Stir the Pot

Set your forearms on top of a stability ball and walk your feet back into a plank, hips level with your shoulders and your body in one straight line. Instead of holding still, move your forearms in a small circle on top of the ball, as if stirring a pot, while keeping your hips and shoulders from rocking or twisting. The instability of the ball is what makes this hard -- your core has to fight to keep your torso rigid while your arms move underneath you. Keep the circles small and controlled; a wobble in your hips means the circle is too big for your current control. Reverse direction after a set.