Recently my training partner and I had a little discussion. It went from pre-exhaustion theories to training splits to bio-mechanics. Example: you'd think that pre-exhausting the biceps before a back workout would be a good way to help isolation, but I don't use that technique, do you? Or training back and biceps together: makes sense and a lot of people do it, but I never do. Why not?
For people who use high intensity training principles, overtraining is an easy thing to do by accident. Generally, you are training the main body part so hard that all the stabilizer and antagonist muscles get worked a little too. And everybody trains so hard, right? :P Here's an example: the biceps are a major factor in rows for the back, so why train them again later in the week when you could lump all the stress into one day and increase recovery time significantly? This is why training back and biceps together, or chest and triceps together, or quads and hamstrings together, is really popular. Granted, there are a lot of bros out there just following whatever split they read in Muscle & Fitness without a single thought to the rationale behind it, but these people are usually also training so poorly that such a split is probably better for them anyways.
But if you're paying attention in the gym and you're consistent, you're also consistently getting better at isolating. You're intensity is constantly evolving and improving. You're always learning how to do something better. So when you get to this point, combining agonist, antagonist, and/or stabilizers in a single workout becomes a bad idea--you can isolate everything well enough to not be indirectly working anything too hard, so why limit yourself? Attempting to train two major bodyparts that are involved with one another becomes problematic no matter how you look at it, especially when you are able to target each muscle group so well. Either you limit yourself on one in order to benefit the other, or you go all out on one and the other is limited by default. There's no way around it.
So why not pre-exhaust instead? You could do a few sets for biceps before you start your back workout and by tiring the biceps out, reduce its involvement in the workout. By doing this you could improve isolation in the back and still not impact the biceps severely enough to limit recovery, right? On paper it looks real good, but unfortunately this falls into the same category as spot-reduction: it just doesn't work that way, honeychild. Just like you can't melt the fat layer over your abs with ab training, you can't fully limit the involvement of secondary movers. The body is a systemic organism, your back and biceps are meant to work together. Despite your best efforts to isolate, your biceps will be helping your back move all that iron. But this involvement, based on my prior assertion, will not negatively affect biceps training at a separate juncture because the biceps is 1) accustomed to aiding in the rotation and contraction of the shoulder girdle and 2) proportionately developed in relation to the back, and therefore should not be overly taxed by even heavy back training.
The simplest solution is usually the best, but no split is going to be perfect. There are a lot of bodyparts that affect each other. Wait, you said that that shouldn't matter, right? Well, there's a difference between training biceps the day before back, and training them at the same time. One is an acute energy problem, one is a chronic recovery problem. You will encounter conflicts like this on every day of your split. Sore triceps will always be a limiting factor on presses, sore biceps will always be a factor on pulls, sore shoulders will affect back and chest, sore quads will affect hamstrings and vice versa... The list goes on, and the multitude of overlapping muscle groups further reinforces the "systemic organism" bit. But sore chest won't affect triceps, sore back won't affect biceps, sore shoulders won't affect quads or hamstrings--see where I'm going with this? You can figure out the rest to fit your schedule.
It looks like intensity and volume will be my next topic since those are both a factor as well.