Figure 1.
a: A theoretical scheme of the foot, the lower leg and thigh, and the force and joint moments that are present during heel strike. During the stance phase the foot exerts a force on the ground (GF, grey, downward arrow). The reaction force to this GF is the Ground Reaction Force (GRF, black upward arrow). The GRF is the force that brakes and propels the human body and that exerts extending and flexing moments on the respective joints. In the first half of the stance phase the GRF brakes the forward velocity, in the second have it propels the body. By measuring this GRF and its joint moment arms the external joint moments can be calculated, as illustrated: MHIP = aHIP •GRF. Like the GRF is the opposite of the force applied to the floor (GF), the external joint moments are the opposite of the moments generated internally around the joints by the muscles (Figure 4a-c). In the figure the curved arrows represent these internal moments. During normal walking velocities, the GRF is the major determinant of the external joint moments; and oppositely muscle function is a major determinant of GRF. 1b: This figure shows the leg and the GRF at about 40% of the stance phase; the GRF is directed in front of the ankle joint and behind the knee joint. Consequently the GRF has a dorsal flexion moment at the ankle joint and a flexion moment at the knee and the internal moments will be plantar flexion and knee joint extension. A forward displacement of the GRF will result in a smaller moment arm at the knee joint and a larger moment arm at the ankle joint, this will result in increase plantar flexion (Figure 4a) and decreased knee joint moments (Figure 4b). The black arrow represents the GRF in HC (GRFHC), the grey one the GRF in DPN (GRFDPN),. The curve arrows represent the internal joint moments at the knee and ankle joint (HC: black curved arrows, DPN: grey curved arrows).