A team of researchers from institutions including Imperial College London and Harvard University found that maximum running speeds in animals are highest in medium-sized species due to limits on muscle contraction speed and extent, challenging previous notions of a single speed limit. Their study, revealing a dual constraint system involving kinetic energy capacity and work capacity limits, suggests animals around 50kg, like cheetahs, reach the highest speeds due to their physical “sweet spot.
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 maximum speed an animal can reach is determined by whichever limit is reached first – and that limit is dictated by an animal’s size. 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.
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. The researchers say this indicates that we should be cautious to estimate the muscular anatomy of extinct animals from data on non-extinct ones. Instead, they suggest the data indicates that extinct giants might have evolved unique muscular anatomies, which warrant more study.