Let us assume that the centre of gravity is moving forward. This could be because we are swaying forward, bending forward, or lifting something heavy.
To bring the centre of gravity back over this small rectangle requires us to change the angle of the ankle minutely. To simplify the analysis we assume that we do not change the angle of the knees or of the hip joints to correct the balance.
This typically happens automatically without coming to the awareness of the actor.
We now have to show how the muscles turn the foot up and down through the ankle. At this point we apply a major simplification by just showing a pair of muscles that control whether the foot turns up or down relative to the whole body through the lower leg, knee, upper leg, hip, etc..
We simplify the action by focusing on the ankle joint, and by ignoring any muscles needed to keep the knee straight or to hold the hip. We also ignore musles required for bending, etc.