Amending previous post to not sound like a jerk (apologies if I did). While I agree that it only takes 24-48 hours for a muscle to recover and be ready for another bout, by "recovery" I mean total body. CNS fatigue is much more likely to occur training a body part at high intensity twice or thrice per week. So, while the muscle may be ready to go again, there is no reason to believe that the central nervous system is. Looking at the OP's plan from another POV, even if he did get away with doing his upper split twice weekly from a recovery standpoint, I would still argue it is a mistake, as he is only doing legs once per week. Unless he wants to look like a lightbulb, he needs to even that out.
ACSM, NSCA, NASM, etc, guidelines all recommend at least two days of training per week, and that's based upon the independent research of each body coming to nearly the same conclusion. If I had access to my ex phys texts right now, I'd be able to cite more of the specific science behind it, so I'll try to get that up here later.
CNS fatigue is a natural response to physical exertion, and is really only a concern when the training volume is extremely high without legitimate recovery periods. Over-training and sustained CNS fatigue is uncommon, and typically associated with the training volumes of high level athletes (though the common exerciser can ail from over-training if the volume is high enough). However, periodization can provide enough rest recovery (active and passive) to allow high training volumes without over-training.
There isn't a whole lot of research into the specific ratios of training volume to recovery time that lead to sustained CNS fatigue and over-training. Most research has instead dealt with symptoms analysis and treatment as well as the physiological mechanisms. However, it looks like a strong symptom precursor to clinically significant CNS fatigue and over-training is the sustained feeling of fatigue, soreness, and depression (physically and psychologically), regardless of recovery period.
So, if an individual finds that resistance training 3 days per week is leaving them feeling fatigued all of the time, then 3 days per week would be too much for that person. It's important to note that muscle soreness and fatigue are two distinctly different things here, and sustained soreness isn't necessarily indicative of over-training. But there is a large portion of the exercising population (athletes especially) that can training for various goals of muscular and cardiovascular development at 3+ days per week and sustain such a volume without clinically significant CNS fatigue.
I do agree with you, however, that the OP should be hitting the upper body, back, core, and legs in equal proportion, for plenty of reasons (injury prevention being chief among them). I just don't believe that once a week per muscle group is enough training to maximize developmental response.
