THE TIME-DEPENDENT MECHANICAL-PROPERTIES OF THE HUMAN HEEL PAD IN THECONTEXT OF LOCOMOTION

Previous measurements of the mechanical properties of the heel pad, es pecially of the energy loss during a cycle of compressive loading and unloading, have given contrasting values according to whether the inve stigators used isolated single impacts (e.g. pendulum tests; energy lo ss approximately 48%) or continuous oscillations (energy loss approxim ately 30%). To investigate this discrepancy, rest periods were inserte d between single compressive cycles, giving intermittent loading as in locomotion. The energy loss, measured as the percentage area of the h ysteresis 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 ener gy loss, Thus, with rest times appropriate to locomotion, the pad is f ar from fully relaxed. The springy heel pad may help to reposition the foot during the transfer of load from the heel to the forefoot. Infor mation is also included on the load-deformation curves for the heel pa d and the way in which these change with rest time. This is presented as equations which may be useful in future models relating the mechani cal properties of the heel to either its structure or its function.