Think back to the last time you stepped off a ledge expecting the ground to be closer than it was. Your foot and knee anticipated the ground at one distance but were straightened and stiff in the moment when your foot actually hit the ground. Consequently, the rather uncomfortable force of this unexpected impact moved up from your foot through your leg and to the rest of your body. In the field of biomechanics, we call this force moving through your body ground reaction force. …