Deformation characteristics of the heel region of the shod foot during a simulated heel strike: the effect of varying midsole hardness.

Impact tests using a pendulum were performed on the shod heel region of nine subjects. Both soft- and hard-soled shoes were used. The deformations involved were calculated from the registered decelerations during impact. Thus, load-deformation cycles were recorded for various impact velocities. In contrast to in vivo force-platform recordings, peak loadings for the soft- and hard-soled conditions differed significantly (614 +/- 29 N vs 864 +/- 49 N, respectively), thus challenging the evidence for compensation at the level of the heel pad. Moreover, computation of the compression of the heel pad in the shoe showed an unexpected inverse relationship between shoe midsole hardness and degree of heel pad compression: the harder the midsole, the smaller the compression (soft shoe 7.6 +/- 0.9 mm; hard shoe 6.7 +/- 0.9 mm). This can be explained by assuming a loading rate dependent stiffness of the heel pad in the shod condition (stiffness in N.m-1 = 51.25x (loading rate in N.s-1)0.76; R2 = 0.90), determined by the visco-elastic nature of the heel pad and the spatial confinement of the heel counter of the shoe.