CHI, K.-J.*; SCHMITT, D.: How can a heel pad perform multiple mechanical roles?

During each step the human foot decelerates and reaccelerates. As a result, the heel pad must cushion impact, stabilize the foot, and return elastic energy. To perform each function, pads must have specific mechanical properties. To cushion impact, the pad has to be compliant and damped. To stabilize the foot, the pad has to be stiff to resist foot displacement. To return elastic energy, the pad has to be resilient. The properties required for these three roles appear to conflict. For example, stiffness, a measure of resistance to deformation, is the opposite of compliance. Similarly, resilience, a measure of elastic return, is the opposite of damping. So the question remains: how can this apparent paradox of pad properties (i.e. the need to be stiff and compliant, resilient and damped) be resolved so that a pad can perform multiple roles in locomotion? To date, only compressive properties of the stabilized heel pads have been examined. It is unknown how the pads behave mechanically in barefoot walking or running. In this study, I synchronously recorded kinetic and kinematic data at 1000 Hz on the load and displacement of human heel pads during walking and running (N=85). I calculated pad stiffness and estimated the energy loss in the pad during impact. My results suggest that: (1) Heel pads are subject to both compression and shear during impact and shear is more significant in running. (2) The resultant pad stiffness depends primarily on the shear component. (3) Heel pads show anisotropic properties: Pad stiffness is greater during rearward shear as compared to forward shear, and the pad dissipates more energy in shear compared to compression. This anisotropy allows a heel pad to perform multiple mechanical roles in locomotion.