Every few years a headline announces that scientists have found the sprint gene. The gene they mean is ACTN3, and the research behind it is more interesting than the headline suggests.
What ACTN3 does
ACTN3 makes a protein called alpha-actinin-3, which sits inside your fast-twitch muscle fibres: the ones that fire when you sprint for a bus, jump or lift something heavy. A common variant, known as R577X, switches that protein off. Carry two copies of the X version (XX) and your fast fibres make no alpha-actinin-3 at all. Your body covers for it with a closely related protein, so people with XX are perfectly healthy and most never know.
The study that made it famous
In 2003, a team in Australia compared the DNA of 436 people from the general population with 107 elite sprint and power athletes and 194 elite endurance athletes. In the general population, 18% were XX. Among the sprinters, only 6% were, and not one of the 35 female sprinters carried XX. The endurance athletes went the other way, with 24% carrying XX.
Source: Yang et al., American Journal of Human Genetics, 2003.
So is it a sprint gene?
It works more like a filter at the very top of sport than an on-off switch. Some of the sprinters in that study were XX and still good enough to compete at elite level, so the variant doesn't rule anyone out. How common it is also depends on your ancestry: the same paper reports XX in about 18% of Europeans, about 25% of Asian populations and under 1% of an African Bantu population.
What ACTN3 really tells you is which way your muscles lean. If you're RR or RX, your fast-twitch fibres carry the protein that elite sprinters almost always have. If you're XX, your genetic profile looks more like the endurance group's.
What to do with your result
If you're RR or RX, power work is where your muscles have the most headroom, so make heavy lifts, jumps and short sprints a regular part of your week. If you're XX, keep the strength work in, because it protects your muscle as you age, but expect longer efforts to feel more natural and lean into them. Whatever your type, your training does far more than this one gene. ACTN3 tells you where to start, not where you'll finish.
Find out your type
Myoform's test reads your whole genome, ACTN3 included, and builds your supplement blend around what it finds. Get your genetic test.
The Decode is for education only and isn't medical advice.