There is an anomaly among the physical features of the animal world. Although traits such as muscle strength, appendage length, longevity, and brain mass tend to increase with body size, maximum running speed peaks for creatures of average size. To find out 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 engine of animals, limit maximum running speed on land. animals.

Lead author Dr. D., from the Department of Bioengineering at Imperial College London. David Labonte said: ‘The fastest animals are not big elephants or tiny ants, but medium-sized animals like cheetahs. “Why does running speed disrupt normal patterns that determine many other aspects of animal anatomy and performance?”

The findings show that there is not a single limit to maximum running speed, as previously thought, but two limits: how fast and how much the muscles contract. The maximum speed an animal can reach is determined by the limit first reached, and this limit is determined by the size of the animal.

Co-author Professor Christopher Clemente, from the University of the Sunshine Coast and the University of Queensland, said: “The key to our model is the understanding that maximum running speed is limited by both the speed at which the muscles can contract and the speed at which they can contract during the match.

“Cheetah-sized animals live in a physical sweet spot where these two limits overlap, around 150 pounds. So these animals are the fastest animals, capable of reaching speeds of up to 65 miles per hour.”

Results are published on: Nature Communication.

limit test

The first limitation, called the “kinetic energy limit,” suggests that muscles in smaller animals are limited by the rate at which they can contract. Because small animals produce greater forces relative to their weight, running for a small animal is like trying to accelerate in low gear on a downhill bike.

A second limit, called the “capacity limit,” suggests that the muscles of larger animals are limited by the amount they can contract. Because larger animals are heavier, their muscles produce less force relative to their weight, and running is more like trying to accelerate while cycling uphill in high gear.

Co-author Dr Peter Bishop, from Harvard University, said: “For large animals such as rhinos or elephants, running can seem like lifting huge weights because their muscles are relatively weaker and gravity requires more effort. As a result of both situations, the animals are forced to slow down as they grow larger.” “

To test the accuracy of their model, the team compared their predictions with data on the speed and size of land animals collected from more than 400 species, from large mammals, birds and lizards to tiny spiders and insects. The model accurately predicted how maximum running speed depends on body size for animals that vary more than 10-fold in body mass, from tiny ticks weighing 0.1 milligrams to six-ton ​​elephants.

Their findings shed light on the physical principles of muscle evolution and could form the basis for future designs of robots suited to the athleticism of top runners. In addition to explaining how fast animals can run, the new model may also provide important clues in understanding differences between animal groups. Large reptiles such as lizards and crocodiles are generally smaller and slower than large mammals.

Co-author Dr Taylor Dick, from the University of Queensland, said: ‘One possible explanation for this could be that limb muscles make up a smaller percentage of body weight in reptiles, which could mean they reach their performance limits at lower body weights. and so to move fast you have to stay small.”

The model, combined with data on modern species, also predicted that land animals weighing more than 40 tons would not be able to move. The heaviest land mammal today is the African elephant, weighing approximately 6.6 tonnes; but some land dinosaurs, such as Patagotitan, probably weighed much more than 40 tons.

The researchers say this shows that we should be careful when predicting the muscular anatomy of extinct animals based on data from non-extinct animals. Instead, they suggest that the data suggest that extinct giants may have developed a unique muscular anatomy that requires further study.

The study raises questions about how giant dinosaurs were able to move, as well as questions that require more focused data collection on specific groups of animals, such as reptiles or spiders. Although only land animals were examined in the study, the researchers will later apply their methods to flying and swimming animals.

Dr Labonte said: “Our research raises many interesting questions about muscle physiology in both extinct animals and living ones, including human athletes. Physical constraints affect swimming and flying animals as well as running animals, and removing these constraints is high on our agenda.” is included.”