Humans are obsessed with speed. We race horses, people, even pigeons – spending billions of dollars engineering new ways to go quicker.
But how does nature dictate how fast an animal can go?
An international team of physicists, biologists and palaeontologists, including Dr Christofer Clemente from the University of the Sunshine Coast, think they have an answer.
One that could even change our understanding of extinct animals. And how robots might move in the future.
In their paper published in Nature Communications, the researchers have developed a physical model of how muscles – the universal animal motor – set limits on land animals’ top running speeds.
Lead author, Dr David Labonte from the Imperial College London, says while many traits such as strength, limb length, lifespan and brain size tend to increase with an animal’s size, maximum running speeds tend to be greatest in medium-sized animals.
“Running speed breaks with the regular patterns that govern most other aspects of animal anatomy and performance,” Dr Labonte said.
“Our model suggests there is not one limit to maximum running speed, as previously thought, but two: how fast vs 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 study raises lots of interesting questions about the muscle physiology of both extinct animals and those that are alive today, including human athletes.”
While it only involved land animals, Dr Labonte said physical constraints affected swimming and flying animals as much as running animals - and unlocking these limits was next on the agenda.
‘Sweet spot’ explains Cheetah’s unrivalled speed
In terms of top speed, smaller animals tend to be restricted by their ‘kinetic energy capacity limit’ - or how quickly their muscles can contract.
They generate large forces relative to their weight, so running for them is a bit like trying to accelerate in a low gear when cycling downhill. Their legs simply can’t go any faster.
The top speed of larger animals tends to be limited by how far their muscles can contract – or the ‘work capacity limit’. Because they’re 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.
UniSC’s Dr Clemente said that when combined with detailed data, their model estimated the optimum transition between these two limits occurred at a body mass of around 54 kg.
“Animals about the size of a cheetah exist in that physical sweet spot, where the kinetic energy and work capacity limit coincide,” Dr Clemente said.
“These animals are consequently the fastest, reaching speeds of more than 100km per hour.”
The model has proved remarkably effective on a wide range of animals, accurately predicting how the maximum running speed varies with body – from tiny 0.1 milligram mites to massive six-tonne elephants.
New insights into dinosaurs... and future robots?
By shedding light on the physical principles behind how muscles evolved, the findings could inform future designs for robots that match the athleticism of the best animal runners.
It could also have implications for understanding extinct species, according to Harvard University’s Dr Peter Bishop.
“One of the exciting aspects of this new model is that it holds promise for better understanding locomotion in extinct species, which has long been a hotly debated topic among palaeontologists,” Dr Bishop said.
Using the model with data from modern species, the authors predicted land animals weighing heavier than 40 tonnes would be unable to move. Yet some land dinosaurs, like the Patagotitan, likely weighed much more than 40 tonnes.
“This indicates extinct giants likely evolved unique muscular anatomies, which warrant further study, and that we should be cautious to estimate the muscular anatomy of extinct animals from data on non-extinct ones,” Dr Bishop said.
Co-author, Dr Taylor Dick from the University of Queensland, said it may also provide critical clues for understanding differences between groups of animals that still exist today.
Large reptiles, such as lizards and crocodiles, are generally smaller and slower than large mammals.
“One possible explanation may be that reptiles have a smaller percentage of their body mass comprising limb muscle, meaning that they hit the work limit at a smaller body mass, and thus have to remain smaller to move quickly,” Dr Dick said.
The research was supported by funds from the Australian Research Council, the Human Frontiers Science Project, the European Research Council, and the Australian Department of Education.
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