Figure 1. Animations showing the peak external valgus knee moment and peak internal hip adduction moment on the lead leg in the early part of the downswing. Two different methods are shown here to cause a large external valgus moment on the lead knee: (a) keeping the knee aligned over the foot while producing a large laterally directed ground reaction force, (b) allowing the knee to collapse into a valgus position while producing a vertical ground reaction force.