During loading response, the GRF falls where relative to the knee axis?

During loading response, the GRF falls where relative to the knee axis?

• A. Posterior to the axis
• B. Anterior to the axis
• C. Medial to the axis only
• D. Through the exact center only

Answer: (A)

The key idea is how the line of action of the ground reaction force (GRF) relative to a joint’s axis creates an external moment at that joint. In loading response—the moment right after heel strike—the GRF vector sits behind the knee joint center. This posterior position generates an external knee flexion moment, meaning the GRF tendency is to bend the knee. The body counteracts this by controlled knee flexion and eccentric activity of the quadriceps to stabilize the knee and absorb shock. If the GRF were anterior to the knee axis, it would produce an external knee extension moment; being exactly through the axis would produce little or no moment. Therefore, the GRF falls posterior to the knee axis during loading response.

The key idea is how the line of action of the ground reaction force (GRF) relative to a joint’s axis creates an external moment at that joint. In loading response—the moment right after heel strike—the GRF vector sits behind the knee joint center. This posterior position generates an external knee flexion moment, meaning the GRF tendency is to bend the knee. The body counteracts this by controlled knee flexion and eccentric activity of the quadriceps to stabilize the knee and absorb shock. If the GRF were anterior to the knee axis, it would produce an external knee extension moment; being exactly through the axis would produce little or no moment. Therefore, the GRF falls posterior to the knee axis during loading response.