Investigation of the optimum heel pad stiffness: a modeling study

Due to the controversy regarding the criterion for diagnosing heel pad (HP) pathology based on evaluating the state of the heel pad stiffness (HPS), this paper intended to apply modeling to understand the influence of the HPS on the mechanical responses of the HP at heelstrike during locomotion, in an attempt to investigate the optimum HPS in terms of the health-related mechanical responses. Two different models, a finite element model and a classical mechanical model, were used to simulate the mechanical responses (force loading and deformation) experienced by the HP at heelstrike. Both excessive force loading and deformation are believed to be detrimental to the heel pad. In the simulation, the corresponding force loading, deformation and net effect (the adding of the normalized force loading and deformation) were calculated for each HPS value. Two models found consistent trends that the stiffer the HP, the greater the force loading and the lower the deformation. In contrast, a softer HP experienced a lower force loading and a greater deformation. Both the force loading and deformation were at medium levels and the net effect was minimal at a HPS value between the highest and lowest values used in the simulation. The modeling result suggested that the optimum HPS should be in a state at which both the force loading and deformation were at medium levels and the net effect was minimal, in terms of the health-related mechanical responses. The abnormal level of HPS, either too high or too low, may correlate to respective pathologies.