One of the major buzzwords or phrases surrounding the P90X program is "muscle confusion". Muscle confusion is a term used, referencing your muscles 'guessing' as to what training stimulus it will introduced to next. The theory, at least, is that if your muscles are constantly learning new stuff and getting introduced to new stimulus, then it will constantly change and therefore burn more calories.
Muscle confusion in P90X refers to a number of common practices, including circuit training, split days, periodization, and even recovery weeks. Probably the best example of muscle confusion comes in the form of periodization.
There should be a large asterisk placed next to "muscle confusion" in the P90X guide, however, because although this may be a relatively new marketing term, this concept has actually been used for many years by athletic trainers - known as periodization.
Traditional periodization models are broken down into cycles. Macrocycles are the largest and usually involve an entire sport year, but can last up to 4 years for an Olympic athlete for example. Macrocycles are made up of 2 or more mesocycles which can last anywhere from several weeks to several months. Going further beyond that, mesocycles are broken up into microcycles which are usually a week long but can be as long as 4 weeks each.
If we look at the breakdown of P90X, which is broken down into 2, 3-week phases, and 1, 4-week phase (with a "transition week" between each phase), we can understand the breakdown of the standard periodization cycles.
In this case, we will refer to the entire 13-week P90X schedule as the macrocycle. Within the macrocycle, the program is broken down into 3 phases. Each phase would therefore be considered a mesocycle. Going a step beyond that, within each phase, or mesocycle, we then break each individual week down into a microcycle.
To properly train any athlete, a well-structured training program will consist of both sport-specific training as well as strength and conditioning training. The emphasis placed on either training modality is entirely based on where the individual stands in not only their sport season, but also in their physical conditioning.
Periodization involves shifting from non-sport specific training (strength and conditioning), that is of high volume and low intensity to low volume, high intensity sport-specific activities. This shift will occur over a period of many weeks to prevent overtraining and optimize performance.
Originally introduced by Russian physiologist Leo Matveyev, periodization is a concept to help prevent individuals from experiencing slower results, plateauing results, or even reversing results which can lead to injury.
Strength and conditioning programs can bring about significant strength gains, but adaptations to any program is inevitable, so periodization is the concept used to help design and schedule a program that will continue to elicit physical growth and changes.
Canadian endocrinologist Hans Selye has since attempted to explain the benefits of periodization by using the General Adaptation Syndrome (GAS) which can be seen above.
When the body experiences a new stress, or a stress that is greater than what is previously experienced, the body enters an alarm or shock phase. During this phase, which may last as long as a few days to a few weeks, the muscles will become tender and sore and the individual will even experience a drop in performance.
During the next phase, also known as the resistance phase, the body begins the process of recovery and repair. During the stress or alarm phase, the body is broken down and therefore must be repaired, that is where the resistance phase comes in. The body recognizes that the stress that was placed on the body needs to be reversed, and then some, so it rebuilds its tissues to be stronger than it was before. This is known as "supercompensation".
If, however, the same stresses are placed on the body for an extended period of time, an exhaustion phase could be reached. This is where the individual may see a return of some of the symptoms experienced in the alarm phase; soreness, fatigue, etc. There is also the potential for staleness, overtraining, and other maladaptations to occur. Not only can a stale exercise program cause overtraining but lack of sleep, poor diet, and excess mental stress can also lead to these conditions.
Most periodization models switch, or change up training every 3-6 weeks. That is why each phase in P90X is 3-4 weeks long. This allows sufficient time for the body to adapt to the stimulus, but not to overtrain. It is also why most individuals who progress through the program seem to have a problem when they reach their transition week as they are feeling great, and at the "top of their game" by the 3rd week of each phase. They wish to keep pressing on, but the schedule changes things up for that very reason - to elicit even more change.