Do you have the right body to be fast?

Do you have the right body to be fast?  The hard truth is your parents set your   speed potential. And look, we know  that's not some hot take.

I mean,   you all point it out to us 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. 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 one are built   for endurance. Great for distance events, but  not for raw speed. Your fast twitch fibers   or type two 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.   Now, 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, well, 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.

[Music] The shot put,  like sprinting, is a sport of pure explosive   power. Look at the power as he literally exploded  across the ring. And when scientists put Gunther'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.

[Applause] [Music] 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 an unfair advantage because they  cover more ground with each stride. 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, I know   what you're thinking. Didn't 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. 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. 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. 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 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. Bolt proved  what matters is training for the body you have.

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. And its job is well, let's  get a simple explanation from the code itself.   Hello there.

I'm ACTN3 and yes, some folks call me  the speed gene. So, how do I make you fast? It's   simple.

My 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 myio.   Whoa. Okay, hold up.

Way too much detail, dude.  Just keep it simple. Sorry about that, folks.

What I do is all about reinforcement for your  fast twitch muscle fibers. Never had much use for   those slow twitch type. Think of it like this.

The  protein I code for acts like adding steel beams to   your muscle's internal scaffolding. So when you  fire those muscles with everything you've got,   bang, that reinforcement cuts the wobble. So  more of your effort turns into speed instead   of useless vibration.

Now, which version of me  you have depends on the copies you inherited. The   R version of my 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 here. 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 folks. So, it seems like if you want to be a world-class  sprinter, you need to be an RR [Music] 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 actin 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  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 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 RAL?  

about eight out of every 10 people watching  this video. What about the ultimate two   copyr advantage? Roughly two and a half billion  people.

Two and a half billion people. Clearly,   that doesn't guarantee you'll be fast. 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? 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.