Impact of mediolateral segmentation on a multi-segment foot model

MacWilliams et al. report their progress on a multi-segment foot model, including mediolateral segmentation, from which they calculate kinematics and kinetics [MacWilliams BA, Cowley M, Nicholson DE. Foot kinematics and kinetics during adolescent gait. Gait Posture 2003;17:214-24]. Their methods are generally well described, and in concept, lead us to new insights regarding foot biomechanics during gait, including kinetics of several joints within the foot itself. However, forces and moments (i.e., force systems) between mediolaterally adjacent segments were ignored in their inverse dynamics calculations. We show theoretically that these omitted force systems instead corrupt the calculated proximal force systems. We also show empirically that physiologically reasonable moment-arms (10-20 mm) and omitted forces (one to four bodyweights) could change both the shape and magnitude of the calculated proximal force systems. In this regard, we demonstrate that these kinetic data need to be viewed with caution. Finally, we extend the concept used in our analysis to general biomechanical modeling.