When energy is considered in terms of (cal/kg/m), as in Section then the equation
has been shown experimentally to roughly model the relationship witnessed in humans between required energy and the average forward walking velocity v . This hyperbolic relationship has an energy minimizing walking velocity of 80 m/min. Figure 3 displays the relationship that we encounter in our experiments which, while reasonably hyperbolic, has a much lower energy minimizing velocity of approximately 12 m/min. A possible conjecture for the disparity with optimal human walking is the lack of the foot effect which provides essentially an extension of the leg when the back heel lifts off of the ground propelling the body forward and reducing the effects of collision.
Figure 3 also compares optimal walking with and without impulsive liftoff forces. The dashdot and dotted lines in indicate the energy relationship for walking with an impulsive liftoff force. A significant energy savings is obtained over walking without such a liftoff force (solid and dashed lines), though there is no noticeable change to the optimal walking speed. The effect of ankle torques, which is also displayed, is small.