P90X has very heavy circuit training influences. Circuit training is great because it allows an individual to accomplish a lot of work in a shorter amount of time by alternating between different muscle groups, oftentimes muscles that oppose one another.
Majority of the work done in the P90X routines are completed by exercising one muscle group, then, while said muscle group has time to rest, the individual then works an opposing or different muscle group. This allows the individual to keep a higher heart rate than compared to traditional resistance training. Not only that, but each resistance workout in P90X is structured so that not only your aerobic energy systems are being pushed, but anaerobically you are being challenged as well.
Circuit training is very similar, in this way, to interval training, where you have short bursts of intense effort followed by a quick rest period. Research has shown the tremendous benefits to circuit and/or interval training for not only healthy body composition, but improvements to the cardiovascular system as well.
There are some drawbacks to circuit training, however. Individuals seeking to get in overall, good athletic shape, can and will benefit from P90X's circuit training, however individuals seeking more specific training for sport, or improving their overall strength, can benefit more from a program designed more specifically to their needs. Traditional strength training, for example, is most beneficial at very low repetition ranges, with longer periods of rest between sets.
Here are some training styles that are utilized in P90X:
Alternating Push and Pull Exercises (ex: Chest and Back)
One style of circuit training is alternating push and pull exercises. This training technique allows the shortening of total workout time by allowing certain muscle groups to rest while their opposing muscles work.
A very common push/pull workout is working the chest and back muscles in the same workout. For example, an individual would complete a set of pushups or bench press and then immediately complete a set of pullups or lat pulldowns. By alternating between pushing and pulling exercises, you ensure that certain muscle groups are not being overworked by working every single set.
Upper and Lower Body Exercises (ex: Legs and Back)
Another form of circuit training is to alternate upper body exercises with lower body exercises. This is especially useful for those individuals that are new to resistance training as they may find doing many different upper or lower body exercises in a row to be too strenuous. This also allows less rest time between exercises cutting down on the total workout time because as the upper body rests from a particular exercise, you can work the lower body, and vice versa.
Not only that, but by cutting down on rest periods between exercises, you automatically add in cardiovascular elements to the workout by having an elevated heart rate almost the entire time!
Supersets (ex: Shoulders and Arms/Chest, Shoulders)
Supersets also cut down on rest time as you generally do one exercise immediately following another. These are usually (not always) used working opposing muscle groups, or agonists to antagonists. For example, an individual would do a biceps curl exercise and as soon as the set is completed, they immediately complete a triceps exercise.
Compound Sets (ex: Back and Biceps)
Compound sets are slightly different in the fact that the intention is to use 2 back-to-back exercises that primarily work the same muscle group to create compounded resistance. For example, an individual could be doing barbell bicep curls, and as soon as the set is complete, pick up a couple of dumbbells and do hammer curls. Slightly different exercises with a bit of a different combination of muscles being worked, but still attacks the biceps.