Pamela Andersons Left Nipple wrote:
Except it's not....
"A bad racing leg feels stiff like a baseball bat. A good one feels bouncy like a diving board. That's because leg designers have to maximize the limb's "energy return," making sure that when the foot hits the ground, as much of that energy as possible gets pushed back toward the sprinter. An efficient racing leg coils on impact and then promptly recoils, propelling the racer toward the finish line. Because of its total lack of shock absorption, walking long distances on a Cheetah is precarious and uncomfortable—this is not an everyday leg. After a race, sprinters change into something that looks a bit more like a human leg for the walk home."
A splendid example of the ignorance to which I referred previously. Why oh why do people insist on arguing about things they know nothing about!
The fact that a prosthesis deforms and recoils does not mean that it isn’t passive. That’s just the way it stores and returns energy. The material bends under the force applied by the runner to store energy and then recoils to release the energy.
In fact, your natural limb works quite the same way, storing and releasing energy to create a “spring” effect by virtue of moving joints and elastic tendons. The natural limb, however, has the additional advantage of active contribution by the muscles. Not to mention the fact that it’s a hell of a lot less awkward to run on.
By the way, I don’t know who produced that quote, but there’s a lot of misinformation in there. The energy return capacity of a prosthetic leg is directly related to it’s mechanical stiffness- the stiffer it is, the more energy it returns. That’s precisely why racing legs don’t absorb shock well. Another advantage of the natural limb is that it constantly changes its level of stiffness to accommodate the situation, simultaneously optimizing both shock absorption and propulsion. Not so with the prosthesis.