Why You Should Exercise Harder

If you want to see changes in time for Summer, flipping through a magazine on the treadmill most days isn't going to cut it. The truth is, you need to be constantly improving your fitness level, most trainers say. That means cranking up the speed, adding one more rep, and pushing past your comfort zone.

Of course, a hard workout will burn more calories than an easy one, but that's not the only reason to sweat it out. Workouts with short, intense intervals, like Tabata, will increase your afterburn — the amount of calories you burn after a workout even when you're doing nothing at all! Plus, increasing those dumbbell sizes or perfecting your kettlebell swing helps build muscle as well. As Tia Falcone, who helped Miss America Nina Davuluri lose 50 pounds, says, "When you're doing weight training, it's not so much about burning calories, it's about gaining as much muscle as possible. All women need to gain as much muscle as humanly possible; that's what's going to make you smaller and it raises your base metabolism permanently." The win here? Muscles help boost metabolism, burning more calories than fat.

Not every workout has to be a hard one, however — it's not healthy or smart to always push your body to its limits. "If you're trying to work out twice a day to lose weight, it's just not effective," explains Yumi Lee, a CrossFit trainer who has worked with Jessica Alba. "Your body will burn out and you'll probably have injury, and then you can't work out for a week. And then you feel awful. It's a vicious cycle." Whether it's two-a-days or taking one intense gym class after another, it's a habit that's not sustainable. So instead of burning out, take trainer Michelle Bridges's advice: schedule six workouts per week, making sure that three of them are intense, two moderate, and one easy. "You don't have to train like an Olympian all the time, but it's [about] building in those habits," she says.

Ready to step it up? Check out this fat-frying cardio and weights workout, our Tabata bikini-prep workout poster, and this intense 60-minute workout circuit.