Theoretical considerations and practical results on the influence of the representation of the foot for the estimation of internal forces with models.

With a hypothetical experiment the differences in internal force estimates due to different mechanical representations of the foot are demonstrated. It is shown that the magnitude of the internal force estimates is dependent on the formulation of the model. A comparative approach (the systematic change of one condition while all other parameters are kept as constant as possible) is suggested to eliminate the influence of systematic errors. The implications of the hypothetical experiment are applied to a practical situation using four different specifications of a generalized three-dimensional six-segment model of the foot. The four specifications vary with respect to the degrees of freedom at the six joints, the distribution algorithm, and the maximum muscle force criterion. The bone-to-bone contact force at the tibio-talar joint during a lateral side shuffle movement executed with two different shoes is used to investigate the influence of the model specifications. The four models estimate distinctly different force magnitudes; however, the result of the comparison between the two shoe conditions is similar for all four models. This is an indication that models should primarily be used for the investigation of the influence of systematically changed external conditions on internal force estimates.