One week, just out of curiosity, I exercised every muscle every day, but not the same exercises for those muscles (about one hour workouts). I found that my shoulders wore out first, then my chest muscles, but my back and leg muscles had enormous capacity as long as I rotated the exercises. I’m wondering if other people have had this experience.
Personally I suspect it says a lot about our evolutionary heritage that our back muscles have more endurance than our shoulder and chest muscles, but I digress.
I have not found a way to overtrain the back. Normally, the elbows give out first. I have worked up to 100 chins a day on top of 4 days of lifting and 1 day of event work, and my back still recovered.
An elite powerlifter with a 900-lb geared bench helped me prepare for my first meet. He had me train my upper back during every single training session. Many other lifters squat heavy every day as well.
I have done hundreds (minimum of 300, sometimes as high as 1000+ if I was bored) of some back movement or another every day for the past two months, and I feel great.