Humans evolved flexible arches to walk upright

Efficiently standing up and walking and running on two feet  stands out among the traits that separates Homo sapiens from great apes—and we can owe a lot of that to a raised medial arch. While crucial, the mechanics behind bipedal walking are still a bit of an evolutionary mystery.  A study published May 30 in the journal Frontiers in Bioengineering and Biotechnology found that helpful and spring-like arches may have evolved for the purpose of helping us walk on two feet.

[Related: Foraging in trees might have pushed human ancestors to walk on two feet.]

The team found that the recoil of a flexible arch repositions in the ankle upright for more efficient walking and is particularly effective for running. 

“We thought originally that the spring-like arch helped to lift the body into the next step,” study co-author and University of Wisconsin-Madison biomechanical engineer Lauren Welte said in a statement. “It turns out that instead, the spring-like arch recoils to help the ankle lift the body.”

The raised arch in the center of the human foot is believed to give hominins more leverage while walking upright. When arch motion is restricted, like it could be in those with more flat feet, running demands more energy from the body. Arch recoil could potentially make our species more efficient by propelling the body’s center of mass forward, essentially making up for the mechanical work that the muscles would have to do otherwise.

In this new study, the team selected seven participants with varying arch mobility and filmed their walking and running patterns with high-speed x-ray motion capture cameras. The team measured the height of each participant’s arch and took CT scans of their right feet. They also created rigid models that were compared to the measured motion of the bones in the foot. Scientists then measured which joints added the most to arch recoil and the contribution of arch recoil to center of mass and ankle propulsion.

Surprisingly, they found that a rigid arch without recoil caused the foot to prematurely leave the ground, likely decreasing the efficiency of the calf muscle. A rigid arch also leaned the ankle bones too far forward. A forward lean looks more like the posture of walking chimpanzees instead of the straight upright stance of a human gait.

A flexible arch helped reposition the ankle upright, allowing the leg to push off the ground more effectively. This effect is greater while running, suggesting that a flexible arch for more efficient running may have been a desired evolutionary trait.

The team also found that a joint between two bones in the medial arch–the navicular and the medial cuneiform–is crucial to flexibility. Investigating the changes in this joint over time could help scientists track the development of bipedalism in our own fossil record. 

[Related: The Monty Python ‘silly walk’ could replace your gym workout.]

“The mobility of our feet seems to allow us to walk and run upright instead of either crouching forward or pushing off into the next step too soon,” study co-author and Queen’s University mechanical and materials engineer Michael Rainbow said in a statement.

These findings and understanding more about arch flexibility could help people who have rigid arches due to illness or injury. Their hypothesis still needs more testing, but could help solve a plethora of modern-day foot dilemmas. 

“Our work suggests that allowing the arch to move during propulsion makes movement more efficient,” said Welte. “If we restrict arch motion, it’s likely that there are corresponding changes in how the other joints function.”