Earlier 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 muscle fibers for use when running. Hirt’s team found that larger animals run out of this fuel more quickly than smaller animals, because it takes them longer to accelerate their heavier bodies. this is known as muscle fatigue. explains why, in theory, a human could have outrun a T-rex.
but günther and his colleagues were skeptical. “I thought we could come up with another explanation,” he says, one that used only the principles of classical physics to explain the speed restrictions. so they built a biomechanical model consisting of more than 40 different parameters related to body design, running geometry, and the balance of competing forces acting on the body.
“The basic idea is that two things limit top speed,” says Robert Rockenfeller, a mathematician at the University of Koblenz-Landau and co-author of 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 drag effects do not increase with mass, it is the dominant speed-limiting factor in smaller animals. “If you were infinitely heavy, you would run infinitely fast, according to air resistance,” says Rockenfeller.
The second property at play, which increases with greater mass, is called inertia, the resistance of an object to acceleration from a state of rest. When running, Rockenfeller says, there’s a time limit for an animal to accelerate its own mass: It’s the duration between mid-sprint, when the foot is flat on the ground, and takeoff, when the foot leaves the ground. this is especially limiting for larger animals: with more mass to propel, it’s harder to overcome inertia. so smaller bodies have the advantage here.
according to the team’s results, the sweet spot for overcoming drag and inertia is around 110 pounds. Not coincidentally, that’s the average weight for cheetahs and pronghorns.
günther’s team was also able to predict the theoretical top speed for different body designs to be 100 kilograms, or about 220 pounds. a house cat of this size could run up to 46 miles per hour; a giant spider, if its legs could somehow support its weight, would top out at 35 miles per hour. unsurprisingly, the average human body design ranks dead last here: at 100 kilograms, we can only go about 24 miles per hour.
but body size isn’t the only feature that comes into play when maximizing speed. in the model, the length of the leg also mattered. animals with longer legs are able to push their bodies further forward before their foot needs to lift off the ground, prolonging the time they have to accelerate between midstance and takeoff.
as for why four-legged animals can run faster than humans, günther says it’s not because we only have two legs, but because our torsos are upright and feel the full force of the gravity. bipedal creatures have evolved much stiffer spinal structures to prioritize balance and stability over speed. however, animals whose trunks are parallel to the ground evolved with more flexible spines that are optimized for prolonged foot contact with the ground.