Under 10 seconds: The science behind Usain BOlt's sprints

Height, genetics or technique, what is it that gives Usain Bolt, the world record holder of the 100, 200 and 4 X 100 meters relay, the extra edge?

Usain Bolt at the Berlin Championships in 2009, where he broke the world record for running 100m in 9.58 seconds.

Think "fastest man on earth" and the name Usain Bolt comes up. Holding multiple world records, he reigns as Olympic Games champion in the 100m, 200m and 4 X 100m relay, three times. For over a decade, his multiple achievements has cemented his name as the greatest sprinter to date. But how does he do it?


Some attribute his success to his height on the basis that he has longer legs and thus longer strides. Some attribute the success to the muscle composition in his legs. Then there are those who analyze his technique. Could it possibly be all three?



Height

Bolt is a towering 6"4 feet. While there is truth that this allows him to have longer strides mid-race, it is actually much harder for him to fast in the beginning. His lower centre of gravity and larger surface area means he has to overcome factors such as air density and increased resistance.


Bolt is taller than most of his competitors, but it is not necessarily an advantage.

Let's look at the 2009 Berlin Championship where Bolt broke the world record for sprinting 100m in 9.58 seconds. To cover that distance in such a short amount of time meant that Bolt accelerated at 12.2 meters per second, and if we go deeper into the physics, he was exerting an average force of 815.8 Newtons. For his height, the acceleration of that magnitude would be countered by a massive air resistance drag, making it an even harder task.


So, it is clearly not his height that gives him that extra edge. In fact, it might be due to his technique.




TECHNIQUE

Bolt does indeed take less strides, but his strides are also powerful, which allows him to accelerate quickly.


He does this by using the rotation of the body around the point of support under the action of gravitational torque, which in simpler terms is the free falling of the body forward, to generate force for the next step.


To achieve that in a very limited frame of space and time is definitely challenging. Just imagine you're about to fall flat on your face, but you resist and generate enough power through your muscles to take the next step. This is known as the Pose method.


Running sequence of Usain Bolt that shows how he free falls to propel himself forward. Image courtesy of Russian Track and Field Magazine.

Bolt also takes a split second to preserve this "free-fall" pose (in frames 12 and 30) which is also known as the Preservation of the Pose to allow for better conservation of momentum of the body. It also allows Bolt to use the gravitational torque to provide the correct angle for acceleration.


Many sprinters use this technique, but Bolt is a master in it. He manages to transform rotational (angular) velocity of the body into a horizontal force, propelling him forward with each step.


Experts say Bolt could be even faster if he manages to increase his average step frequency to the level of his rivals - about 4.5 steps per second. His 100m result could be 9.11 seconds!




GENETICS

We looked at Bolt's height and his technique. Both seem to play into the reason of why he can achieve superhuman speed. So, does his genetics really matter at this point?


Speed is influenced by a range of factors, one of which is muscle composition. Muscles are made of slow-twitch and fast-twitch fibers, which have different functions. Slow-twitch fibers contract at a rate of about 15mm per second, whereas fast-twitch fibers contract at 40-45mm per second. It becomes obvious that fast-twitch muscle fibers are required for most rapid movements such as sprinting. Most muscles have an even balance of both muscle fibers.


Here's the big secret: Bolt hit the genetic lottery. His legs have a higher proportion of fast-twitch to slow-twtich fibres of 80:20%. This composition can give a speed of 99.9% higher than the general population.


One may argue that strength and endurance training can help shape undifferentiated fibers into slow- or fast-twitch fibers, but science has shown that the best runners were born with an imbalance of one or the other. This does suggest that our genetics predispose whether we can become elite marathoners or sprinters.


With a disadvantageous height that reduces step frequency, Bolt countered that by perfecting the running technique. That would have put him up to level with other shorter, higher-paced runners. But his genetics provided him that extra edge to achieve a maximum speed of 44.72km/h - a true superhuman status.



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