We now have to show how muscles attached to the hip, the upper leg, and the lower leg can coordinate in a walk. At this point we show one pair of muscles that control whether the upper leg lifts relative to the hip. We show another pair of muscles that rotate the lower leg relative to the upper leg using the knee. We apply another major simplification by ignoring the ankle. We also assume that the walk is in a straight line without turns.
We simplify the action by focusing on the hip joint, and by ignoring any muscles needed to keep the knee straight or to hold the ankle. We also ignore musles required for balancing, etc.