Metabolic Program
P90X is Circuit Training and falls into the Cross Fit arena.
It is a more of a fat burning and general conditioning program.
It is not a very effective program for increasing mass.
P90X Counter Producte To Increasing Mass
The key to increasing muscle mass is diet. You have consume more calories than you burn.
P90X is all about burning calories. This defeats the purpose of increasing mass.
P90X Cardio Endurance Program
As NeilPearson noted, P90X is really a cardio endurance program. Endurance programs do little to promote hypertrophy, grow muscle.
As with most cardio endurance programs, high repetitions are performed with short rest periods (about 30-60 seconds).
Hypertrophy Training
To increase muscle mass, keep the repetition range in the 8-12 area with rest periods of about 2 minutes between sets.
P90X Ineffective Mass Program
Any exercise program will increase muscle mass to some extent, even P90X.
However, P90X is the wrong tool for the hypertrophy job.
Analogy
Using P90X to increase muscle mass is like using a crescent wrench to hammer an nail into a board.
Why not use the right tool? A hammer for the job.
The same applies to increaseing muscle mass. Why not use a Hypertrophy Program rather than a Cardio Endurance Program?
Kenny Croxdale