# Why can't Usain Bolt run faster than a domestic cat?

Do you remember Jamaican runner Usain Bolt, the only living person to hold 8 Olympic gold medals? Although, Bolt has retired since 2017, but he still holds the title of the fastest runner on the planet, with a speed of approximately 44 km / h, running 100m in just 9.58 seconds.

Such a speed for us is very terrible, but in reality it is not equal to the maximum speed of domestic cats (48 km / h). And if faced with cheetahs (up to 130 km/h) and wildebeest (nearly 100 km/h), Bolt can only 'breathe smoke'.

Many people believe that the important factor determining the speed of a person, an animal is the size of the muscle group: the stronger, the faster. This conclusion holds true only in a few cases, but in a race in the wild, an elephant never beats an antelope. So what is really the key factor?

Recently, a team of scientists led by biomechanical researcher Michael Günther of the University of Stuttgart designed a complex model that included 40 different parameters such as body size, leg length, muscle density, running track, physical impact… to answer the above question.

Usain Bolt - The fastest man on the planet.

## Factors that determine victory or defeat

'The basic idea is to delve into the two factors that limit speed. The first is air resistance - the opposing force acting on each leg as we move forward. Air resistance is not influenced by mass, so it is the limiting factor in speed in small animals. Based on the above theory, if you have a lot of weight, you will run very fast,' said Robert Rockenfeller, a mathematician at the University of Koblenz-Landau and co-author of the study.

The second factor is that inertia - the force an object resists when it accelerates from rest - increases with mass. According to Rockenfeller, when running, there is a time limit for the animal to accelerate its own mass: the time between when the foot is raised above the surface of the ground, and when the foot has left the ground. This is a very limited feature in large animals, because of their large mass the acceleration time will be shorter than in small animals.

According to the results, the best body mass to overcome air resistance and inertia is in the 50kg range. This is the average weight in cheetahs as well as wildebeest. A 100kg domestic cat can run as fast as 74km/h. A spider of similar weight, if its legs are strong enough to keep its body from falling, can reach 56km/h. However, with a person weighing 100 kg, it can only reach 37km/h.

But body size isn't the only trait that governs speed maximization. Leg length is also important. Animals with longer legs are able to propel their bodies further forward, prolonging body mass acceleration.

## Why do four-legged animals run faster than humans?

Not because we only have 2 legs, but because the body lies perpendicular to the ground when standing upright, this is the position that bears all the gravity. We evolved on two legs to prioritize balance and stability over speed. Four-legged animals have a flexible skeleton that can bend and stretch to help them prolong the time their feet are in contact with the ground, so they definitely run faster than humans.

A four-legged animal with a flexible skeleton.