There's the heart and muscles. Assuming the same power output a lower cadence (bigger gear) will demand more of the muscles and a higher cadence will demand more of the heart/aerobic system. The trick is calling on the right system for a given situation and ideally both would simultaneously exhaust at the end of the ride/race. I try to push my aerobic system more for as long as possible (high cadence TTs for instance) then when the end is near I'll push a big gear until my legs scream to stop...then go a little bit more. GL