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Windshield wiper

Lie on your back with knees bent to 90-degree angles. Straighten your arms by your sides, and lengthen your fingertips. Press the backs of your shoulders against a mat, and slide them down away from your ears. Focusing on the deep waist muscles, inhale and slowly move your knees to the right, then exhale and return to starting position. Repeat on the left; that’s 1 rep. Do 5–8 reps.

Next: Plank on the ball