It's hard to argue with the success of the werewolf workout. The shirtless stars of Twilight: New Moon have the kind of ripped physiques that make even the most dedicated gym rats howl at the moon with envy. And they owe their buffed bods to a controversial fitness fad called "muscle confusion."
"It's changing up your workouts on a very regular basis to make sure that your body doesn't get used to the task," explains Ray Love, president of Fitness Plus, a gym in Calgary.
It may sound straightforward, but the technique has divided the fitness community. One trainer even called it "the biggest muscle building fallacy in bodybuilding."
Adherents argue that when doing the same workout over and over, the body's muscles dull to the routine, and gains made in size and strength eventually plateau. By constantly mixing up exercises, sets and reps, muscles become "confused," forcing them to work harder and therefore burn more calories.
The promise of quick results has won over many trainers and people looking to get in shape - especially Hollywood stars, who often have very little time to transform their bodies.
Chaske Spencer, who put on 20 pounds of muscle in just three weeks to play a werewolf in New Moon , sang the praises of muscle confusion to OK Magazine earlier this month. His trainer also helped create the six-pack abs and bulked-up biceps featured in the film 300 .
"You can do 100 push-ups, but the next day maybe do 100 pull-ups. … You still use the muscle, but in a different form. It's faster," Mr. Spencer said.
"We work out the circuit, which is where you go from chest to arms, shoulders, legs and stomach. You go from one set to the next set to the next set. … We would only break when we're done with the full set. You get a minute or two break, and do it five or six times. It helps burn fat, too, fast."
Muscle confusion is also at the heart of P90X, a program developed by Beachbody, which comes with the tagline, "Go from regular to ripped in 90 days." Demi Moore, Pink and Sheryl Crow are celebrity fans.
There's plenty of science to support muscle confusion's claims, says Matt Heath, a neuroscientist in the school of kinesiology at the University of Western Ontario.
"It's very, very well established in the cognitive sciences, this idea that practice" he says. "It's not just about muscle strength when you're lifting a weight. It's about the strategy you employ in order to recruit all the different … motor neurons that ultimately go on to fire off and tell the muscle to contract. You tell those motor neurons more efficiently how to fire and that's just communicated to the muscle, and so therefore the muscle can produce more force allowing you to lift more weight."
Mark Smishek, president of the Canadian Bodybuilding Federation, is one fitness expert who has his doubts.
"A muscle is a muscle. It just flexes and contracts," he says. "It doesn't have a brain and you really can't confuse it, so that term, 'muscle confusion,' is really silly."
Echoing others in his field, he says the best fitness results are gained by sticking to a routine.
"You probably have to do the same thing over and over again for probably six to eight weeks before you actually adapt really well to that fitness regimen," says Robert Vigars, a professor of sports biomechanics at the University of Western Ontario. "If I keep mixing up my routines, so much that I'm only blitzing one particular muscle group periodically, that muscle group's not going to develop that much."
Rob Belisle, a professional bodybuilder in Edmonton, says that muscle confusion has been known in bodybuilding circles since at least the time that Arnold Schwarzenegger was pumping iron. While it's attracted followers ever since, Mr. Belisle prefers to keep with the same workout regimen.
"When I get ready for a show I like to stick to the same thing," says Mr. Belisle, winner of the open middleweight class and the overall masters at this year's Canadian National Bodybuilding Championships.
Still, mixing things up does have one benefit that can't be denied, especially when it comes to the often tedious experience of trying to get in shape.
"It doesn't get boring," Mr. Belisle says.