As I ruptured my achilles tendon 4 weeks ago, I have been working out 6x/week with a pure upper-body-only workout. Now, watching my shitty legs actually whither is a humbling experience, but I have learned something interesting about so-called "overtraining" in my upper body. I am working out each muscle group in my upper body 3x/week (I use a push/pull split) and I was worried about overtraining, so I was taking it easy every other workout. For the last few weeks though, I have been going to failure with increasing weights each and every day. My body, far from crying uncle, is actually responding and I am getting stronger/leaner and (hopefully) bigger. Now, I am (for now) a complete natty and have a fairly dialed in diet (175-250g protein/day, low carbs, etc -- not ready YET Gal/NoOne). And have been getting at least 8 hours of sleep a night (it's hard to do anything with a single leg and crutches).
I always believed in overtraining, but it was likely that I was just falling off somewhere else (sleep, nutrition).
Once i get my leg back, I am going to just go bonkers with a 3day push/pill/legs split. Fuck overtraining.