The Sprinter Gene: What ACTN3 Actually Tells You About Fitness

ACTN3 rs1815739 determines whether your fast-twitch muscle fibers make a protein called alpha-actinin-3. People with at least one working copy tend to show up more often among power and sprint athletes, while people with two non-working copies show up more often among endurance athletes. That's the whole finding. It is real, it is replicated, and it is nowhere near enough to tell you what sport you should play.

What ACTN3 actually does

ACTN3 codes for alpha-actinin-3, a structural protein found in fast-twitch (type II) muscle fibers, the fibers responsible for quick, forceful contractions like a sprint start or a heavy deadlift. The rs1815739 variant, often called R577X, is a stop-codon mutation. One version of the gene lets you produce a working protein. The other version doesn't, so people with two copies of that variant produce none of it at all in their fast-twitch fibers.

This is the marker Biome's DNA panel reports under "power vs endurance," and it's rated a moderate-evidence trait, meaning the research is real and repeated but not strong enough to function as a prediction tool on its own.

The evidence: genotype frequencies differ, but not by a landslide

A 2024 systematic review and meta-analysis published in Sports Medicine Open pooled existing studies comparing ACTN3 R577X genotypes across power athletes, endurance athletes, and non-athletes, and found the genotype distribution does differ between power and endurance groups, which is the core evidence behind the "sprinter gene" label (NIH/PMC, 2024).

A separate 2025 narrative review in Frontiers in Physiology states plainly that the "ACTN3 R577X are significantly more frequent in power athletes than in endurance athletes" (NIH/PMC, 2025). That's a real, quotable finding, and it's also the most that can honestly be said. It's a frequency shift across groups of athletes, not a rule that predicts any one person.

A 2021 study in Genes looked at the practical side: whether ACE and ACTN3 genotypes tracked with how much people improved on explosive-strength exercises after training. The researchers found genotype was "associated with the improvement of explosive strength" during a structured training program, tying the gene to training response rather than just to who ends up in elite competition (NIH/PMC, 2021). That's a more useful angle for an ordinary gym-goer than "which genotype do elite sprinters have," because it speaks to how you might respond to training, which matters more day-to-day than who you're competing against.

A 2023 study of professional soccer players examined ACTN3 rs1815739 alongside a second gene, PPARA-alpha rs4253778, and reported that the genetic variations studied were "more closely associated with improvements in endurance (YOYO INT 2) and maximal" performance measures in that squad (NIH/PMC, 2023). Worth noting: this was one team's worth of professional players, and soccer itself blends power and endurance demands, so it's a narrower and messier picture than the sprinter-versus-marathoner framing consumer genetics companies like to use.

Why one gene can't predict your sport

Here's the part that gets left out of most "get your fitness genes tested" marketing. Every study above is comparing group frequencies, not predicting individuals. Plenty of endurance athletes carry the "power" version of ACTN3. Plenty of power athletes carry the "endurance" version. Usain Bolt's genotype at this one locus tells you nothing you couldn't learn faster by watching him run.

Athletic performance is polygenic and heavily environmental: training history, muscle fiber type distribution across dozens of other genes, cardiovascular capacity, tendon stiffness, coaching, nutrition, and motivation all matter more than any single SNP. The 2025 Frontiers review frames ACTN3 as one of several "selected genetic determinants" worth studying, not the determinant, and treats it alongside other genes affecting endurance and strength rather than as a standalone answer (NIH/PMC, 2025).

What a power-vs-endurance result is actually useful for

Think of an ACTN3 result the way you'd think of knowing your resting heart rate is a bit low: a genuine data point, not a verdict. If your result shows the genotype associated with reduced alpha-actinin-3 production, that doesn't mean you can't sprint or lift heavy. It means the population-level research leans toward endurance-oriented training responding well for people with your genotype, on average, across large groups. It is a nudge for curiosity, maybe a reason to notice how your body responds to interval training versus long steady efforts. It is not a diagnosis, a talent scan, or a substitute for actually training and seeing what happens.

How to read any "fitness gene" claim

A few questions are worth asking before trusting any single-marker fitness claim, whether it comes from a DNA test, a magazine article, or a coach:

1. Is this a group-frequency finding or an individual-prediction claim? Almost everything in this field is the former.
2. How many genes does the underlying research say are actually involved? If a review lists several genetic determinants (as the 2025 Frontiers review does), a one-gene test is only covering part of the picture.
3. Was the study done in elite athletes, general population samples, or one specific team, like the 2023 soccer cohort? Findings from a narrow group don't automatically generalize.
4. Does the source use hedged language like "associated with" rather than "causes" or "determines"? The honest research uses the hedged version.

FAQ

Does ACTN3 determine if you're a sprinter or a distance runner? No. It's associated with a genotype frequency difference between power and endurance athlete groups, meaning the "power" version shows up more often among sprinters and the other version shows up more often among endurance athletes, but individual outcomes vary widely and other genes and training matter as much or more (NIH/PMC, 2024).

What is ACTN3 rs1815739 (R577X)? It's a variant in the ACTN3 gene that determines whether your fast-twitch muscle fibers produce the protein alpha-actinin-3. One version produces a working protein; the other produces none in those fibers.

Can a DNA test tell me which sport I'm genetically suited for? Not reliably from ACTN3 alone. The research behind it looks at group averages across many athletes, not individual predictions, and performance depends on training, other genes, and physiology the test doesn't capture.

Does training response to ACTN3 genotype actually differ? There's some evidence for it. A 2021 study in Genes found ACTN3 and ACE genotypes were associated with differences in explosive-strength improvement after a training program, which is a more direct, practical link than the athlete-group comparisons (NIH/PMC, 2021).

Is ACTN3 the only gene linked to power versus endurance performance? No. Reviews on the topic, including a 2025 narrative review in Frontiers in Physiology, discuss multiple genetic determinants of endurance and strength performance, of which ACTN3 is one of the more studied (NIH/PMC, 2025).