How did the human foot develop?

Our foot is surprisingly different

Amazing discovery: Contrary to popular belief, the human foot gets its stability primarily from its transverse arch - the lateral curvature of the metatarsus. It is this that ensures that our feet do not lose their shape despite the enormous stress they are exposed to when rolling, as researchers report in the specialist magazine "Nature". This finding is not only of practical importance, it also sheds a whole new light on the evolution of the human foot and the upright gait.

Our foot is an ingenious construction made by nature - and unique for the human species and their upright gait. While the flat feet of the great apes bend in the middle with every step, our foot remains stiff there. Researchers previously suspected the reason for this in the longitudinal arch - the arched arch between the heel and ball of the foot. Held by a network of tendons, this arch supports the metatarsus and gives it elastic strength - at least that's what people thought.

Curvatures in the test

But that's not the whole story, as Madhusudhan Venkadesan of Yale University in New Haven and his colleagues have found out. Because the human foot has a second arch: the transverse arch in the metatarsus. And this component, which has so far hardly been studied biomechanically, is even more important for the strength of our feet than the longitudinal arch, as experiments reveal.

For their study, the researchers first used a virtual model and a simplified replica of the human foot to investigate the relationship between curvature and stiffness of the arch of the foot. Venkadesan and his team changed the vault curvature of these models and determined the forces acting perpendicular to them it could withstand.

Cross is more important than lengthways

The surprising result: if only the curvature of the longitudinal arch changed, contrary to expectations, this hardly had any effect on the stability of the foot. It was different with the transverse arch of the foot model: "We found that plastic models and simulations with more pronounced transverse arching are more difficult to bend than flatter ones," reports co-author Mahesh Bandi from the Okinawa Institute of Science and Technology.

This was confirmed by additional bending tests and force measurements with the feet of human corpses. If the transverse tendons in the metatarsus were cut, the transverse arch sagged and the foot lost its stability, as the researchers found. According to their measurements, the transverse arch contributes more than 40 percent to the stiffness of the foot, while the longitudinal arch only around 23 percent.

Common picture turned upside down

This means: the image of our feet and their biomechanical functioning, which has been common for almost a century, is outdated. Because instead of the longitudinal arch, the transverse arch plays the main role for the particular stability of our feet. The scientists compare its operating principle to the curvature of a banknote or a slice of pizza: if you press the sides upwards slightly, the end does not hang down, but remains straight.

“The same is true in the foot,” says Venkadesan. “Of course it's not as easy there as with a sheet of paper because many other tissues and structures are involved, but the principle is the same.” When we put our weight on the bale shift, the weight pushes the metatarsal bones apart and tightens the tendons of the transverse arch. This makes the metatarsus stiff and ensures that it does not give in to the pressure by buckling.

Stress on the human foot when running. © Yale Biomechanics and Control Lab

New view of pre-human gang

This could also shed new light on the evolution of the human foot and the gait of our ancestors. Because the footprints in the African Laetoli show that the pre-humans Australopithecus afarensis already 3.4 million years ago walked similarly to modern humans, although this species did not have a longitudinal arch in the foot. On the other hand, in contrast to great apes, the Australopithecus already had a slight transverse arch, which could have given its foot the stability it needs to walk upright.

"Our results suggest that a human-like transverse vault could have developed around 2.5 million years ago - 1.5 million years before the genus Homo emerged," says Venkadesan. "This could have been a decisive step on the way to anatomically modern humans." (Nature, 2020; doi: 10.1038 / s41586-020-2053-y)

Source: Yale University, Okinawa Institute of Science and Technology

February 27, 2020

- Nadja Podbregar