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Humans used to be thought to have the longest strides of any primate for their height. But, now, chimpanzees take 25% more strides than us due to their swiveling hips that rotate by up to 61deg each time they take a step. This is to compensate for their shorter legs and crouched posture.
Even the tallest human beings take very short strides compared to the strides of their closest primate relatives. Nathan Thompson, U.S. Institute of Technology, states that humans don't have very long strides because they are small. However, scientists had believed that the stride of a human was too long to be efficient until recently. Thompson explained that this misconception became apparent only after he started looking into the literature. Thompson was intrigued by the length of chimpanzees' strides and began to investigate how they rotate their pelvises when walking. He was intrigued by this possibility and decided to compare humans and chimpanzees walking at different speeds. His discovery, published in Journal of Experimental Biology, shows that the strides of chimps are 25% longer than those of humans (for their height), due to their swiveling hips which extend their strides 5.4x more than the tiny wiggles we do when walking.
Thompson says that "working with people and animals always presents its challenges." Thompson spent many years working with Susan Larson, Brigitte demes and Matthew O'Neill at Midwestern University, U.S., to get the chimpanzees used to walking on their own, while filming the animals in 3D. Working with human walkers was not without its difficulties. Thompson recalls one volunteer who was constantly having fits of giggles when she tried to walk on the treadmill naked. Thompson says that it was a strange sensation and they couldn't help but do so.
After Danielle Rubinstein and William Parrella O'Donnell had reconstructed the human's gait and hip movements in 3D, they compared the humans to chimpanzees. They found that although their legs were 112% longer, the strides of the humans were 26.7% shorter. The chimpanzees were able to move their hips between 28-61 degrees, whereas the humans could only twist their pelvis by 8 degrees. The chimpanzees also had an advantage when the team measured how far their pelvic rotation extended their stride length. Their stride length was 5.4 times longer than that of humans due to their smaller size.
Thompson says that chimpanzees use pelvic turns to squeeze as much stride length as possible. He explains that monkeys and apes walk on crouching legs, which naturally shortens their stride. "I don’t think there are many options other than rotating their pelvis, given their anatomy constraints."
Why have people stopped swiveling their hips, when they could increase their stride length? Thompson suggests that extreme hip rotations might cause our natural swing of our legs and arms to be thrown out. This could make walking more efficient and force our muscles to work harder. Thompson says that while scientists believed humans would have the longest stride possible for efficiency for many decades, it is now clear that our stride length is significantly shorter than our closest cousins. This suggests that there may be other factors that have had an impact on how we walk. He suggests that humans have been subject to selective pressure over 7 million years for economic bipedalism. This means that it has been very time-consuming to test the benefits and costs of different walking styles. It might be worthwhile to walk with shorter strides to make up any energy lost.
Further research by Chimpanzees sheds light on the origins of human walking
More information: Nathan E. Thompson and co, The loss of 'pelvic step in human evolution, Journal of Experimental Biology (2021). Journal Information: Journal of Experimental Biology Nathan E. Thompson and co, The loss of 'pelvic' in human evolution (2021). DOI: 10.1242/jeb.240440
