What makes human feet so stiff? Human feet have an arched shape, which has a role in its stiffness. There are two arches, the medial longitudinal arch (MLA) and the transverse tarsal arch (TTA). Prior research has focused almost entirely on the MLA alone, but our study shows that the TTA is an even bigger contributor to the foot’s stiffness.
The MLA runs from heel to toe along the inside the of foot and is seen in the view shown here. Past research has quantified how it affects stiffness using many different methods, including three-point bending tests on cadaveric feet.
The TTA is located around the middle of the foot. The tarsal bones (see below for the anatomy) are arranged in an arc that spans the width of the foot. The metatarsals that attach to the tarsals are therefore also arranged in an arc at one end. Our study shows that the TTA has an even bigger role in the foot’s stiffness than the MLA.
See the Physics Section for an explanation of the how the TTA increases stiffness.
The two arches of the human foot help to stiffen the foot against the forces acting on it during locomotion.
Each foot has 26 to 28 bones and the two feet make up almost 25% of all the bones in the human body.
The long, ray-like bones that run from the middle of the foot to the toes are called the metatarsals. Right behind them, toward the heel, are a set of bones with complicated geometry, known as the tarsal bones or simply the tarsals. In non-human primates, it is mostly these tarsal bones that severely bend when they walk upright.
Medial Longitudinal Arch (MLA)
The plantar fascia, running from heel to toe, contributes around 25% to the foot’s stiffness through the MLA.
Anatomical image showing the plantar fascia is from Primal Pictures.
The elastic tissues spanning the MLA, especially the plantar fascia, create a bow-and-string structure. The bow is formed by the MLA and involves many ligaments that gives it stiffness. In addition, the string that is formed by the plantar fascia provide additional stiffness. Higher the MLA, greater the stiffness contribution of the plantar fascia.
This mechanical function of the MLA and plantar fascia is illustrated with the overlay showing a segmented model with a spring.
The MLA has been the main topic of study for nearly a century and current definitions of the foot’s shape are based upon the MLA.
Transverse Tarsal Arch (TTA)
Upon cutting the tissue between the metatarsals, the foot’s three-point bending stiffness nearly halved.
Because of the TTA, trying to bend the foot is expected to stretch the tissues between the metatarsals and increase stiffness. We measured stiffness in the foot using three-point bending tests on cadaveric feet. We cut the tissue between the metatarsals, much like the demonstration on a mechanical mimic at the top of this page to test the role of the TTA. The stiffness nearly halved when we cut the tissue. Prior work has shown that the plantar fascia contribute around 25%, thus making the TTA one of the biggest contributors to the foot’s stiffness.