A tale of two arches

Think of your foot as a slice of pizza. Not all the time: just while we consider how and why it does was it does.

Humans have evolved stiff, arched feet that are essential for effective upright walking, and which allow us to apply forces that exceed our bodyweight. In contrast, other primates, such as chimpanzees, gorillas and macaques, have relatively flexible, flat feet. 

Researchers have long debated how the structure of the human foot creates stiffness, with most studies focussing on the medial longitudinal arch (MLA), which runs from the heel to the ball of the foot.

However, a new study by a team of researchers from the US, Japan, the UK and Australia has found that the transverse tarsal arch (TTA), which runs across the foot, is responsible for more than 40% of the stiffness.

It’s a significant finding not just for those with a deep interest in feet, but also for those who treat foot disorders and potentially for others who design prosthetic limbs or robots they want to behave like humans.

The research, which was led by Madhusudhan Venkadesan from Yale University, US, and is published in the journal Nature, was quite an undertaking.

The authors first examined the evolution of the TTA across primates, including extinct hominin species, and found that only the genus Homo had fully developed MLA and TTA. 

This, they say, suggests the combination of the two adjacent arches produces stiffness along the length of the foot. And there were several stages in the evolution of the human foot to enable efficient walking and running. 

“By analysing fossils, we track the evolution of the curvature parameter among extinct hominins and show that a human-like transverse arch was a key step in the evolution of human bipedalism that predates the genus Homo by at least 1.5 million years,” they write.

They then performed bending tests on human feet, discovering that the TTA’s impact may be similar to that of bending a sheet of paper parallel to the width to stiffen it lengthways.

Or, as Glen Lichtwark and Luke Kelly from Australia’s University of Queensland say in an accompanying News & Views article: “An analogy for this proposed stiffening mechanism is the way that a pizza slice becomes less floppy if the slice’s outer crust is curled up.”

Venkadesan and colleagues emphasise that this alone cannot be used reliably to infer movement capabilities, and other mechanisms might stiffen the foot sufficiently to allow a human-like gait – a point noted by Lichtwark and Kelly.

“The range of curvature of the arch of human feet suggested by Venkadesan et al. would indicate that a nearly twofold change in stiffness is possible as a result of natural variation in curvature of the transverse arch from one person to the next,” they write. 

“However, any relationship between transverse-arch curvature and stiffness is probably not enough to completely explain the regulation of foot stiffness, and other factors will also need to be considered…”

Among the questions if and how the relationship between MLA and TTA varies depending on whether people have high arches or “flat feet”.

So, there’s still work to be done, but important steps have been taken.