“If this explanation holds up, it should also be true in other environmental mediums,” he says. The most accurate way to study running behavior in animals would be to implant mechanical sensors inside their muscles and track them as they move in their natural environment-but this raises obvious logistical challenges and ethical concerns, Günther says.Ĭloyed also looks forward to seeing how this analysis will be expanded, particularly to other locomotive modes like flying and swimming. Cloyed says it would require catching animals and observing them in a laboratory, or using high-quality videos of them sprinting, to analyze the biomechanics of their movements. But all of the scientists note that doing so will be a challenge. Günther and Rockenfeller agree that experiments are needed to verify their conclusions, and they feel they have presented a comprehensive model for other researchers to test in the future. Animals whose trunks are parallel to the ground, however, evolved with more flexible spines that are optimized for prolonged foot contact with the earth. Bipedal creatures have evolved with much more rigid spinal structures to prioritize balance and stability over speed. Animals with longer legs are able to push their bodies farther forward before their foot must leave the ground, prolonging the time they have to accelerate between midstance and liftoff.Īs for why four-legged animals can run faster than humans, Günther says this isn’t because we only have two legs, but because our torsos are positioned upright and feel the full force of gravity. Unsurprisingly, the average human body design comes in last place here: At 100 kilograms, we can only reach about 24 miles per hour.īut body size isn’t the only feature that comes into play when maximizing speed. A house cat this size could run up to 46 miles per hour a giant spider, if its legs could somehow sustain its weight, would top out at 35 miles per hour. Günther’s team was also able to predict theoretical speed maximums for different body designs at 100 kilograms, or about 220 pounds. Not coincidentally, that’s the average weight of both cheetahs and pronghorns. So smaller bodies have the advantage here.Īccording to the team’s results, the sweet spot for overcoming air drag and inertia lies at around 110 pounds. This is especially limiting for larger animals-with more mass to push forward, it's harder to overcome inertia. When running, Rockenfeller says, there is a time limit for an animal to accelerate its own mass: It’s the duration between midstance, when the foot is flat on the ground, to liftoff, when the foot leaves the ground. The second property at play, which does increase with greater mass, is called inertia, the resistance of an object to accelerate from a state of rest. “If you were infinitely heavy, you would run infinitely fast, according to air drag,” Rockenfeller says. Since the effects of drag don’t increase with mass, it’s the dominating factor capping speed in smaller animals. The first is air resistance, or drag, the opposing force acting on each leg as it tries to push the body forward. “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.
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