By peoplestaff225
Updated March 05, 2015 11:21 AM
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As a former U.S. Marine, Erin Oprea is dedicated to fitness. Now a Nashville-based personal trainer with clients like Carrie Underwood, she uses her expertise to help those with busy lives stay fit. Visit her site or follow her on Twitter @erinoprea. Then come back to PEOPLE.com every Thursday to master a new workout move.

Instead of your typical plank, all you need to create a strong core is just a little help from a big medicine ball.

There are all kinds of reasons you may not want to put your hands on the floor for planks; getting your hands dirty, the ground is cold, the TV is positioned too high, it’s too easy and stable. But all of these can be overcome by adding the ball into your repertoire and bringing your knee to your opposite elbow.

Getting yourself slightly off-balance and driving that knee up will really target your oblique abs (those hard to reach side abs). Plus, with the right playlist, you can pump those legs to the beat of your favorite song to make this just about the best move ever.

Stability Plank Knee Crunch

1. Get into plank form with your hands on the ball and abs, legs and bottom squeezed real tight.

2. Drive your right knee up, keeping your hips out of the air, bringing your knee to your chest. Make sure your hips stay square, facing the ground.

3. Repeat using the left knee and continue going back and forth. Try and go at a decent pace like a mountain climber and keep your back flat to maintain that form.

4. Perform the plank for 30 seconds to 1 minute to get that heart rate up and tone that core.