Speaking of rechargeable battery, one idea that has crossed my mind is wether there is wasted energy during walking or running that can be recovered to recharge the battery.
Your feet aren't made from rubber and you aren't bouncing on them when walking; you're dampening the impact shock. This suggests the energy of the "downstroke" is being wasted.
Achilles tendon is elastic in a way that does store downstroke energy, to release it later during the same step as weight moves forward. The energy saved is considerable and making use of this when running instead of dropping heel first gives unbelievable results in distance before exhaustion.
(Basically every tendon is this way, hence why it's a bad idea to stretch dynamically as you're just creating useless tendon fatigue, instead doing either static stretching, plyo, or explosive iso)
