If you’ve never thought about those two extremities at the end of your legs, you’re missing out on a rather remarkable part of your anatomy.
It’s one of the features that sets humans apart from other primates, and enables you to walk and run on two legs so effectively.
I’m talking of course, about your feet.
When you’re looking at one of your feet, you’re probably aware of your longitudinal arch, which runs from your heel to the ball of your foot.
But you might not pay so much attention to your transverse arch, which runs across your foot.
New research, published today in the journal Nature, suggests this previously overlooked part of the human foot could play a major role in enabling you to walk and run so well.
Uniquely human feet
There are two characteristics of your feet that are uniquely human.
Firstly, your feet have a pronounced arched shape. Other primates have much flatter feet.
These arches are made of bone, but are supported by other structures in your foot like ligaments, fascia and other elastic tissues.
“Think of the foot as an arch-shaped skeleton that is held together by elastic tissues,” said lead author of the paper and mechanical engineer Madhusudhan Venkadesan of Yale University.
Secondly, your feet are much stiffer than those of other primates.
“The foot feels large forces from the ground in every step when we walk or run. These forces can be several times our body weight,” Dr Venkadesan said.
“Despite that, you can barely see the human foot bend.”
Whereas the feet of other primates bend quite significantly somewhere in the middle of their foot, Dr Venkadesan said.
Our feet absorb energy when they hit the ground to prevent really high forces in our bodies, and store that energy so it can be used later, said biomechanist Glen Lichtwark of the University of Queensland, who wasn’t involved in the study.
That means they have the stiffness and the flexibility to be both a lever and a spring.
What makes your feet so stiff?
Most of the past research looking at what makes human feet so stiff, and hence able to walk and run so effectively, has focussed on the role of the longitudinal arch.
However that feature alone is not sufficient to explain foot stiffness.
Dr Venkadesan and his colleagues instead focussed on the transverse arch and measured the bending stiffness of this structure.
They first looked at what effect the curvature of a thin sheet had on the stiffness of that sheet, and then looked at whether this principle held in the real world using computer simulations and experiments on curved blocks of material known as shells.
“We then proceeded to test the theory in more foot-like mechanical structures and eventually in real human feet [from cadavers],” Dr Venkadesan said.
By cutting the transverse tissues between the bones of the midfoot, they were able to show that the transverse arch is responsible for more than 40 per cent of the foot’s stiffness.
This far outweighs the contribution of the longitudinal arch, which previous research has shown was responsible for 23 per cent.
How human feet evolved
Dr Venkadesan and colleagues used this knowledge to look at the evolution of human-like walking.
“By applying this technique to fossils from extinct species that were part of our ancestry, we traced how the transverse arch evolved,” he said.
While evidence suggests that our ancestors may have first walked upright some 6 million years ago, or even earlier, we don’t see clear examples of a human-like heel-to-toe walking style until 3.66 million years ago with the footprints of Australopithecus afarensis.
“Our study places a transverse arch that is similar to humans’ at around 3.4 million years ago,” Dr Venkadesan said.
“[But] more fossils are needed … to draw firmer conclusions.”
Interestingly the longitudinal arch appears much later in the fossil record, probably only in the genus Homo.
From models to the real world
Biomedical engineer William Parr of the Surgical and Orthopaedic Research Laboratory at UNSW Sydney praised the paper for coming “full circle”.
The researchers have formulated a model and also validated the empirical results they’ve got from testing the model in the lab, Dr Parr said.
“What this means is that you’re very confident in the validity of the results that they presented.”
Dr Lichtwark said it’s surprising the intriguing concept hadn’t come up before.
But he cautions jumping to conclusions of how important the curvature of the transverse arch is as a mechanism, particularly in human foot health.
“Human feet are really variable … but even though the foot structure for instance can be quite different from one person to the next they can still have the same performance,” he said.
There could be other compensatory mechanisms that contribute to the stiffness of the arch, like the muscles or the ligaments.
Dr Lichtwark said he can see this work being applied in areas like shoe design.
“If we can now take into account things like curvature, you can imagine that we’d be able to better prescribe footwear and orthotics.”
Dr Parr said it might also lead to the development of new footwear which aims to control the stiffness of the shoe in the transverse plane, not just the longitudinal.
This finding could also have a significant impact in prosthetic limbs and robotic design.
“One of the biggest problems is a lot of these designs are single limb,” Dr Lichtwark said.
“Whereas if we can take advantage of the same sort of design they use in their paper … then we can actually create a foot which is much more similar to a human foot.”