Memorial Day is gone and so is your trim waistline. You don't have time to fool around with silly ab routines now. You need something that works.

So how about the best ab exercise known to man? Or at least to a reputable scientific journal?

It's called a combination swiss ball pike and roll-out.

Sounds like a medieval torture device. But it's actually easy to learn and extremely efficient. In fact, you can probably get a killer ab workout by doing this move fewer than 10 times.

"If someone was looking at one exercise and wanted to make sure he was getting the most abdominal work done in a way that makes him look better and function better," says trainer Nick Tumminello, "this is the exercise to do."

To start the pike/roll-out, simply hold yourself in a pushup position with your feet on a swiss ball. (To make the exercise easier, move the swiss ball towards your belly button) With your body in a plank (straight) position, keep your legs straight and push your hips towards the ceiling while keeping your back flat. After straightening your hips and coming back to the start position, push your body backwards on the ball until your arms are fully extended in front of you and your legs are fully extended behind you.

Here's the video, courtesy of Nick Tumminello:

Why does this work so well? As the Journal of Orthopaedic and Sports Physical Therapy states:

"The roll-out and pike were the most effective exercises in activating upper and lower rectus abdominis, external and internal obliques, and latissimus dorsi muscles, while minimizing lumbar paraspinals and rectus femoris activity."

Translation: The exercise forces your abdominals to work the hardest while your lower back and hip flexors -- areas that can take over in other traditional ab exercises -- are almost completely left out. So while a crunch works your abs but only with help from other areas, the roll-out pike isolates your abs much more effectively.

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Remember, this one isn't easy. Don't expect to get dozens (or even a dozen) reps right away.

But if you keep at it, you can expect to get dozens of compliments right away. Or at least by July 4th.