Scientists exploring the evolution of bipedal walking have long assumed that the raised arch of the foot helps humans walk by acting as a lever that propels the body forward. However, a new study published in the journal Frontiers in Bioengineering and Biotechnology has found that the recoil of the flexible arch repositions the ankle upright for more effective walking.
In running, these effects are even greater, suggesting that the ability to run efficiently may have been a selective pressure for a flexible arch which made walking more efficient too. Besides shedding new light on human evolution, these findings could have medical applications, by helping doctors improve treatments for patients’ locomotory problems.
“We thought originally that the spring-like arch helped to lift the body into the next step,” said study lead author Lauren Welte, an associate research fellow in Foot Biomechanics at the University of Wisconsin-Madison who conducted the research during a postdoctoral fellowship at Queen’s University. “It turns out that instead, the spring-like arch recoils to help the ankle lift the body.”
The evolution of the raised medial arch sets us apart from great apes, and is crucial for our capacity for bipedal walking. Although the precise mechanisms by which it operates are still unclear, this arch is thought to give hominins more leverage when walking, and to make them more efficient runners by propelling the center mass of the body forward, or by making up for mechanical work that muscles would otherwise have to do.
To test these hypotheses, the scientists enrolled seven participants with varying arch mobility, and asked them to walk or run while their feet were filmed by high-speed x-ray motion capture cameras.
To examine the effect of arch mobility on adjacent joints, the researchers created rigid models and compared them to the measured motion of the foot bones. In addition, they measured which joints contributed the most to arch recoil, along with the contribution of arch recoil to center of mass and ankle propulsion.
Contrary to their expectations that arch recoil would assist in lifting the body by acting as a rigid lever, the experts found that a rigid arch lacking recoil led to early detachment of the foot from the ground. This early detachment likely decreased the efficiency of the calf muscles.
Alternatively, the absence of arch recoil caused the ankle bones to lean excessively forward, resembling the posture seen in walking chimpanzees rather than the upright stance characteristic of human gait. By contrast, a flexible arch played a crucial role in realigning the ankle to an upright position, enabling more effective leg push-off from the ground.
This effect was even more pronounced during running, indicating that efficient running may have exerted evolutionary pressure in favor of the flexible arch. Finally, the scientists discovered that the joint between two bones in the medial arch – the navicular and the medial cuneiform – was crucial to the arch’s flexibility.
“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,” explained senior author Michael Rainbow, an associate professor of Mechanical and Materials Engineering at Queen’s University.
These findings could open therapeutic pathways for people whose arches are rigid due to injury or illness. In such cases, supporting the flexibility of the arch could help improve overall mobility.
“Our work suggests that allowing the arch to move during propulsion makes movement more efficient. If we restrict arch motion, it’s likely that there are corresponding changes in how the other joints function,” Welte explained.
“At this stage, our hypothesis requires further testing because we need to verify that differences in foot mobility across the population lead to the kinds of changes we see in our limited sample. That said, our work sets the stage for an exciting new avenue of investigation,” Rainbow concluded.