Cheetahs' Unrivalled Speed Explained by Their 'Sweet Spot' Size, Finds Imperial Study
Published:10 May2024    Source:Imperial College London
There's a discrepancy in the animal kingdom. While many key traits such as strength, limb length, lifespan and brain size tend to increase with animals' size, maximum running speeds tend to be greatest in medium-sized animals. To explore why, an international team of researchers including Imperial, Harvard University, The University of Queensland and The University of the Sunshine Coast, developed a physical model of how muscles, the universal animal motor, set limits on land animals' top running speeds. To test the accuracy of their model, the team compared its predictions to data on land animal speed and size collected from more than 400 species, from large mammals, birds and lizards to tiny spiders and insects.
Their findings suggest that there is not one limit to maximum running speed, as previously thought, but two: how fast versus by how far, muscles contract. The first limit, termed the 'kinetic energy capacity limit', suggests that the muscles of smaller animals are restrained by how quickly they can contract. Because small animals generate large forces relative to their weight, running for a small animal is a bit like trying to accelerate in a low gear when cycling downhill. The second limit, called the 'work capacity limit', suggests that the muscles of larger animals are restrained by how far their muscles can contract. Because large animals are heavier, their muscles produce less force in relation to their weight, and running is more akin to trying to accelerate when cycling up a hill in a high gear.

The maximum speed an animal can reach is determined by whichever limit is reached first -- and that limit is dictated by an animal's size. Animals about the size of a cheetah exist in a physical sweet spot at around 50kg, where these two limits coincide. These animals are consequently the fastest, reaching speeds of up to 65 miles per hour. Their findings shed light on the physical principles behind how muscles evolved and could inform future designs for robots that match the athleticism of the best animal runners. In addition to explaining how fast animals can run, the new model may also provide critical clues for understanding differences between groups of animals. Large reptiles, such as lizards and crocodiles, are generally smaller and slower than large mammals.