As a guy who tends to switch back and forth between periods of high activity (such as long-distance hiking) followed by months at a time of largely sedentary desk jobs, I've started to notice something about my body: I have two equilibrium states.
By this I mean ...
Picture a graph mapping exertion (X axis) against comfort (Y axis). The graph obviously flatlines at high exertions (I've pushed my body too hard, and muscle ache grinds everything to a halt), and at extremely low exertions (enforced bed rest weakens everything and the lack of mobility starts to drive your muscles nuts). But there's a peak somewhere in the range of mostly-sedentary activity, where my body adapts to its low demands and I feel okay even though my strength and mobility are crap; and there's a second peak somewhere in the physical-fitness range, where everything's working, and I'm limber and rugged.
If I'm at the sedentary peak and I want to move toward the fitness peak, it hurts during the upward transition (trying to limber up stiff, little-used muscles). And if I'm at the fitness peak and slacking off, it hurts during the downward transition (where all my finely-tuned muscles overcompensate for the trivial work I'm doing, and I have to do a LOT of stretching to keep from spending all day sore). So the graph has a shape resembling an M.
Edited to add: This post would have been both a lot clearer and a lot wittier if I'd just posted the damn picture instead of writing the thousand words. Gotta reinstall Photoshop one of these days. :-p