We simplify the action by focusing on the hip joint and the ankle of the foot that carries the weight. This foot therefore does not change position relative to the stage. The knee joint of this foot is also assumed not to change its angle.
This foot and hip make a right-angle triangle with the stage as one side, the foot (ankle) - knee - hip joint as the second side, and the line from the hip joint and perpendicular to the stage as the third. The triangle is in the plane defined by the direction of travel.
The hip, therefore, by changing its angle, changes the triangle and thus the position of the hip relative to the stage, since the weight-bearing foot does not slide along the stage.
Ignoring the contribution of rolling over the heel at the start, and of pushing of the ball of the foot or the toes at the end, the highest point for the weight-bearing hip-joint is when the leg is directly below the hip.