Make the two-dumbbell version even harder by alternating arms-keeping one up while lowering the other back. Hold a dumbbell with a hand-over grip as you lie with your head, neck, and upper back in contact with a ball. Bridge your hips up so your thighs are parallel to the floor and your knees are bent 90 degrees. Lift your arms directly over your chest, lower them behind your head, and do a standard pullover.
