The mechanical properties of the human heel pad: A paradox resolved

h viuo and in vitro mechanical testing of the human heel pad gave apparently different properties for this structure: the in uioo stiffness is about six times lower, whereas the percentage of energy dissipation is about three times higher (up to 95% loss). It was postulated that this divergence must be ascribed to the lower leg being involved in in uiuo heel pad testing. This hypothesis is presently evaluated by applying the two experimental procedures formerly used in the in uiuo (an instrumented pendulum) and in vitro (an Instron servo-hydraulic testing machine) investigations on the same isolated heel pad samples. Instron load-deformation cycles mimicking pendulum impacts (i.e. 'first loop-half cycles') are first evaluated and then compared to real pendulum impacts. When performed properly, the pendulum test procedure reveals the same mechanics for isolated heel pads as the Instron does. The load-deformation loops are basically identical. Thus similar non-linear stilInesses (about 900 kN m-i at body weight) and comparable amounts of energy dissipation (46.5655%) are found with both types of test, still being largely different from the former in uiuo results (150 kN m-t and 95%, respectively). Therefore, the present findings support the hypothesis that the presence of the entire lower leg in in uiuo tests indeed influences the outcome of the measurements. It must be concluded that the previously published in uiuo data, if interpreted for the heel pad alone, implied not only an incorrectly low resilience but also a value far too low for stiffness.