Final answer:
A ground reaction force occurring behind the knee joint creates a knee extension moment, which tends to straighten the knee.
Step-by-step explanation:
If a ground reaction force occurs behind or posterior to the knee joint, it will create a knee extension moment. This means the force is applied in such a way that it would tend to straighten the knee rather than bend it. Extension moments are generally associated with forces applied behind the axis of rotation, which in the case of the knee, is at the joint itself. When muscles contract to counteract this force, they produce a compensatory effect allowing for the motion of extension. These principles are crucial for understanding joint mechanics and can be applied in various situations, such as sports movements or rehabilitation exercises.