Our feet apply large forces on the ground every time we walk or run. These forces can be several times our bodyweight, and at push-off, most of the force is concentrated on the front of the foot, known as the ball of the foot. In order for us to push in this manner, it is important for the foot to maintain its shape. That is the role of stiffness, a property that helps to maintain shape. Stiff feet were important for the evolution of bipedalism, and humans are unique among primates to possess such stiffness. So, we studied the human foot to understand where its stiffness comes from.
Loads on the foot
As the videos above show, the ground forces are quite large at the moment of push-off. Just when the front of the foot pushes on the ground to launch into the next step, the ground forces are in excess of our bodyweight.
At the same time as the forces act on the front of the foot, the ankle itself is fairly firm. It does not flop around. This combination of forces on the foot, from the ankle on the heel-end and from the ground at the front, resembles that shown in the picture below.
This pattern of loads is called three-point bending. The body pushes down on the foot and other forces such as from the Achilles tendon pull up. This results in a torque acting at the ankle in addition to the downward force applied by the rest of the body.
How to measure stiffness?
Static stiffness in three-point bending is measured by applying a displacement and measuring the force required to apply that displacement.
Foot stiffness is not a static quantity because muscles of the foot can modify its stiffness. But it does so around a baseline, which is measured by static stiffness measurements of cadaveric feet. The loading conditions, with the rollers on the heel and a slippery top on the forefront, mimic the forces felt by the foot in locomotion.