This can actually be an alternative green vehicle.... if it works!
Here is the idea: When we jump, our leg muscles are pushing us up and or forward. The stronger they are, the higher or faster we can go.
We go back down, we are putting that energy back to the ground. The result is a bit of wasted energy, and a hit back from the ground to us (which is also unpleasant).
Now if we use a couple of springs with the right level of elasticity (which will depend on each person's weight and height) we can achieve two things:
1. The energy our body's weight is pushing down will be absorbed by the spring, and so we experience less bounce pressure in our legs (pleasant feeling)
2. We can use the same energy for jumping again (or next step of running) and so jump higher or faster.
I am not talking about these:
The problem with these is that they are not a natural fit for how "we jump" or "we run" because they are replacing the nature of the end of our legs by appending the spring to our legs.
What I am talking about is building some sort of "boots" that will add a relatively weak spring in parallel to our legs. In other words, our legs and brain are still doing the main thing, leading the jump or running, but we are effectively making our leg muscles stronger.
Any one interested in working on a proto type? I wish I had time!