The next question, following this framework, is that in order for a suspended leg to load a spring, it will be necessary to apply a force against some other par

The next question, following this framework, is that in order for a suspended leg to load a spring, it will be necessary to apply a force against some other part of the body -- so where does it go? On a bicycle this is the other foot, which is made possible by the pedals. But with a spring affixed to the leg, the most likely result is a force against the dorsal surface of the thigh -- not exactly a pleasant idea (try pushing two fingers into the back of your leg if you're curious). So then you are inventing something of an exoskeleton to manage this. Cycling is extremely efficient -- around 80% of the energy expended by the body is converted to forward motion, even in the simplest configuration, and this can be enhanced further under some circumstances.