Purpose: The hamstring compact position with exercise ball is a progression of last week's hamstring compact position, as it teaches swimmers (specifically breast swimmers) how to differentiate their hips from their core. This is more difficult than last week's exercise since it uses an unstable surface and positions them horizontally, more like swimming. Make sure the athlete has mastered the hamstring compact position before initiating this exercise, ensuring appropriateness of the exercise. Directions: Start with your hands and forearms on the bottom of an exercise ball. Next, posteriorly tilt or 'tuck under' your pelvis, tightening your abdominals and roll out on the ball. Then, have a partner loop a towel around the back of the swimmers calves and slightly pull on their calves, forcing them to contract their hips (hamstrings) without loosing their streamline position. Perform for 30 seconds - 2 minutes.