Prosthetic limbs are usually constructed to withstand immense forces. There is a considerable degree of technology involved in making a standard leg, let alone one for a runner, which usually includes carbon fibre tubing and specifically designed carbon foot, that can absorb impact and return the stored energy in a controllable way. Then there’s the stump cup, which usually includes specific materials to minimise rubbing and often a non-return valve that allows the cup to suck onto the stump. I’ve been into it and the whole area very complex.
That’s not to say that it’s impossible to 3D print an entire leg. Just that it’s probably a whole lot harder that one might think. One only needs to look at the components involved to see that they’re made to aerospace standards…
It might be an idea to get in touch with e-nable, who provide 3D printed hands for people. They may be working on something.