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Stand on your right foot with your hands clasped behind your head. Extend and lift your left leg behind you as high as you can without leaning forward. Brace your abs in tight as you bend your left knee and bring it in front of your body. Trace a large circle with your knee from right to left (clockwise), keeping your upper body as still as possible. Extend your left leg back behind your body, trying not to touch the floor. Do 15 reps with your left leg, 15 with the right.