In this post we analyze the big 3 biological traits that determine how fast you can run and why “bad genetics” might be the wrong diagnosis. The hard truth is that your parents set your speed potential. But are you actually slow, or have you just not unlocked the hardware you were born with? In this deep dive, we break down the genetics of sprinting to answer the ultimate question: “Were you born fast, or can you build speed?” We investigate the “big 3” factors that athletes obsess over:

1. Muscle fiber type. Why the ratio of fast-twitch vs slow-twitch matters less than the size of the fibers (featuring the Werner Günthör case study).
2. Biomechanics. How leg length and structure impact your ceiling.
3. The “speed gene” (ACTN3). The truth about the R allele, the X variant, and what 1% of performance actually looks like.

If you are tired of wondering if you have the “right body” to be elite, this article breaks down the science of your physiological limits, and your hidden potential.

Do you have the right body to be fast? The hard truth is your parents set your speed potential. We know that’s not some hot take. Everyone points it out all the time. But what are the specific physical traits that are supposed to make you fast? Scientists have measured it all: leg length, muscle fibers, bone structure,  and even the so-called speed gene. So, let’s find out if you have what it takes to be fast.

Do you have enough fast-twitch muscle fibers?

Some people are born to run far and some people are born to run fast.

• If your race is measured in minutes or hours, you need muscles that refuse to die: thin, relentless fibers built for a slow burn.
• But if you want to be fast, you need something radically different. You need muscles that burn like dynamite.

Every skeletal muscle in your body is actually a mix of two different primary fiber types: slow twitch and fast twitch.

• Slow twitch or type I are built  for endurance. Great for distance events, but not for raw speed.
• Your fast twitch fibers or type II are what make you fast. They drive explosive acceleration and top speed, but they fade quickly. These are the fibers that win the 100, the 200, and the 400.

Here’s what supposedly determines your speed potential. Everyone is born with a different ratio of these two fibers. And the thinking goes, if you were born with, say, 70% fast twitch, you’re a sprinter. You won the genetic lottery for speed. But if you landed on the other end with 70% slow twitch, you’re an endurance athlete. Or are you?

It turns out that your percentage of fast twitch muscle fibers has almost no connection with your sprinting ability. In 2025, a landmark study decided to figure out what really makes an elite sprinter fast. And what they found, it went against everything we’ve been told. There was no significant connection between a sprinter’s performance and their genetic percentage of fast twitch fibers. What they discovered is that the key to speed isn’t the number of fast twitch fibers you have. It’s about how much physical space they take up inside the muscle. It’s about their size. And that’s the crucial difference. Because while you can’t change the number of fibers you were born with, you can absolutely change their size. Proof of this comes from one of the most powerful athletes in history: Werner Günthör (3-time world champion).

The shot put, like sprinting, is a sport of pure explosive power. When scientists put Günthör’s body under the microscope, they found something that seemed to defy logic. Genetically, he had the muscle profile of an elite endurance athlete dominated by 60% slow twitch fibers, which means that based on muscle fiber type, he should have been a distance runner. So, how did he become one of the most powerful athletes in history? Through years of specific explosive training, his smaller number of fast twitch fibers grew to be almost three times larger than his slow twitch ones. The result, those few explosive fibers ended up dominating almost 70% of his total muscle area. He wasn’t born with the right muscle fibers for his sport. He built the right body.

Your percentage of fast twitch muscle fibers has almost no connection with your sprinting ability.

CSA (Cross sectional area) = How much space fast twitch fibers take up

Do you have the right leg length?

So, you can change your muscles, but what about your bones? It seems obvious that being born with the right leg length means you’ve won a part of the genetic lottery for speed. It´s simple. Longer legs are unfair advantage because they cover more ground with each stride. But there´s just one problem: long legs don´t make you faster.

Scientists studied elite sprinters and found absolutely zero connection between leg length or leg to height ratio and speed. Now, maybe what you´re thinking: “Didn´t Usain Bolt prove longer legs are the advantage?” Well, here´s the thing: longer legs can cover more ground per stride. But you pay a triple penalty for that extra length:

1. MOMENT OF INERTIA. The first and most obvious is at the start. Accelerating and controlling longer legs is harder. Think of it like swinging a sledgehammer versus a regular hammer.
2. STIFFNESS DEMAND. Longer legs mean there is more distance between ground impact and your joints. So, you need more force to stay stiff or they compress and you slow down.
3. GROUND CONTACT. At top speed, all sprinters get around 90 milliseconds on the ground. And longer ankles create a mechanical disadvantage. Your muscles have to work harder to generate the same ground force in that split-second window.

What made Usain Bolt legendary wasn´t his leg length. It was doing what biomechanics said was impossible. He trained relentlessly to turn his disadvantage into the most unbeatable weapon in sprinting. So yes, you have the right leg length. So does everyone else. Usain Bolt proved what matters is training for the body you have.

Do you have the right DNA?

Now we get to the big one, the source code. You can’t change or train your way out of your DNA. Because deep inside your genetic code, scientists have found what they call the speed gene. Its real name is ACTN3.

