This maybe intresting for you:
http://www.apnea.nl/UK-Columnwk4.htm
Personally I think that your body needs a certain amount of time to learn and addapt, and this time cannot be rushed to much, or it will fireback, just like a 'burn-out' hapens to the work-a-holick.
Static training, is certainly heavy on the body when you practicing near max breathholds. And a max is different every time, some day's it may be 5, othe days only 4.
To counter this I changed my trainings to be 'effort' based. I practice O2 schedules like: 50% effort, 2 min rest, 50%, 2 min rest, 70% , 2 min rest, 90%, 2 min rest, 90%. Where the 50% means I come up with the _Idea_ I could do twice as long.
This means I still train nice when I'm having a 'bad' day. I'll let my buddy write down the times, and I read them after the whole schedule. This way I focus on learning and feeling, instead of the stupit beating the clock.
Nowaday's I train static only once a week, but in practice dynamic 3 times a week in addition. And that has also apnea dives, although short ones. Seperately to that I train balance, focus an stretching and overall stamina by cycling.
Good luck with your research, it's a intresting question, I've heard stories of people training to much and see their times and spirit sink way back, needing a year to recover from it.
My friend owning
www.apnea.nl likes to call it:
Static Burnout Syndrome (SBS) he's researching the same thing.
Maybe it's nice to team up?
Good luck,
Carlos.