Humans gave up rotating hips for a smoother stride

Humans are the only mammals to be habitually bipedal, and the evolution of bipedalism over the past seven million years has led to many changes in our skeletal structure and gait. A new study has revealed that rotating hips are not an advantage in human locomotion.

Until recently, it was believed that the relatively long legs of humans gave them an advantage in terms of a longer stride that made walking locomotion more efficient.

However, in the results of a study published by The Company of Biologists, Nathan Thompson of NYIT finds that chimpanzees have a 25 percent longer stride length than humans, when height is taken into consideration.

“Standardized by size, humans really don’t have long strides,” says Thompson, after investigating the strides of humans and chimpanzees as they walked at different speeds.

Thompson and colleagues Brigitte Demes, Susan Larson (both at Stony Brook University, USA) and Matthew O’Neill (Midwestern University, USA) habituated chimpanzees to walking upright on their back legs while they filmed the animals in 3-D.

When they scaled the humans and chimpanzees to have matching heights, they found that although the humans’ legs were 112 percent longer than the legs of chimpanzees, their strides were 26,7 percent shorter.

Upon closer investigation, Thompson observed that chimpanzees lift their hips considerably on the side that takes the stride. This is known as a pelvic step or pelvic rotation. 

While chimpanzees rotate their hips between 28 and 61 degrees, humans show a paltry 8-degree rotation while striding. A human keeps his or her pelvis relatively stable while walking, although as walking speed increases, the pelvic step contributes more to the total step length.

When the researchers analyzed how much rotating hips increased the stride length of a chimpanzee, they found that it extended the length by 5.4 times more, taking into account a chimp’s height. 

“I think that chimpanzees use pelvic rotations to try to squeeze every bit of stride length out, otherwise their strides would be – absolutely – very small,’ says Thompson, explaining that apes and monkeys tend to walk on bent legs that naturally shorten their stride; 

“I don’t think there are a lot of options other than rotating the pelvis, given their anatomical constraints.”

Humans have always been considered to be very efficient walkers and to have the longest possible stride. However, now that their stride size has been shown to be relatively shorter than that of their nearest living relatives, the researchers consider that other factors must have been important in the evolution of the human gait. 

Thompson suggests that walking with an exaggerated pelvic rotation may increase stride length but could also disrupt the natural swing of our arms and legs, which helps to balance us. Ditching the rotating hips may have also helped to stabilize a human’s center of mass, which would mean that fewer muscles are active while walking and so the energetic costs are reduced. 

“Humans have had about seven million years of selective pressure for economical bipedalism; this means that there has been a lot of time to experiment with the costs and benefits, so it might be worth it to walk with slightly shorter strides, because whatever energy we lose, we might make up elsewhere,” concludes Thompson.

The study is published in the Journal of Experimental Biology.

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By Alison Bosman, Earth.com Staff Writer