12,736 subscribers a community for 7 yearslast post today [+]
P90X emphasizes "muscle confusion", a method of cross-training and 'periodization' which is achieved through switching the order of exercises and incorporating new and varied movements. Muscle confusion prevents the body from adapting to exercises over time, resulting in continual improvement without plateaus.
Other sub-reddits that P90X'ers might find of interest...
GERMAN POTATO SOUP!