When you think of the fastest animal, a cheetah might come to mind. Or an antelope. But it likely isn't a mouse or an elephant. It has long intrigued scientists that the fastest animals are of an intermediate body size, not the largest or smallest. Body mass, more than any other physical characteristic, is useful for describing how animals of different sizes move, especially when it comes to speed. Recently, Christofer Clemente and Taylor Dick from at the University of Queensland, Australia, with Friedl De Groote from KU Leuven, Belgium, sought to understand why intermediate sized animals are the fastest.
However, instead of focusing on four-legged animals the team decided to simulate a two-legged human creature instead, ranging in size from 100 g (a mouse) to nearly 1000 kg (a water buffalo), because there are many more simulations of movement of humans than of four-legged animals. The team then calculated how the simulated people could run at different speeds. To do this, they worked out which gait, such as running versus walking, would provide the most stability so the simulated person didn't trip or fall as they moved. During the running simulations, they measured the forces produced by the muscles and the amount of energy the simulated people were using for their size to move. The researchers then compared all of the simulated humans, ranking them more highly if they were large and used less energy to run at the same speed as a smaller simulated human using the same amount of energy at the same speed.
It turned out that the simulated human that used the least energy to move at the highest speed weighs roughly 60 kg, matching the body mass of the fastest land animals. Although the speeds of the fastest and lowest speed simulated people matched well with those of four-legged animals, the simulated humans had a top speed of roughly 6 m s−1 which is almost four times slower than the fastest animals. The difference in top speed is partially accounted for by the differences between two- versus four-legged running.
Clemente and colleagues then looked at the impact forces generated when an animal's limbs contact the ground, to understand how these forces influence speed and stability in animals of different sizes. Their simulations showed that as body size increased, the forces exerted on the ground also increased, putting extra demand on muscles. For simulated humans larger than approximately 60 kg, the calculations suggested that reducing speed might avoid placing strain on muscles. To explore this, the team then calculated how much spare muscle strength an animal has beyond that which is needed for normal movement. They found that the hip and ankle muscles in larger animals were operating near their limits, while as the simulated animals grew larger, their knee muscles operated further below the maximum force they can output. The researchers also noticed that the larger simulated humans tended to stand more upright, reducing muscle strain by allowing their weight to be supported with less muscle effort. Meanwhile, the lighter simulated humans adopted a more crouched posture, similar to smaller four-legged creatures and our most ancient human ancestors.
Overall, this study shows that the happy medium, which allows medium sized animals to run faster than smaller and larger creatures, is the result of bigger animals maxing out on muscle force production and smaller animals maxing out on the force they can produce when striking the ground. Medium animals balance these two factors to reach the highest speed. Most impressively, Clemente and colleagues’ discovery reveals that this holds regardless of whether you're on four limbs or two.
