The ground reaction vector during normal stance shows some interesting behaviour at heel strike, which you can see in this picture or movie. Notice that the vector is almost vertical at first, before quickly flipping backwards, i.e. the antero-posterior shear force is briefly zero before going strongly negative (posterior). This event is accompanied by a spike on the load trace, which is often called the heel-strike transient.
Notice that whilst it is vertical, the vector passes approximately through the ankle joint, so the ankle moment at this time is close to zero. The size of this transient very much depends on the characteristics of the force platform and associated data acquisition system used.
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