By Joel Woldt, M.S., John P. Porcari, Ph.D., Scott Doberstein, M.S., Jeff Steffen, Ph.D., and Carl Foster, Ph.D., with Mark Anders

If you watch television, chances are you’ve seen the P90X infomercial. Since 2004, the now ubiquitous advertisement has inspired viewers with compelling before- and after-P90X photos, moving testimonials from regular folks, and snippets of trainer Tony Horton’s no-nonsense workouts, all of which has convinced legions of people to pick up the phone and plunk down $120 for the 12-DVD set. In fact, as of November 2010 more than 3 million copies of P90X (a.k.a. Power 90 Extreme), which is dubbed “The Ultimate 90-day Home Fitness Boot Camp,” have been sold, making it America’s most popular home exercise program for the past seven years running.

HORTON-ISMS Eleven phrases that trainer Tony Horton uses famously in the P90X workout videos: “Bring it!” “The warm-up’s going to kill you. That’s a beautiful thing!” “Let’s X it up.” “I’m talking supersonic X-style fit.” “Oh yeah, gonna burn off some goo today! Get rid of the goop because we don’t want it.” “He makes Gumby look like the Tin Man!” “Oh, Mommy!” “Like a pterodactyl backing out of trouble.” “Oh, dude, this is going to hurt so much.” "Yes, I know it’s hard! It’s supposed to be!” “Keep your bucket nearby my friends, because this routine is X City.” Have a hankering for more Horton-isms? The sequel, P90X2, will be on sale by late December 2011.

Horton, the quirky yet motivating star and creator of P90X, first made a name for himself as a trainer for Hollywood’s elite, including actors Sean Connery and Ewan McGregor, musicians Sheryl Crow, Usher and Bruce Springsteen, and ‘80s rocker Billy Idol. It was Idol who gave Horton the nickname “Muscle Confucius,” a tip of the hat to Horton’s favorite fitness concept, muscle confusion, which is at the heart of the P90X program. Exercise scientists call it daily undulating periodization, which is the principle that by changing up workouts regularly, the body doesn’t have time to adapt and become efficient at the exercise. This, in turn, means the body burns more fat and builds more muscle, more quickly.

Those who stick with the very intense P90X program and “keep pressing play” (as P90X devotees say) six sessions per week for the full 90 days are a loyal group. And although there is plenty of anecdotal evidence that suggests the program is effective, no scientific studies have been published to determine the true energy cost and exercise intensity of P90X. Until now.



The Study

For this exclusive study, the American Council on Exercise enlisted a research team from the University of Wisconsin, La Crosse. Led by John Porcari, Ph.D., and Joel Woldt, M.S., the researchers from the University’s Exercise and Health Program recruited 16 healthy subjects, ages 19 to 26, all of whom exercised regularly and had experience either with P90X or similar circuit-style weight-training and aerobic workouts.

To first establish fitness baselines, researchers performed maximal exercise testing on each subject using a motorized treadmill to determine max heart rate and VO 2 max. The subjects then completed up to three practice sessions with each of the four P90X workouts chosen for this study: Legs & Back, Plyometrics, Cardio X, and Chest, Shoulders & Triceps. Although P90X is comprised of a dozen different workouts that alternate from day to day over the 90-day training period, researchers chose to analyze these four because they best represent P90X as a whole.

Once each subject was proficient in the workouts as deemed by the research team, actual testing began. The subjects exercised to the best of their abilities (e.g., performing the maximum amount of repetitions possible using a weight of their choice) throughout the warm-up, conditioning phase and cool-down, as researchers kept tabs on subjects’ heart rates (HR) by taking recordings at one-minute intervals throughout the workout. At the end of each workout, ratings of perceived exertion (RPE) were recorded while the heart-rate data was plugged into HR/VO 2 regression equations that were created from the maximal exercise tests to determine the predicted VO 2 that the subject exercised at during that session. Due to the intensity and wide range of motion required by P90X, subjects were unable to wear bulky metabolic testing equipment. Caloric expenditure was later calculated from the same VO 2 data.

At least 48 hours of rest was given to subjects between each workout test.

