This study focused on development of a multi-body model of a lower extremity for representation of shearing and bending responses at the knee joint in lateral impacts to a leg and its application to an analysis of the effect of boundary conditions and ligament properties on these responses. The model was validated against published results of previously conducted experiments using postmortem human subjects. To assure the reliability of this validation, reanalysis of the experimental results was done to understand how strongly these results were affected by the type of initial injury. A parameter study of the lower extremity model suggested that in the initial impact phase, shearing responses at the knee joint are unlikely to be influenced by the friction between the foot and ground. However, the effect of this friction may not be negligible if the analysis is focused on a time window longer than that associated with the initial injury occurrence.