Stir-the-Pot

Place both forearms on top of a fitness ball and assume a plank position with your body in a straight line and your feet just wider than shoulder-width. Move your arms in arches or semi-circles instead of making full circles as if you’re stirring a big pot. Alternate between left-to-right arches and right-to-left arches. Making full circles is unnecessary and awkward. Eliminating the bottom end of the circle takes away the aspect of the exercise when your arms are fully underneath you, which is the easiest portion of the exercise on your abs, but the most awkward on your shoulders. – Nick Tumminello