Biological legs vs Cheetah prosthetic legs - which is better for running? EDIT: I misspelled "which" in the subject line, how embarrassing. You may have heard of Oscar Pistorius, a double below-knee amputee from South Africa who recently won the silver medal in the 400m at the South African senior athletics championships (did I mention everyone else in the race was running on his natural legs?). However, he may be excluded from further competing in "able-bodied" events because some feel that his artificial legs give him an unfair advantage by virtue of being longer then natural legs. Others say that his legs are a disadvantage, since unlike natural legs, they are basically just springs and can not generate energy. Here is the statement about this that seems to be widely circulating: To me, these statements seem badly incomplete at best, and purely nonsensical at worst. Supposedly, the natrual ankle returns 250% of the stored energy. BUT HOW CAN YOU EVEN TALK ABOUT THE ENERGY RETURN OF A SYSTEM WHICH NOT ONLY STORES ENERGY, BUT ALSO GENERATES IT? I would think the generated energy wouldn't depend too much on the stored energy, so if there's little stored energy, then the generated energy can be ten times greater than the stored energy. On the other hand, if there's a lot of stored energy, then the energy generated would be much less than the energy stored. Anyway, is the ankle even good at storing energy? Try jumping of a height of only two feet or so and landing on your feet WITHOUT bending any other joints. You probably can't do this. I think the reason for this is that your body knows that your feet alone simply cannot handle this impact and it brings your knees and hips into action to absorb the shock. So it seems that the natural ankle isn't very good at all at absorbing and storing energy. On the other hand, if you were jumping on very springy artificial feet, you could conceivably jump from the second floor, land without bending any joints, and be thrown back up to the second floor (slightly lower due to energy absorption). Of course, I was discussing jumping, and running is not the same thing as jumping, but thankfully, basic mechanics applies to running as well. So what's going on here? What in the world do people mean when they say that the biological ankle returns 250% of the stored energy?