The time-dependent mechanical properties of the human heel pad in the context of locomotion.

Previous measurements of the mechanical properties of the heel pad, especially of the energy loss during a cycle of compressive loading and unloading, have given contrasting values according to whether the investigators used isolated single impacts (e.g. pendulum tests; energy loss approximately 48%) or continuous oscillations (energy loss approximately 30%). To investigate this discrepancy, rest periods were inserted between single compressive cycles, giving intermittent loading as in locomotion. The energy loss, measured as the percentage area of the hysteresis loop, was found to change linearly with the logarithm of the rest time. It was approximately 33% when the rest time was 1 s. Each 10-fold increase in the rest time added approximately 3.7% to the energy loss. Thus, with rest times appropriate to locomotion, the pad is far from fully relaxed. The springy heel pad may help to reposition the foot during the transfer of load from the heel to the forefoot. Information is also included on the load-deformation curves for the heel pad and the way in which these change with rest time. This is presented as equations which may be useful in future models relating the mechanical properties of the heel to either its structure or its function.