So, how does ACTN3 make you fast? It’s  simple. Its job is to provide the instructions to  build a structural protein called alpha-actinin-3 that sits at the Z disk to cross-link actin filaments, increasing the stiffness of the myiofibrils. What it does is all about reinforcement for your fast twitch muscle fibers. It never had much use for those slow twitch type. Think of it like this. The protein it codes for acts like adding steel beams to your muscle’s internal scaffolding. So when you fire those muscles with everything you’ve got, that reinforcement cuts the wobble. So more of your effort turns into speed instead of useless vibration.

Which version of ACTN3 you have depends on the copies you inherited.

• The R version of its instructions tell your cells to build those steel beams. Full reinforcement.
• The X version has what we call a stop code on, basically a big old stop sign that tells your cells don’t build alpha-actinin-3. So they don’t.

You get one copy from each parent. So you can be RR (double reinforcement), RX (single reinforcement) or XX (which is no reinforcement). And that’s the code.

So, it seems like if you want to be a world-class sprinter, you need to be an RR or at least an RX. Except that’s not the whole story. XX athletes can still be world class because the body has a backup plan. Fast twitch fibers compensate by up regulating a different protein: alpha-actinin-2. The muscle becomes more efficient and more fatigue resistant. Here’s the proof:

• Scientists studied the genetics of elite sprinters from Jamaica, an epicenter of sprinting dominance. Even in this hyper elite group, they found that 2 to 3% of the sprinters were XX. It’s a small number, but it’s not zero.
• In another study, scientists documented an elite long jumper with a near Olympic gold distance of 8.26 m. His genotype was XX.

So, let’s be clear. Does having the perfect RR gene give you a statistical edge? Yes, the data is undeniable. A 2024 meta analysis found that power athletes, including sprinters, have about 50% higher odds of possessing the RR genotype than non-athletes. It’s a real statistical advantage. But here’s the catch. After reviewing the performance of 346 elite sprinters, it turns out that this gene explains less than 1% of sprint performance.

So, how many people have this so-called unfair advantage with at least one copy of the R allele? About 8 out of every 10 people reading this article. What about the ultimate two copy RR advantage? Roughly 2.5 billion people. Clearly, that doesn’t guarantee you’ll be fast.

• % of people with at least 1 copy of R allele ➔ ~ 8 out of 10 people
• % of people with 2 copies of R allele ➔ ~ 3 out of 10 people

Your body isn’t a simple machine controlled by one switch. The NIH genetics database identifies more than 150 genetic variations linked to athletic performance. And that’s just what we know right now. The idea of a single speed gene is a massive oversimplification of what it truly takes to be fast. So where does that leave us?

+150 genetic links to performance. (Source: NHGRI-EBI GWAS CATALOG)

To be an Olympic level sprinter, you have to win the genetic lottery. That’s a fact. As we said at the start, your parents set your speed potential. The truth is, for most people, that potential is way higher than they realize. And almost nobody ever really finds out what that ceiling is. Every single day, an athlete who maximizes their potential beats someone with better genes who doesn’t. And here’s the thing, there’s a whole universe of success that isn’t competing at the Olympics. It’s winning a state championship. It’s making the team. It’s feeling what it’s like to be faster than you’ve ever been.

Bibliographic references:

• Outperform. (2025). Do you have the right body to be fast? [Video file]. YouTube. https://youtu.be/hZXrWyi8xwE?si=XkjBXekIqvN_DNbJ
• Methenitis, S., Stasinaki, A.-N., Mpampoulis, T., Papadopoulos, C., Papadimas, G., Zaras, N., & Terzis, G. (2025). Sprinters’ and Marathon Runners’ Performances Are Better Explained by Muscle Fibers’ Percentage Cross-Sectional Area than Any Other Parameter of Muscle Fiber Composition. Sports, 13(3), 74. https://doi.org/10.3390/sports13030074
• Tomita, D., Suga, T., Terada, M., Tanaka, T., Miyake, Y., Ueno, H., Otsuka, M., Nagano, A., & Isaka, T. (2020). A pilot study on a potential relationship between leg bone length and sprint performance in sprinters; are there any event-related differences in 100-m and 400-m sprints?. BMC research notes, 13(1), 297. https://doi.org/10.1186/s13104-020-05140-z
• Scott, R. A., Irving, R., Irwin, L., Morrison, E., Charlton, V., Austin, K., Tladi, D., Deason, M., Headley, S. A., Kolkhorst, F. W., Yang, N., North, K., & Pitsiladis, Y. P. (2010). ACTN3 and ACE genotypes in elite Jamaican and US sprinters. Medicine and science in sports and exercise, 42(1), 107–112. https://doi.org/10.1249/MSS.0b013e3181ae2bc0
• El Ouali, E. M., Barthelemy, B., Del Coso, J., Hackney, A. C., Laher, I., Govindasamy, K., Mesfioui, A., Granacher, U., & Zouhal, H. (2024). A Systematic Review and Meta-analysis of the Association Between ACTN3 R577X Genotypes and Performance in Endurance Versus Power Athletes and Non-athletes. Sports medicine – open, 10(1), 37. https://doi.org/10.1186/s40798-024-00711-x