The fastest animals are neither large elephants nor tiny ants, but intermediately sized, like cheetahs (Acinonyx jubatus). Why does running speed break with the regular patterns that govern most other aspects of animal anatomy and performance? New research suggests 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 key to our model is understanding that maximum running speed is constrained both by how fast muscles contract, as well as by how much they can shorten during a contraction,” said Professor Christofer Clemente, a researcher at the University of the Sunshine Coast and the University of Queensland.
“Animals about the size of a cheetah exist in a physical sweet spot at around 50 kg, where these two limits coincide. These animals are consequently the fastest, reaching speeds of up to 105 km per hour (65 mph).”
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.
“For large animals like rhinos or elephants, running might feel like lifting an enormous weight, because their muscles are relatively weaker and gravity demands a larger cost,” said Dr. Peter Bishop, a researcher at Harvard University.
“As a result of…
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