3 and ACE: The “Speed Gene” and the “Endurance Gene,” Explained

Why do some people take to sprinting and heavy lifting almost naturally, while others thrive on long, steady endurance work? Genetics is a real part of the answer — some research estimates that over 70% of elite athletic ability has a genetic basis, with the rest coming from how genetics interacts with training, nutrition, and environment.

ACTN3: often called the “speed gene”

The ACTN3 gene carries a variation known as R577X. Carriers of the R allele produce a protein that supports optimal fast-twitch muscle fiber performance — useful for strength and speed. People carrying two copies of the X variant (XX genotype) produce little to none of this protein, and interestingly, that variant shows up more often in endurance athletes, suggesting it may carry its own advantage for stamina-based sports.

ACE: the endurance-and-strength gene

The ACE gene has an insertion/deletion (I/D) polymorphism. The II genotype tends to reduce cardiac afterload during exercise and improve how efficiently the heart and blood vessels work together — an advantage for endurance. The D allele, by contrast, is more associated with greater muscle volume and rapid-twitch fiber proportion — an advantage for strength.

What the research shows when you combine them

A systematic review and meta-analysis found associations between the ACE II genotype and endurance events, and between the ACTN3 R allele and power events. A case-control study of elite Korean athletes found that the combined ACTN3 RX+XX and ACE II genotype pairing was associated with enhanced endurance performance — suggesting these genes don't work in isolation.

What this means for your training

None of this determines your ceiling — genetics is one input among many, and training, recovery, and consistency still do most of the work. But knowing whether your biology leans toward power or endurance is exactly the kind of signal that shapes smarter programming: how much volume you can recover from, whether your plan should emphasize strength blocks or aerobic base-building, and where plateaus are more likely to show up. That's the kind of adaptive, genetics-informed programming built into our Premium and Elite coaching tiers.

Sources

• The Association of Sport Performance with ACE and ACTN3 Genetic Polymorphisms: A Systematic Review and Meta-Analysis — pmc.ncbi.nlm.nih.gov/articles/PMC3554644

• Association between Complex ACTN3 and ACE Gene Polymorphisms and Elite Endurance Sports in Koreans — pmc.ncbi.nlm.nih.gov/articles/PMC11431688

• Prevalence of Polymorphism and Post-Training Expression of ACTN3 (R/X) and ACE (I/D) Genes in CrossFit Athletes — pmc.ncbi.nlm.nih.gov/articles/PMC10001917