i am not a bio(major) but i have a doubt:: when we climb a flight of stairs obviously we are putting ourselves in a higher gravitational potential and hence higher energy state. so, we need to expend some sort of biochemical energy and hence feel a bit tired. but, when we get down even we feel tired(not that much) and according to me we even expend energy in this case.now, again considering the gravitational potential energy, we now need to gain gravitational potential energy(consider that we climb up and down the stairs with the same velocity so no change in kinetic energy)1. isn't this a violation of the energy conservation?? if not, then let me know how our cells/muscles store the potential energy gained by climbing down??