Humans have lost their swing. Chimpanzees and other great apes swing their hips when they walk, but modern humans don鈥檛. This means our strides are shorter than those of chimpanzees, even though our legs are proportionally longer.
鈥淲e鈥檝e always had this idea that evolution has been acting on fossil humans to make strides longer and longer,鈥� says at the New York Institute of Technology. But, in fact, 鈥渉umans right now could make our strides longer, but we don鈥檛.鈥�
A defining feature of humans and other hominins is that we are bipedal, walking upright on two legs. This contrasts with chimpanzees, our closest living relatives, which can walk on two legs, but also on four legs by 鈥渒nuckle-walking鈥�. The transition to habitual bipedality was one of the most crucial steps in our evolutionary past. But it isn鈥檛 clear why our ancestors started walking that way. 鈥淭hat鈥檚 still the million-dollar question,鈥� says Thompson.
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One idea is that walking bipedally and having long legs allows us to take bigger strides. 鈥淭he idea is that humans have these really long strides and that鈥檚 part of what makes us a really economical biped,鈥� says Thompson. But studies of humans walking show that human strides are quite short, given our leg length.
To find out what is happening, Thompson and his colleagues asked 10 volunteers to walk on treadmills, using motion capture to record their precise movements. They compared this to existing data on chimpanzee gait.
Compared with the chimps, the humans rotated their pelvises much less. 鈥淲e found chimpanzees, when they鈥檙e bipedal, they rotate their pelvis like three to four times more than humans do,鈥� says Thompson. That gave them longer strides.
Humans are anatomically capable of rotating their pelvises while walking, says Thompson. Race walkers, who must go as fast as possible while always having one foot in contact with the ground, do it a lot. But for some reason, we don鈥檛 routinely do it.
It isn鈥檛 clear why, but Thompson says larger pelvic rotations might make it harder for humans to balance. It would be possible to compensate, but that would use more energy, offsetting the benefit of longer strides. 鈥淗umans have had 7 million years to play with bipedalism and try to find the most optimal solution,鈥� he says.
Journal of Experimental Biology
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