# The fastest man on Earth physics

The legs of the world's fastest man (Usain Bolt) can develop up to 5 times the force compared to those of normal people. What stops him from being faster? A normal healthy high-school student can complete the 100 metres in 11-13 seconds while Usain Bolt takes 9.58 seconds. If Usain Bolt's legs can exert so much more force than a high-school student, then how is it that the student can complete the race in 11-13 seconds?

Weight=94kg Acceleration=12.517m/s

F = ma = 94 x 12.517 = 1183N (correct me if I'm wrong)

So Usain Bolt's legs can exert 1183N on the ground.

• Please provide a source for the assertion in the first sentence. – rghome Jul 5 at 7:19
• Your number 12.517 m/s looks like a velocity, not an acceleration, and where does it come from? – Ben Crowell Jul 5 at 12:03
• – Ben Crowell Jul 5 at 12:39
• Thank you, i dont remember from which website i got that acceleration from, some other websites claim it was 10.44m/s, also 815.7N is too little, according to a youtube video it was 500pound of force exerted each time one of his legs hit the ground. – Zheer Jul 5 at 14:35
• What makes me wonder is that usain bolt force lifts too much weight at the gym compared to most people,he still runs only a bit faster despite that force, isnt force what makes us faster, also his bodyweight is near average. – Zheer Jul 5 at 14:37

Human legs carry their actuation muscles along their length, which establishes their rotary inertia about the knee and hip joint. this inertia limits the speed with which the runner can swing his or her legs back and forth, which limits the speed at which they can run.

Adding muscle mass for more strength adds inertia as well, so the limiting effect cannot be overcome with bigger muscles.

Animals capable of high-speed running (horses, deer, antelopes, rabbits, cats, ostriches, etc.) have their actuation muscles mostly in the upper parts of their legs, close to the main mass of their bodies (and connected to the lower leg and foot bones with long tendons) and not in the lower parts of their legs. This lets them cycle their legs back and forth more quickly than if their leg muscles were distributed down the length of their legs.

• Thank you, then why force doesnt help in this situation i mean usain bolt's legs can exert much greater force than a normal persons legs can exert, i believe usain bolt can squat 600-700lbs while an average high school student can only squat 100-150lbs – Zheer Jul 5 at 6:06
• because it's not just static force that propels a runner, but a force moving through a distance. – niels nielsen Jul 5 at 6:13
• What about farmers carry exercise, when you move weights for distance, the world record is 400kg for 20m, if you move less weight you can walk faster. – Zheer Jul 5 at 6:34
• walking is not the same thing as running; the dynamics are quite different. When carrying a large weight, your speed is not limited by how quickly you can swing your legs back and forth. – niels nielsen Jul 5 at 7:21

Force at a point in time is all very well, but most of the time a running man's feet (even Usain's) are not touching anything and exerting no force at all. You need to calculate what the average force is over the total time he is running.

I would expect that the 1 or two second slower high schooler's are exerting a lot less force when their feet are on the ground, but that they actually spend more time than Usain with a foot on the ground. This would increase their average force over the race.

• Correct,but what about his leg swinging, what about the muscles in his leg, he has very powerful muscles, his muscles will reduce the ground contact time much better than highschool students, his leg extensor and flexor muscles help him. – Zheer Jul 5 at 16:38

Humans have two types of muscle fibres: slow twitch and fast twitch. Athletes who have mainly fast twitch fibres are more suited to sprint events, while those with mainly slow twitch fibres are best suited to endurance events like marathons. The reason Hussain Bolt has more powerful legs than the vast majority of high school students is that nature has endowed him with more than 80 percent fast twitch fibres in his leg muscles. In addition, of course, he trains harder than most students and has longer legs.

• Thank you, but im talking about his speed. – Zheer Jul 5 at 15:59
• Any sports scientist will tell you that the number of fast twitch muscle fibres in the sprinters legs is very relevant to his speed. Perhaps I should have made that clearer in my answer, but I thought it was so obvious as to need no further elaboration. – Michael Walsby Jul 5 at 16:04
• Correct, fast twitch muscle help you generate more force and speed, but it wasnt about how he generated the force, it was about how the force affected the speed, i mean someone like usain bolt has stronger legs and more force then why he runs only a bit faster than high school students. – Zheer Jul 5 at 16:31