Some people take up running and immediately fall in love with the mile repeats and the burn of an all-out 100-meter sprint. Others find their groove somewhere around mile 10 of a long, slow run and could go for hours. If you’ve ever wondered why your body seems to gravitate toward one over the other, it’s not just about training or willpower. Part of the answer is sitting in your DNA.
There’s a gene that’s been nicknamed the “speed gene” by sports scientists, and it plays a real role in whether your muscles lean toward explosive power or steady endurance. It won’t turn you into an Olympic sprinter or a marathon champion on its own, but it does help explain why certain types of training click for some people and feel like a slog for others.
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What Causes Your Natural Muscle Type
Your muscles are made up of different types of fibers, and the mix you were born with has a lot to do with what your body is naturally good at. Broadly, there are two main categories: fast-twitch fibers, which contract quickly and generate a lot of power but tire out fast, and slow-twitch fibers, which contract more slowly but can keep going for a long time without fatiguing.
The ACTN3 Gene
The gene most closely tied to this is called ACTN3. It provides instructions for making a protein found specifically in fast-twitch muscle fibers, where it helps those fibers generate quick, powerful contractions. Everyone has two copies of this gene, one from each parent, and there are two main versions, or alleles: the “R” version, which produces a fully functional protein, and the “X” version, which does not.
People with two R copies (RR) tend to produce plenty of the fast-twitch protein, which is associated with better performance in power and sprint-based activities. People with two X copies (XX) don’t produce a functional version of the protein at all, and this genotype shows up more often in elite endurance athletes, since their muscles may rely more heavily on slow-twitch fibers. People with one of each (RX) tend to fall somewhere in the middle, often with a more balanced mix of fiber types.
It’s worth being clear that this isn’t a light switch. ACTN3 is one gene among many that influence athletic performance, and factors like VO2 max, tendon stiffness, training history, and plain old motivation all play a role too. But ACTN3 is one of the more well-studied and consistent genetic markers researchers have found when comparing sprint and power athletes to endurance athletes.
How Common Is Each Muscle Type
Research across different populations has found that roughly 18 percent of people carry the XX genotype (no functional fast-twitch protein), while around 30 percent carry RR, and the remaining half or so carry the mixed RX version. The exact numbers shift somewhat depending on ancestry, since allele frequencies vary across populations, but the general pattern holds: most people land somewhere in the middle, with smaller groups skewed clearly toward one end or the other.
Interestingly, the XX genotype has essentially never been found in elite sprinters in the studies that have looked for it, while it shows up at a noticeably higher rate among elite endurance athletes than in the general population. That’s a striking pattern for a single gene, and it’s a big part of why ACTN3 gets so much attention in sports genetics research.
Does Your Muscle Type Affect Your Health
For the most part, your ACTN3 status isn’t something to worry about health-wise. It’s more of a performance trait than a medical one. That said, some research has looked at whether the XX genotype might be associated with a slightly different injury profile or a modestly reduced baseline muscle strength in older adults, since fast-twitch fibers also matter for things like fall prevention and quick reactive movements later in life. None of this is cause for alarm. It just means your training approach might reasonably shift as you age, regardless of which version of the gene you carry.
What Your Muscle Type Means for You
If you’ve ever felt like sprint workouts leave you gasping while your friend barely breaks a sweat, or the reverse, where you can run forever but have never enjoyed a track workout in your life, your genetics may be part of the reason why. If you’re curious whether your own muscle fiber tendencies lean toward power, endurance, or somewhere in between, a DNA-based fitness report can give you a clearer picture, so you can build a training plan that works with your body instead of against it.
Frequently Asked Questions
Can I Change My ACTN3 Genotype Through Training?
No, your genotype is fixed for life. However, training absolutely still matters. Even people with the XX genotype can build meaningful strength and power through the right training, and people with RR can absolutely train themselves into strong endurance athletes. Genetics sets a tendency, not a ceiling.
Does This Mean I Shouldn’t Bother With Sprint Training if I Have the XX Genotype?
Not at all. Sprint and power training offer benefits like bone density, strength, and metabolic health regardless of your natural fiber-type leanings. Your genes might mean you have to work a little harder to see the same power gains as someone with RR, but the health benefits are there either way.
Is ACTN3 the Only Gene That Affects Athletic Performance?
Far from it. Researchers have identified dozens of genes linked to traits like oxygen use, tendon elasticity, pain tolerance, and recovery speed. ACTN3 is simply one of the most consistently studied and most talked about because its effect on fast-twitch muscle function is so direct.
Can I Find Out My ACTN3 Status Without a Lab Test?
Not reliably. There’s no obvious physical sign that tells you for certain whether you’re RR, RX, or XX. Some people guess based on whether they were naturally better at sprinting or distance running as kids, but that’s an imperfect proxy at best. A DNA test is the only way to know for sure.
Do Elite Athletes Always Match Their “Ideal” Genotype?
No. Plenty of successful sprinters and endurance athletes don’t carry the “expected” genotype for their sport. Genetics nudges the odds, but training, technique, mindset, and simple love of the sport still carry enormous weight.
Whether you’re built more for the 100-meter dash or the long, quiet miles of a marathon, both types of movement have something to offer. Knowing your natural leanings just means you get to train a little smarter, and maybe finally forgive yourself for hating those sprint intervals.
