How to do it: Grab a pair of dumbbells and lie on your back on a Swiss ball that your middle and upper back are on the ball. Raise your hips so your body forms a straight line from your knees to your shoulders. Hold the dumbbells over your forehead, with your arms straight and your palms facing each other [A]. Without moving your upper arms, bend your elbows and lower the dumbbells until your forearms are beyond parallel to the floor [B]. Pause, then lift the weights back to the start.