Where does the potential energy go when a limbed animal goes downhill?

Question

I am looking for a definitive answer on where the energy goes when a limbed animal goes downhill. For convenience, let's discuss a bipedal, e.g. human.

When going uphill, it's clear that the potential energy ultimately comes from the ATP-->ADP citric acid cycle. But where does it go when going downhill?

Here are possibilities I see:

1. The potential energy is returned to chemical energy via some fashion
2. The potential energy is turned into heat by friction in bunched up muscle sheathes sliding past each other, and it is carried away by blood
3. The potential energy is turned into heat by strain deformation in the muscles, tendons, and ligaments. Again, this heat is carried away by blood.

I don't believe #1 is the case, but am not a domain expert so I leave it for completeness. Plus, there's severe competitive advantage if an organism evolves a way to do, what is for all intents and purposes, regen braking.

I suspect it's #2 just because of the shear amount of heat which needs to be carried away. #3 would require a fair amount of localized heat generation, and tendons and ligaments are not, AFAIK, typically well served by the cardio-vascular system.

Answer 1

Our muscle system has no capability of collecting energy on its reverse cycle.

Our muscles have two states: on, off. On the off state, the muscle relaxes back to its unexcited position but does not have any built-in physiological system to absorb or collect any energy if work is done on it. The idea may have crossed your mind from electric cars but they do have electronics and battery to collect and store energy during deceleration.

For example, if you attach a frog leg muscle to a lever and try to contract it ofter extension no energy will be retained in it.

Also, the skeletomuscular system is always working, to maintain our balance, gait, breaking the stepwise falls into coordinated moves.