Why Even the Fastest Human Can’t Outrun Your House Cat

This weekend, the fastest sprinters on the planet came together at the Tokyo Olympics to compete for the gold in the 100-meter dash. Lamont Marcell Jacobs crossed the finish line in 9.80 seconds to bring Italy its first gold in the event. In the women’s race, Jamaica won the gold, silver, and bronze—a clean sweep led by Elaine Thompson-Herah, who smashed through a 33-year-old Olympic women’s record with a time of 10.61 seconds.

But neither of them could touch the legacy of Jamaica’s eight-time Olympic gold medalist Usain Bolt, who retired in 2017 but still boasts the title of fastest human alive. Bolt ran the 100 meters in 9.58 seconds. Maxing out at about 27 miles per hour, that’s just under the top speed of a house cat. (Yes, a house cat.) In a race against cheetahs and pronghorns, the fastest animals in the world, Bolt wouldn’t stand a chance.

You might think how fast an animal can go depends on the size of its muscles: more strength, more speed. While that’s true to a certain extent, an elephant will never outrun a gazelle. So what really determines maximum speed?

Recently, a group of scientists led by biomechanist Michael Günther, then affiliated with the University of Stuttgart, set out to determine the laws of nature that govern maximum running speeds in the animal kingdom. In a new study published last week in the Journal of Theoretical Biology, they present a complex model factoring in size, leg length, muscle density, and more to discover which body design elements are the most important for optimizing speed. 

This research provides insight into the biological evolution of legged animals and their corresponding gaits, and it could be used by ecologists to understand how speed constraints on animal movement inform population, habitat selection, and community dynamics in different species. For roboticists and biomedical engineers, learning about nature’s optimal body structures for speed could further improve the designs of bipedal walking machines and prosthetics.

“It’s about understanding the reasons for evolution, and why and how it shapes the body,” Günther says of the project’s goal. “If you ask this question mechanistically, then you can really add to the understanding of how body design is shaped by evolutionary requirements—for example, being fast.”

Previous work in this area, led by Myriam Hirt of the German Center for Integrative Biodiversity Research, found that the key to speed had to do with an animal’s metabolism, the process by which the body converts nutrients into fuel, a finite amount of which is stored in the muscle fibers for use when sprinting. Hirt’s team found that larger animals run out of this fuel more quickly than smaller animals do, because it takes them more time to accelerate their heavier bodies. This is known as muscle fatigue. It explains why, theoretically, a human could have outrun a Tyrannosaurus rex.

But Günther and his colleagues were skeptical. “I thought we might be able to give another explanation,” he says, one that used only the principles of classical physics to explain speed constraints. So they built a biomechanical model consisting of over 40 different parameters relating to body design, the geometry of running, and the balance of competing forces acting on the body.

“The basic idea is that two things limit maximum speed,” says Robert Rockenfeller, a mathematician at the University of Koblenz-Landau who coauthored the study. The first is air resistance, or drag, the opposing force acting on each leg as it tries to push the body forward. Since the effects of drag don’t increase with mass, it’s the dominating factor capping speed in smaller animals. “If you were infinitely heavy, you would run infinitely fast, according to air drag,” Rockenfeller says.

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