Starting Position: Stand holding a medicine ball with your feet hip-width apart. Hold the ball relatively close to the your body slightly below chest height, positioning your hands at the midline of your body. Contract your abdominal / core muscles to stiffen your torso, holding it vertical to the floor. Depress and retract your scapulae (pull your shoulder down and back) without arching your low back
Movement: Exhale and slowly rotate your torso in one direction, keeping your upper arms at your sides, elbows bent and hands positioned at the midline of your body. Your head, chest and torso should all move together and avoid leaning your torso in the direction of your movement. Hold this end position briefly before turning to rotate in the opposite direction.
Exercise Variation: To increase the exercise intensity, fully extend your arms to shoulder height and keep your arms in this position throughout the exercise. The longer lever this creates increases the loading on the spine, requiring the core muscles to work harder.