Ground reaction force (GRF)
Magnitude and direction of GRF change continuously during stance phase – changes in the direction and magnitude of GRF influences muscle activity and the maintenance of equilibrium.
During static stance – the GRF vector passes anterior to the ankle and knee, posterior to the hip. During gait, determination of the position of the GRF relative to the ankle, knee and hip allow for an approximation of the external moments that are occurring at each joint.
eg if GRF is posterior to the knee, an external knee flexor moment is created – this must be countered by a knee extensor moment (provided by knee extensor muscles).
eg if GRF is lateral to the subtalar joint axis, an external pronation moment is created at the subtalar joint – this must be countered by a supinatory moment (this can be provided by the posterior tibial muscle or a foot orthoses).
GRF is measured by a force plate.