The influence of heel pad confinement on heel pad mechanical properties

Abstract

This study examined the effects of different amounts of heel pad confinement on the mechanical behaviour of the human heel pad. Confinement can be manipulated in two ways: containment defined as the act of heel pad confinement by an external device (representing the heel counter) from the impact surface superiorly, and exposure defined as the amount of heel pad left unconfined distally before the start of the containment device. It is hypothesised that with greater containment, the heel pad will demonstrate less maximal deformation and increased heel pad stiffness compared with the uncontained heel pad. With increasing exposure, it is hypothesised that the heel pad will demonstrate greater maximum deformation and decreased heel pad stiffness compared with the uncontained heel pad. Cadaver heel pads were compressed using loading profiles based on vertical ground reaction force profiles collected from 11 subjects running at their preferred running velocities. Ten cadaver heel pads were each loaded 60 times to peak forces scaled to body weight with a constant time interval between loadings, under different conditions of containment and exposure. Statistical comparisons indicated that fully containing the human heel pad significantly alters the heel pad mechanical behaviour by decreasing the maximum deformation and increasing its stiffness. Thus, these results supported the first hypothesis. Statistical comparisons indicated that partially exposing human heel pad significantly alters the heel pad mechanical behaviour by decreasing the maximum deformation and increasing its stiffness, to a greater extent than fully containing the heel pad. These results are the reverse of the predictions of the second hypothesis. Overall these results have implications for heel pad mechanical behaviour when shod, and for footwear design.