Force-deformation properties of the human heel pad during barefoot walking

INTRODUCTION: The plantar heel pad is a specialized fibroadipose tissue that attenuates and, in part, dissipates the impact energy associated with heel strike. Although a near-maximal deformation of the heel pad has been shown during running, an in vivo measurement of the deformation and structural properties of the heel pad during walking remains largely unexplored. This study used a fluoroscope, synchronized with a pressure platform, to obtain force-deformation data for the heel pad during walking. METHODS: Dynamic lateral foot radiographs were acquired from 6 male and 10 female adults (mean ± SD; age = 45 ± 10 yr, height = 1.66 ± 0.10 m, and weight = 80.7 ± 10.8 kg) while walking barefoot at preferred speeds. The inferior aspect of the calcaneus was digitized, and the sagittal thickness and deformation of the heel pad relative to the support surface were calculated. A simultaneous measurement of the 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) of the heel pad was 10 times lower than its final stiffness (212 ± 125 N·mm), and on average, only 1.0 J of energy was dissipated by the heel pad with each step during walking. Peak deformation (10.3 mm) approached that predicted for the limit of pain tolerance (10.7 mm). CONCLUSION: These findings suggest that the heel pad operates close to its pain threshold even at speeds encountered during barefoot walking and provides insight as to why barefoot runners may adopt "forefoot" strike patterns that minimize heel loading.