Background The plantar heel pad is a specialised fibroadipose tissue that attenuates and dissipates, in part, the impact energy associated with heel strike. Although near maximal deformation of the heel pad has been shown to occur during running,1 in vivo measurement of the mechanical properties of the heel pad during walking remain largely unexplored. This study used a digital fluoroscope, synchronised with a pressure plate, to obtain force–deformation data for the heel pad during walking.
Methods Dynamic lateral foot radiographs were acquired from sixteen healthy adults while walking at their preferred speed. Sagittal thickness and deformation of the heel pad relative to the support surface were calculated. Simultaneous measurement of peak force beneath the heel was used to estimate the principal structural properties of the heel pad.
Results Transient loading profiles associated with walking induced rapidly changing deformation rates in the heel pad and resulted in irregular load–deformation curves. The initial stiffness (32±11 N.mm–1) of the heel pad was an order of magnitude lower than its final stiffness (212±125 N.mm–1) and only 1.0 J of energy, on average, was dissipated by the heel pad during walking. Peak deformation (10.3 mm) approached that predicted for the limit of pain tolerance (10.7 mm).
Discussion/Conclusions Peak deformation of the heel pad approached that predicted for the limit of pain tolerance,2 suggesting the heel pad operates close to its pain threshold even at speeds encountered during walking. These findings raise questions as to the function of the heel pad during gait, highlight the need for alternative energy dissipating mechanisms during high impact activities often encountered in sport, and provide insight as to why barefoot runners adopt ‘forefoot’ strike patterns that minimise heel loading.
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