Swiss Ball Plank

Rest your forearms on top of the ball, shoulder-width apart, and walk your feet back until your body forms a straight line from your shoulders to your ankles. The ball wants to roll in every direction, so your core is working constantly just to keep your forearms from sliding. Small adjustments are normal — micro-corrections with your shoulders and hips are exactly what makes this harder than a plank on the floor. Keep your hips level rather than letting them dip toward the ball or pike up to fight the instability.