Don’t Do the Plyometrics, Kenpo X, or Cardio X WorkoutsIt's not that Vin Miller's recommendation is necessarily incorrect, it's just that it's not universally applicable. Oddly, he earlier criticizes the program for focusing too much on glamor muscles and not enough on a "truly healthy and balanced lifestyle", yet it is anaerobic work, not aerobic work, that gets the most conspicuous aesthetic results.
When proper safety precautions are observed, plyometrics is a great way to improve performance and injury resistance. However, the P90X Plyometrics workout is more of a long calorie burning session than a true plyometrics workout. Along with Kenpo X and Cardio X, these workouts are very similar to aerobics, step, or spin bike classes which means that they’re relatively high in intensity and are a significant physiological burden that can easily wear down the body and require more time to recover from, especially when done on a regular basis.
If the goal is to build upper body muscle mass, intense cardio is potentially counterproductive, especially when done in long intervals and/or high frequencies. But it's crucial piece of overall conditioning for athletes, and not just for professionals. As a recreational athlete, pullups and pushups aren't going to do me as much good when I'm playing soccer or ultimate frisbee as endurance work will do. Body builders suck at most team sports like basketball and 7-on-7 because all that mass is heavy and demands a lot of oxygen when exerted.
During plyo, Tony Horton states that the workout really helps him step up his game on the court, and it is undoubtedly true--plyometrics makes one a better basketball player.
Vin Miller suggests as an alternative to intense cardio going on a brisk walk or an easy bike ride and getting one's heart rate up in the 55%-75% range. For general health, there probably aren't any negative consequences in getting out there and doing what my 55 year-old mother does each morning, but it's not going to do a thing to help an athlete (which I define here as anyone who pushes his physical capacities to their limits--in addition to playing sports, it also applies to those who figure skate, rock climb, etc). To realize real gains, an athlete needs to push into the 90% range.