He turned the lights up, then sat down to observe.
He turned the temperature down, then sat down to observe.
He ordained two 5-minute breaks instead of their preferred two 10-minute breaks. Productivity increased.
He shortened their workday 30 minutes, then again, then back to the original length. Productivity increased, increased, and increased again.
What the hell happened here?
(1) Worker productivity increased in all instances simply because they were being observed.
(2) Workers were increasingly aware of their own productivity, and this made them more productive (I'll call this the Powertap effect).
(3) Workers responded to increased expectations ("the Pygmalion effect").
(4) Humans in tedious conditions perform better when changes--any changes--are made to their environment (the "novelty effect").
P90x applies the principles observed in the Hawthorne Works factory to your body. It's creed: "change in routine increases muscle productivity." Maximize change to maximize muscle productivity. That's the idea.
The question is: is constant change ideal?
There are currently no studies on P90x, and a Google Scholar search on "muscle confusion" yielded exactly zero relevant results. There are a lot of testimonials, people whose fitness improves. But we're not interested in whether muscle confusion improves fitness--anything that stresses muscles and raises your heartrate does that. We're interested in whether it is optimal--whether it's right for you and I, semi-trained cyclists.
In this case, we'd have to compare it to periodization training.
The principles of periodization are the same as "muscle confusion," except for one thing: duration of focus. In P90x, you switch focus within workouts; in periodization training, you switch focus every week or several weeks.
I don't think P90x is ideal for a simple reason: you have to have a routine before you can change a routine. In other words, instead of changing focus every 30 seconds or even every day, it's better to focus for longer periods on one thing.
This is not just a psychological truism; it's also physiologically substantiated. Your body doesn't adapt to a focus for at least several weeks. For example, if you start squatting today, you won't begin to plateau for at least two months. You'll improve steadily, seeing large gains in strength. It doesn't make sense, day-to-day, to "confuse" your muscles by switching your routine next week when the whole point of exercise is to focus on one element of your overall fitness for as long as you can, bring it to the point of plateau, and then switch to another element. That's why periodization allows elite athletes to build fitness over six months.
Because P90x is all over the place, almost every day, it doesn't allow the kind of focus that periodization allows. For this reason, and until I see evidence of its benefits, I'll stick to what has been shown to yield results over time.