Yoga tests your flexibility.
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P90X is an extreme exercise program that incorporates diverse disciplines like weight training plyometrics and yoga. If you're inflexible, the 90-minute yoga routine in P90X may scare you off. It's advisable to do the yoga routine if you can, but if not there are still benefits to doing the program without it.
According to Beachbody
The "P90X Fitness Guide" recommends against skipping yoga, or any other workouts. The guide says that missing workouts "will adversely affect your energy, recovery and results." Specifically, it states that yoga "is a vital part of extreme fitness and an integral part of P90X." According to Beachbody you should follow the P90X schedule for one complete 90-day cycle. After 90 days you can assess the program and begin adjusting it to fit your own needs, potentially skipping some workouts or making substitutions.
After 90 Days
After you've completed 90 days of P90X you should continue staying active to maintain your results, but you don't need to do all the workouts in their entirety simply to stay fit. While you can skip some workouts in a maintenance program, the "P90X Fitness Guide" recommends doing the yoga workout at least once per week. This is similar to the schedule during the program which calls for yoga once or twice per week depending on the week of the schedule.
Getting Enough Exercise
The Centers for Disease Control and Prevention recommends getting at least 150 weekly minutes of moderate-intensity aerobic exercise or 75 minutes of vigorous-intensity aerobic exercise. The CDC also recommends getting two resistance workouts that work all your major muscle groups. Resistance workouts can include yoga, but you can also use weight exercises like those found in other P90X workouts. Even without doing yoga, P90X will meet the CDC's exercise recommendations.
When it comes down to it, the decision of whether to do the yoga portion of P90X or to skip it, is up to you. The program intends for you to do all the workouts to get the maximum benefits from the program, but it's an extreme program and you can still get significant benefits from doing a partial version of P90X that meets the CDC's exercise guidelines. What's most important is that you choose a workout routine that fits your needs and that you're able to stick with.