What keeps our foot stable

footprint

Footprint in the sand (Image: M. Venkadesan)

Our foot is a unique construction of nature – and less researched than you might think. Because researchers have now discovered surprising things about the structures that give the foot its stiffness. Their experiments reveal that, contrary to previous assumptions, it is not the longitudinal vault that plays the main role, but the previously neglected transverse vault. Only this ensures that our foot does not lose its shape despite the enormous strain when rolling. This knowledge is not only of practical importance for orthopedic surgeons, it also sheds new light on the evolution of the human foot and upright gait.

Our foot enables us to stand securely on two legs and to walk upright, even withstanding jumps, sprints and sudden changes of direction. The foot remains dimensionally stable and stiff, although every time it rolls off, forces are placed on our forefoot that can correspond to a multiple of our body weight. The reason for this was previously assumed by anatomists and biomechanics in the longitudinal arch of our foot – the arched arch that extends from the ball of the foot to the hind foot. Held by a network of stable tendons, this arch supports the midfoot and gives it elastic strength. According to studies, the longitudinal arch is responsible for around 25 percent of foot stability. But the human foot still has a second, transverse arch, the role of which has hardly been investigated, as Madhusudhan Venkadesan from Yale University in New Haven and his colleagues explain.

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Scheme of the foot and its arch. (Image: M. Venkadesan)

Arch test

To find out whether and to what extent the transverse arch of the foot contributes to its strength, Venkadesan and his team have now carried out a number of different experiments. First, they examined the relationship between curvature and stiffness, both virtually and real, using a simplified model of the arch of the foot. This consisted of three rods connected with feathers to replace the metatarsals and tendons. The researchers now changed the curvature of this construct and determined which perpendicular forces it could withstand. It turned out that the curvature and the connecting transverse ligaments of this midfoot model are decisive for its strength. “We have found that plastic models and simulations with a more pronounced transverse curvature are more difficult to bend than flat ones,” reports co-author Mahesh Bandi from the OIST Graduate University in Japan. “In contrast, an increase in the curvature of the longitudinal vault had little effect on the rigidity.”

However, the question remained unanswered whether this connection also applies to the much more complex ensemble of the human foot. Venkadesan and his colleagues therefore carried out additional experiments with the feet of corpses. They recorded the effect of force and flexibility once with intact tendons and once with severed transverse tendons – if these tendons are missing, the transverse arch also loses its curvature. “The experiments show that the tissues between the metatarsals contribute substantially to the foot stiffness, and this more than the previously determined contribution of the longitudinal arch and its tendons of around 23 percent,” report the researchers. Accordingly, the transverse arch contributes more than 40 percent to the flexural rigidity of the human foot.

A new look at the foot of the people in front

The scientists compare the principle of action of the transverse vault to the bulging of a banknote: if it is curved, it is difficult to push it down at one end. “The same applies to the foot,” says Venkadesan. “Of course it is 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 shift our weight onto the balls of the foot, the weight increases the pressure on the metatarsals and tightens them Tendons of the transverse vault. This makes the midfoot stiff and ensures that it does not give in to pressure by buckling. According to the researchers, their discovery upset the image of our foot and its biomechanical functioning that has been valid for almost a century. Because instead of the much-studied longitudinal arch, the previously neglected transverse arch plays the main role in the special stability of our feet.

This could also shed new light on the evolution of the human foot and the gait of our ancestors. Because the footprints in Laetoli, Africa, show that the prehistoric Australopithecus afarensis was similar to modern humans 3.4 million years ago, even though this species had no longitudinal arch in the foot. However, unlike apes, the Australopithecus already had a slight transverse arch, which could have given its foot the necessary stability for walking upright. Our results indicate that a human-like transverse vault could have developed around 2.5 million years ago – 1.5 million years before the genus Homo was created, ”says Venkadesan. “This could have been a crucial step on the way to anatomically modern humans.”

Source: Madhusudhan Venkadesan (Yale University, New Haven) et al., Nature, doi: 10.1038 / s41586-020-2053-y

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