Readers here for any length of time know that I also run with a similar set-up, although I only have one "store-bought" leg.
I've set out to run it again since then, but unfortunately, something has usually stalled my efforts, usually fit problems with the leg as it got older. And after I was sidelined for a while for other leg surgery last year, it wouldn't fit at all afterwards and a new one had to be custom made, a process that took months and we're still working out fine-tuning and adjustment issues that are keeping me from doing much serious running with it.
As one who knows all about these devices in a way that Michael Johnson can and hopefully will never understand, I can tell you that, yes, when they are fitting and working perfectly, they give you some extra spring. Maybe it's as much or even more than you get from a natural leg and ankle, but it's often offset by fit problems. If it's even a millimeter or two out of whack height-wise, it's not going to be as efficient and damage in the form of bruising or abrasions to the leg stump may occur. Sometimes the pain is a warning for me to stop, but other times the damage will start in one of those areas where there aren't many functioning nerves left, and when that happens, by the time I notice it, enough trauma has occurred to inhibit even walking for a few days. Oh, and adding to the game, my leg changes shape while running as a result of fluid loss through dehydration. Yours does too, and so does Michael Johnson's, but when mine does, now the leg doesn't fit as tight. It begins to slip and move, and at best it's efficiency is diminished. At worst, it moves far enough out of place that damage starts to occur, but that's rare. It also slips when I perspire and the socket liner starts to fill up with sweat. Now add to that the fact that it doesn't corner like a normal foot, and that it can't compensate for holes, rises and other sudden changes in topography like a regular ankle does, and when all is said and done, I'm grateful for my running foot but I have to give the advantage to regular feet at the end of the day.
On a perfect day when everything is working just like it's supposed to on a track, perhaps Oscar Pistorius might gain a bit in terms of energy return and speed. But if he does--and that's a pretty big "if"--it's almost assuredly cancelled out by fit problems and fatigue. Remember, that leg is still 5lbs or so of dead weight, and the calf muscle is totally useless if there's even one there. I have one, but it's no longer attached to anything at the distal end so it's effectively worthless.
Given a choice, would Oscar Pistorius trade the nebulous "advantage" that he allegedly gets from those prostethics for real feet? Hell, I know that I would. And as one who can fully appreciate the extra hard work that Oscar had to do to learn to run and run efficiently with those legs, especially to the degree that he has, I like to tell Michael Johnson to go fuck a duck. And if Johnson still has a jealousy problem over these legs, he can always go to the surgeon's and get one or two for himself. Funny how I don't see him rushing out to do that, though. He apparently doesn't want to raise himself up that badly; he just wants to bring potential betters down instead. Jackass.
