Possible breakthrough for podiatry and foot biomechanics
The human foot is a complex nexus of 26 bones, 33 joints, and over 100 muscles, ligaments, and tendons. Its intricate design allows the foot to carry the body upright effortlessly; our short toes enable us to run, and our arch and heel allow our feet to evenly distribute weight, especially on uneven ground.
Nature published a study this week arguing that the arch that runs over the top of our foot is more important to our mobility than we have previously given it credit for. The authors provide evidence that this previously underappreciated feature- the transverse tarsal arch– accounts for more than 40% of the stiffness of the foot.
Their paper could mean a breakthrough in the world of foot biomechanics. Prosthetists will be able to create more accurate designs and orthopaedic doctors and podiatrists will have a better understanding of the human foot and how to treat it.
Testing the waters
The findings, compiled by researchers from the United States, Japan, and the United Kingdom, reveal that this upper arch collaborates with the lower arch running beneath the foot called the medial longitudinal arch. This arch-team makes up the stiffness of the feet- a phenomenon that is unique to humans. For example, primates have more flexible feet that allow them to grasp tree branches. This feature is what allows us to take off running without falling over.
The paper’s main author, Madhusudhan Venkadesan, a mechanical engineering Yale professor, said that their findings surprised them, as the focus has always been on the long arch beneath the foot.
To test that the transverse-arch curvature may help prevent the bending of the foot and increase its stiffness, the researchers used a simple analogy- that this purported stiffening process is much like the way a pizza becomes less floppy if its outer crust curls upwards.
Initially, the team took a theoretical approach by modelling an elastic shell. They found that by increasing the transverse curvature, the stiffness of the shell increased along its breadth. This confirms that there is a marked transition point beyond which the amount of curvature directly impacts the vertical stiffness.
Venkadesan et al. then used a human cadaver specimen to test the relevance to the stiffness of the human foot arch. The specimen was thawed after freezing to combat rigor mortis and then ligaments were cut along the transverse arch. The team then assessed how the foot deformed when pressure was applied to it. This experiment found that stiffness reduced by over 40%.
In comparison, previous research found that cutting ligaments along the medial longitudinal arch reduced stiffness by 23%. This suggests that the transverse arch plays a more crucial role in the stiffening of the feet.
Breakthrough for orthopaedics and podiatry
What does this discovery mean for people with noticeably flat feet? This research indicates that they have potentially less stiffness in their feet than people with higher arches. However, it is also possible that flat-footed individuals have sufficient transversal arch curvature to compensate for their low median arches, enabling them to effectively walk and run.
More research into the transverse arch is needed to determine whether it contributes to locomotor performance, and how this impacts individual foot stiffness. These findings will have significant implications for prosthetists and the field of orthopaedics and podiatry.
Venkadesan and his team believe that it is conceivable that new treatments focused on the contribution of the transverse arch could be developed for several foot disorders.