Just thought I would butt in here and add my 2 pence worth.
Training frequency...
It is all down to fatigue. You can thrash a bodypart in one workout, or you can spread the fatigue out across multiple workouts during one week. The difference being that if you are thrashing that bodypart in one workout then your CNS is getting fried and you will need 5-7 days before you train that bodypart again. Now for the majority of people this isn't optimal since the muscle itself can actually recover far faster. In which case we spread the work out throughout the week. Not only that but it usually enables you to get far more out of the mass building compound exercises. For example...
A Typical leg day on a one part per week split might look like the following:
Squats
Lunges
Hamstring curls
Leg extensions
Calf raises
God knows what else etc...etc....
If you did all that three times per week to failure then yes you would probably be overtraining pretty quickly. But when you look it at what is really going to be putting the mass on here? Yep, the squats. (Although lunges are great also). So what do we do? Drop the accessory isolation work (or at least keep it to a minimum) and instead up the frequency for the squats. (Usually with a lighter squat day for the second day if we are doing 3 full body's per week)
We are now squatting 2 or even 3 times per week. Since we arent blasting our CNS with set after set of isolation work we can get away with training the muscle more often and more effectively.
Don't get me wrong bodypart splits definately have their place and I personally still think they are a great introduction to weight lifting (though i'm sure some will disagree), but once a person knows what they are doing and is able to successfully manage their fatigue, there are far more optimal training methods. Then as we get more advanced still and linear progress is just no longer possible, we then move on to periodisation etc...
Good old Bill starr's 5x5 involves squatting 3 times per week and I wouldn't hesitate to say that particular routine has put more mass on natural athletes over the last 30 years than any other.
Basically as you increase the frequency of training you decrease the volume each day to compensate or you periodise the program. Think of it as accumulated fatigue. It's all a case of finding what you respond to best. Right now I am following DFHT with fantastic results.... In fact accidently. I have only been intending to maintain my current weight as I have been happy with it for a while now. Unfortunately DFHT has thrown a substantial amount on lean mass on me over the last few months without me intentionally bulking.
If you want to take a look at what DFHT involves check out this link:
http://forum.mesomorphosis.com/showthread.php?t=134233013
Scroll down to the section on Dual Factor Hypertrophy Training. If anyone decides to give this routine a shot remember that the unloading weeks are are
just as important as the loading weeks. If you don't unload then you will likely end up overtraining pretty quickly on this routine... Thats what periodisation is all about afterall.