Originally Posted by BendtheBar
I can't speak for others, but my joints are the first to go. I feel 10 years younger now that I'm not doing physical labor. I use to wake up unable to feel my hands.
Joints are usually the most problematic, from what I hear.
That said, I think there is a distinct difference between overtraining (which I don't believe exists) and burning out. If you don't listen to your body and slowly adapt your volume to how your body cycles itself, you can get injured. That's why if you're squatting 3x a week and one day you feel weak and doing lots of volume seems impossible, you ease up for that day and do something else. If you train like a robot, you break down like one.
Theoretically, it is impossible to overtrain even if your intensity of training exceeds your recovery capacity. In that case, your body will not be capable of using maximal weights because of how fatigued it is (although they might FEEL like a max). As it tries to adapt, you'll get weaker, but only for a certain time.
In the end, listening your body and easing into volume is the way to go, IMO.