This principle has been starting to get some momentum finally, but I know there are still some nay-sayers, so I want to hopefully bring light to what "overtraining" is since people take that word out fo context.
Overtraining isn't training until your body starts to feel excruciating pain. Granted, you'll be the sorest you ever possibly have felt before, even days after, but it's basically training your muscles to their ABSOLUTE limit! The focus is not on sets, or reps, or even time (little to no rest periods). The focus is more of trying to reach your 1RM, and pushing that 1RM so by the next workout, you can do 2 or 3 reps of your 1RM, creating decently fast progress and a new way to shock your body into growth.
"So what do I do, if I don't count reps or sets or rest periods?" You may be asking. What your shooting for is listening to your body. Lift that weight until you can't anymore! Starting out, I started with lifting 50% of my 1RM until I couldn't anymore, and kept upping the weight till I reached my 1RM, and worked my way down the weights again (classic pyramid set). I eventually worked my way down to 30 pounds and just kept lifting it till I couldn't anymore. If you did the previous sets correctly, even the 30 pounds won't take long to burn out the muscle group. There's also a finisher set called the "5-5-5", where you do 5 bottom-range reps, followed by 5 top-range reps, and finally 5 full-range reps. Sounds easy, but when you been lifting all the weights until you couldn't lift anymore, it's a challenge!
Anyways, a lot of people confused by how this works, and to be specific, it focuses on TUT. Time Under Tension has been proven to build strong, functional, and fast muscle. The slower the tempo on your lifts, the more TUT there is. Less rest periods ALSO mean more TUT, since the muscles don't have time to recover completely and are still under tension. Also with no rep ranges, your not restricting how far your muscles can go, making them a LOT stronger and more durable. Overtraining is about pushing yourself to the absolute limit, which means not putting a number on what you do.
I do it, and it helped me gain 3 inches on my arms alone, and I can't wait to phase in chest, then finally legs. If you asked me how many sets I do, I'll answer, "I dunno, but I worked out for almost an hour and a half, and I'm BEAT! Probably will be for a few days too."
This sort of routine will make you do more reps than you normally did in a week! Last week I did over 550 reps just with biceps! I have no injuries, I was SORE a few days, but I went right back at it a few days later. You can either put this sort of routine in your regimen, or not, but it wouldn't hurt to try at least before you shoot it down, right?