Why do we regain energy quickly after a workout?

[Mallignamius]

After lifting a few weights, or running, or whatever, we need to rest a while before we can get going again. I can lift weights for a while, then I need to rest a few minutes before I can continue. I don't need to refuel (eat) for some time to have more energy later.

What is going on inside the body that lets us return to work so quickly? It only takes a minute or two, and I can lift the same weights almost the same number of times. Of course I'm pacing myself, maxing out. But I would like to understand what this is, where I can find more information to study it further.

I know it's physiology, has to do with energy, but that's about it.

EDIT: Whoops. I guess this belongs in Homework & Coursework Questions> Other Sciences
Sorry.

[grant9076]

The way I understand it is that the ADP molecules are recycled back into ATP. Because this is a non-spontaneous reaction, it is combined with oxidation of fats in the citric acid cycle to bring about oxidative phosphorylation.

In addition, the lactic acid is transported in the blood back to the liver where it is converted to pyruvic acid and then back to glucose (gluconeogenesis via the cori cycle). This is also a non-spontaneous reaction which is also combined with aerobic lipolysis.

This means that much of what you burn during a weight lifting session is fat (between sets). The aerobic lipolysis also explains the oxygen debt and the heavy breathing between sets.

Here is a source that I came across. It is pretty concise but it is not well documented. There might be other more well documented sources out there.
http://www.spartafit.com/articles/energy%20systems.php

[Mallignamius]

Excellent. That at least gives me sources to study so I can understand this. Thanks!