The tweak: Most of the studies on HIIT have involved one type of cardio activity, but Gibala says that this approach can also be applied to weight lifting or circuit training. Jessica Matthews, an exercise physiologist with the American Council on Exercise, recommends interval training that includes full-body power moves, like burpees (those killer push-up/jump-up combos), kettlebell swings, plank-ups and medicine ball slams. This is the basic idea behind CrossFit classes, Insanity DVDs and some boot-camp classes.
At home, Matthews suggests setting up a circuit, like this one, of 6 to 8 exercises that challenge the entire body. Perform each move for 30 seconds, followed by 30 seconds of active recovery (like jogging in place). Once you've completed all the exercises, rest for 1 minute and then do the entire circuit one or two more times (total workout time: less than 30 minutes). Matthews adds that while researchers like Gibala gauge fitness by measuring enzymes in the muscles and the body's maximum oxygen intake (VO2 max), most of us non-scientists are mainly looking for slimmer thighs and more-sculpted physiques. The time it takes to get accomplish those goals varies widely by individual ...but, as any trainer will tell you, usually takes longer than two weeks.
The fine print: The military recently noted a disproportionate number of injuries from CrossFit, Gym Jones Insanity and P90X, resulting in lost duty time, medical treatment and rehab. Some of the riskiest moves are power cleans (bodybuilder barbell lifts) and handstand push-ups. If you, like some of these troops, don't already know how to safely execute a perfect lunge, then you may have trouble doing a series of them...fast ...while swinging kettlebells. Look for a gym with intro classes that can teach you the basics. Another mistake: pushing too hard. Matthews says that she has seen people get hooked on total-body HIIT-style workouts and refuse to take days off, which can eventually lead to overuse injuries. (And, finally, Gibala says that none of his studies have tested the benefits of HIIT for more than a few months ...but by that time, you should already be fitter, stronger and healthier).
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