Cheetah runs faster because of its physical ‘sweet spot’

It’s common knowledge that medium-sized animals like cheetahs, tigers or even dogs are known to be faster than bigger animals like elephants, or even smaller ants. However, there are exceptions.

An interdisciplinary team of scientists provide a biomechanical explanation – combining biology with mechanics (as in physics). Apparently, their findings can inform robot designs, inspired by animal biomechanics.

“The key to our model is understanding that maximum running speed is constrained both by how fast muscles contract, as well as by how much they can shorten during a contraction,” said Professor Christofer Clemente, a biomechanics researcher from University of the Sunshine Coast and The University of Queensland, in Australia, in a press release.

Their paper was published in Nature Communications

Their model uses two physical constraints. The first is by kinetic energy capacity, dictated by how fast do the muscles contract to generate forces much bigger than its own body weight. The second limit is by work capacity, dictated by how far the muscle contracts.

“For large animals like rhinos or elephants, running might feel like lifting an enormous weight, because their muscles are relatively weaker and gravity demands a larger cost,” said Peter Bishop, a biomechanics researcher at Harvard University, US. “As a result of both, animals eventually have to slow down as they get bigger.”

Apparently, this model offers an explanation as to why crocodiles, despite being medium-sized in a sense, aren’t so quick.

“One possible explanation for this may be that limb muscle is a smaller percentage of reptiles’ bodies, by weight, meaning that they hit the work limit at a smaller body weight, and thus have to remain small to move quickly,” said Taylor Dick, a biomechanics researcher at The University of Queensland, Australia.

Cheetahs, in contrast, which can attain a maximum speed of 65 km/hr, hits the physical sweet spot of 50 kg, when the two constraints set by kinetic energy and work capacity are equal.

The team tested their hypothesis against data gathered from 400 species of various sizes and body weights, including mites weighing just 0.1 mg, to six-tonne elephants.

The model doesn’t just offer explanations to known facts about speeds in present animals. It can be extrapolated to extinct animals such as dinosaurs, although with some caveats.

It predicts land animals heavier than 40 tonnes would be immobilized. The heaviest land animal, the African elephant, weighs around 6.6 tonnes. Although it was known that land-based dinosaurs such as the Patagotitan, possibly exceeded even 40 tonnes.

And this is where researchers erred on the side of caution, saying that their model was based on known anatomies of non-extinct animals. In fact, extinct giants might have evolved unique muscular anatomies, thus warranting more study.

This research was funded by the Australian Research Council, the international Human Frontier Science Program, and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program.

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