Consider a human walking.
Pay attention to the side view and what happens with the foot, ankle, and knee. As the planted foot goes underneath the body it starts extending, lifting the heel off the ground and then the toes giving a final thrust. After that thrust happens, the knee is bending allowing the toes to clear the ground, then the upper leg swings forward with the lower following and swinging forward to bring it in a direct line with the upper leg so the foot is forward of the body, in position to plant the heel for the next step.
The lower leg is acting like a pendulum. If there were no muscles there at all the walking motion would still work, as you can see with someone who has a prosthetic limb after an above-the-knee amputation where the entire step is accomplished purely by the motion of the upper leg. Because the lower limb is acting like a free-swinging pendulum, there's very little energy required on the part of the lower leg muscles after the foot is lifted in order to get ready for the next step. Essentially, as the upper leg swings forward the lower leg is swung forward for free. Then, as it's planted, the body travels over the top because of the motion of the upper leg.
The only major exertion on the part of the lower leg is that final toe-thrust. And note that the upper leg and lower leg are straight when it happens, allowing the force to be transmitted as efficiently as possible to the whole body. If the knee bent as that thrust was happening, there'd be an inefficient energy transfer.
That's why things like crouch-running or walking can't be sustained: the always-bent knee means less efficient energy transfer. A person in reasonable shape can walk continuously for hours. Someone forced to walk with their knees even slightly bent the entire time might be able to go a few hundred meters and probably be in agony by the end.
Now look at your backwards knee. The lower leg can't exploit the pendulum effect because it will be required to be pulled upward in order to swing forward. While the human lower leg swings forward and is stopped without muscular effort from going further by the knee joint, in the backwards knee you need muscles to straighten the leg on the step and keep it straight because the natural inclination would be for the joint to fold forward. The muscles are forced to be working all the time.
Now think about what happens on the last part of the step, as the toes generate the thrust as the hell is lifted. Well, how is the heel lifted? Again looking at the normal knee: just as the toe thrust is finished, the upper leg swings forward, causing the knee to bend. This shortens the overall effective length of the leg, allowing the foot to just clear the ground as it swings forward.
On the backwards knee, that can't happen. There's no smooth, energy-efficient way to lift the lower leg in order to clear the ground as it moves forward. The only way I can see it happening is that reverse-kneed biped would have to continually hop, pushing off hard enough so there's enough room to clear the ground so the ankle and knee can be bent to pull them up to clear the ground. This will inevitably result in a lot more energy directed at moving the body up and down instead of forward, thus a lot less energy efficient overall.
There might be one way around the bouncing torso problem, and that's if the reverse-kneed biped never actually straightened the knee, keeping it flexed continuously to basically act as a shock absorber to minimize the amount of upward thrust directed into the torso. But then that again runs into the energy efficiency problem: a significant portion of the force created by the feet and toes isn't being used for forward motion and is lost.
Going back to the point I made about a prosthetic lower leg and how in a normal human knee it doesn't require anything other than the motion of the upper leg in order to function, a prosthetic lower leg on the part backwards-knee biped would require some kind of powered mechanism in order to function. That demonstrates the difference in energy efficiency between the two: one can function without power, one could not.