45 minutes cardio Workout

You heard right — this circuit cooks, and you'll have the skinny to prove it. We dared top Hollywood trainer Jeanette Jenkins to design a routine that would fry your ass and firm you fast. Result: You'll melt about 500 calories doing her mix of multipart toners and kick-butt cardio. We know, because we put it to the test. Jenkins, who trains A-listers like Kelly Rowland, Pink, and Alicia Keys, led a group of FITNESS readers equipped with heart rate monitors through a session to clock their calorie burn. "This workout gets results because it challenges you at every level, " Jenkins says. "Each set maxes out your muscles, while the intervals keep your metabolism stoked on high." Check out her fierce physique as she demos the moves, then get cracking! Have a set of five- to eight-pound dumbbells handy and do the circuit three times through — four if you've got the stamina. Rock this workout three to four days a week for a flab-to-fab transformation this month.