At heel strike the vertical ground reaction force is located anterior to the axis of rotation of the knee joint. Momentarily, this creates an external knee extension moment during the first percent of stance. To counteract this moment, an internal knee flexion moment is generated to assist with stabilizing the knee joint as it prepares for weight acceptance. Once the foot is flat the vertical ground reaction force moves posterior to the knee joint as the trunk glides over the stance foot, resulting in an external knee flexion moment. This is counteracted by an internal knee extension moment which continues through the rest of stance. This knee extension moment assists with weight acceptance and deceleration of the forward motion of the center of mass.Neptune RR, Zajac FE, Kautz SA. Muscle force redistributes segmental power for body progression during walking. Gait Posture. 2004;19(2):194-205.
