Now, do these prosthetic devices propel him forward better than legs? Why not just measure his stride. Surely if he was being propelled, his stride would be longer (is that an advantage necessarily anyway).
Or, is he expending less effort because of this spring effect? Well, this type of thing can be measured as well to a certain extent.
In the end, I kinda doubt we are going to see double amputees dominating the olympics anytime soon because of this amazing advantage they now have.
Who knows, maybe parents will be chopping off their childrens legs so they can later win olympic gold.
"It is no measure of health to be well adjusted to a profoundly sick society.” ~ Jiddu Krishnamurti