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Lie with your ankles balanced on top of a stability ball, legs together, while your head, shoulders and arms are flat on the floor. Your body should form a bridge from your feet to your upper spine. From this position, flex your hamstrings to bend your knees and roll the ball toward you, with your heels approaching your glutes. When your knees are fully bent, squeeze the contraction for a count, then slowly roll the ball back to the start. Repeat for reps.