Kirsch—who also works with Kate Upton and Heidi Klum—recently shared an abs-blasting routine on Instagram using a weighted medicine ball and his own body mass that's as core shaking as they come. (He's not the only one who swears by the tool for a killer workout, either—Karlie Kloss and Cindy Crawford use 'em on the regular, too.) Scroll down to see the trainer's go-to exercises. Hint: They're ones you already know and love—just with a twist.
1. Spiderman plank
Get into a plank position, pressing a weighted medicine ball into the floor with your hands. Keeping your core engaged, take turns bringing each knee out to the side and toward the ball, making sure you're bringing your legs back into a plank position between sides.
2. Jack knives
Lying on the mat, hold the medicine ball in your hands. Keeping both your arms and legs straight, raise your upper body and lower body at the same time until the ball meets your calves. Slowly lower back down, hovering your legs a few inches in the air and tapping the floor with the ball above your head.
3. Ball slams
Keeping a strong stance, bring the medicine ball over your head and, using all your core strength, slam it onto the floor. As it bounces back up, grab it and repeat.
Take your abs workout anywhere with this 2-minute sequence. Or, find out how Shay Mitchell turns her treadmill into a Pilates reformer for a hardcore sweat sesh.
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