Is that what you were after?
nope. I know how muscles work, tear and repair
can't see that once a week is anywhere close to optimal though. muscles do not take a week to repair after a training session
Don't know the science behind it.
I know that it's tried and tested though and recommended by all but the elite body builders.
I guess a lot of it is to do with how hard you train. When I was training properly a single session would completely fuck up one body part. Usually to the point of not being able to do anything with it for the rest of the night, often to the point of being cramped and unable to move it, occasionally to the point of being physically sick.
If you train that hard you can't physically do anything else. It's just not possible. It'd be like asking someone to run 2 marathons in a day.
When you can only physically cope with training one bodypart per day it becomes a matter of time as to how regularly you can train that bodypart. You have 5 major muscle groups which equates to 5 days on and 2 days off.
Unless you're not training particularly hard it's just not possible to fit more in. The way I train at the moment I could happily do 2 body parts in a single session because I'm not training very hard. That will change though.
Elite body builders will train twice in a day thus being able to train each body part twice in a week or more. They will train at 7 in the morning then eat and sleep before training again in the afternoon. Then they eat more and sleep more. They do it as a job though so they have the time.
I suppose the answer to your question is that it isn't optimal with a primary goal of muscle building but it is optimal for someone who tries to lead a normal life alongside it.