Stand with your hips, knees and toes facing forward. Bend your left knee and step back with your right leg into a lunge position.
Keeping your weight over your left heel, reach your arms overhead. Quickly pull your right knee into your chest as you bring your left elbow to the right knee and squeeze your abs [shown], then return to lunge position.
Do 10 to 15 reps, building up to 30. Switch sides and repeat.