While many key traits such as strength, limb length, lifespan and brain size tend to increase with animals’ size, maximum running speeds tend to be greatest in medium-sized animals. Empirical data show that maximum running speed increases up to a critical body mass and then decreases — the fastest runners are of intermediate size. The findings suggest that there is not one limit to maximum running speed, as previously thought, but two: how fast versus by how far, muscles contract. The maximum speed an animal can reach is determined by whichever limit is reached first — and that limit is dictated by an animal’s size. The maximum running speed is constrained both by how fast muscles contract, as well as by how much they can shorten during a contraction. Animals about the size of a cheetah exist in a physical sweet spot at around 50kg, where these two limits coincide. These animals are consequently the fastest. The first limit, termed the kinetic energy capacity limit, suggests that the muscles of smaller animals are restrained by how quickly they can contract. Because small animals generate large forces relative to their weight, running for a small animal is a bit like trying to accelerate in a low gear when cycling downhill. The second limit, called the work capacity limit, suggests that the muscles of larger animals are restrained by how far their muscles can contract. Because large animals are heavier, their muscles produce less force in relation to their weight, and running is more akin to trying to accelerate when cycling up a hill in a high gear.
