And yes, I realize "mental energy reserve" isn't a scientific term, it's just a term I'm using for lack of something better to label something I experience.
Re: the football team though, no matter how energized they are, that team will not be able to lift more than their 1RM (1 rep max), they won't be able to lift their 10RM (10 rep max) more than 10 times (maybe a few more on a good day). Eventually there are physical limits to their ability. Similarly if that team had not slept for 48 hours, or had done non-stop math problems for 18 hours and was asked to continue doing them.