A phrase like 'spring in your step' is usually meant to evoke enthusiasm or happiness but a new study finds that its mechanism, the spring-like arch in our feet, did help us walk on two feet. Just in a different way than previously believed.

Most believe that the raised arch of the foot helps us walk by acting as a lever which helps to lift the body into the next step by propelling the body forward but the new work argues that the recoil of the flexible arch repositions the ankle upright for more effective walking - and the effects in running are even greater.

While a flexible arch and its link to selective pressure in evolution is academically intriguing, it could also help doctors improve treatments for present-day patients’ foot problems.

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The raised medial arch is crucial to bipedal walking, it is a key way we are different from apes.  When arch motion is restricted, running demands more energy so it is thought to give hominins more leverage when walking upright, making up for mechanical work that muscles would otherwise have to do.

To investigate the hypotheses, the team selected seven participants with varying arch mobility, who walked and ran while their feet were being filmed by high-speed x-ray motion capture cameras. The height of each participant’s arch was measured, and their right feet were CT-scanned. The scientists created rigid models and compared them to the measured motion of the foot bones to test the effect of arch mobility on adjacent joints. They also measured which joints contributed the most to arch recoil, and the contribution of arch recoil to center of mass and ankle propulsion.

They discovered that a rigid arch without recoil either caused the foot to leave the ground early, likely decreasing the efficiency of the calf muscles, or leaned the ankle bones too far forward. The forward lean mirrors the posture of walking chimpanzees, rather than the upright stance characteristic of human gait. The flexible arch helped reposition the ankle upright, which allows the leg to push off the ground more effectively. This effect is even greater when running, suggesting that efficient running may have been an evolutionary pressure in favor of the flexible arch.