Are you still using the leg-curl machine? Then you should know this: "It's not the best way to work your hamstrings," says Alwyn Cosgrove, C.S.C.S., Men's Health fitness adviser and co-owner of Results Fitness in Santa Clarita, California. "In fact, it's not even close."

A quick explanation: The leg-curl machine targets your hamstrings because it requires you to flex your knees against a resistance. This is called knee flexion. (Get it?) Knee flexion is certainly one of the jobs of your hamstrings -- but it's not the most important one. At least not in terms of why you need your hamstrings to be strong in the real world. (For a real-world fitness plan that fits even the busiest schedule, try The Zero Excuses Workout.)

"The main function of your hamstrings is to extend your hips," says Cosgrove. If you need a visual, think of the movement you use when you thrust your hips. An example: When you sprint, you forcefully thrust -- or extend -- your hips each time you push off the ground with your foot. This helps you propel your body forward.

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You perform hip extension in plenty of exercises -- the stepup, deadlift, Romanian deadlift, hip raise, and even the squat, to name a few. So if you want to strengthen your hamstrings, these exercises are all better choices than the leg-curl machine. After all, you rarely have to flex your knees against a resistance in every day life.

That said, if your knee flexion is weak, it’s certainly smart to strengthen this function of your hamstrings. However, there’s an even better exercise for this than the leg-curl machine. It’s called the Swiss-ball hip raise and leg curl. This movement actually requires both knee flexion and hip extension. So it works two of your hamstring functions at once. "It doesn’t matter that you aren't 'isolating' knee flexion," says Cosgrove. "With this exercise, the weakest link automatically gets the best training effect.” (Make sure The 8 Scariest Restaurant Meals aren't the weak link in your diet.)

And though you may think it's a simple movement, few people in the gym get this exercise right. So watch the video below to learn how to do the Swiss-ball hip raise and leg curl with perfect form.

Oh, and a note to those who feel this exercise is too easy. Remember: You can always use progressions to make an exercise harder. In this case, you can perform the same movement, but with just one leg on the ball instead of two. (You hold the other in the air.) That's tough by almost anyone's standards.

Want to learn more great moves to sculpt your body? Then check out The Men's Health Big Book of Exercises and The Women's Health Big Book of Exercises, where you'll find full-color photos of more than 500 exercises, and dozens of great workouts.

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