Place yourself on a stability ball with the ball supporting your upper back region. Holding two dumbbells, keep your abdominals tight and press the dumbbells straight up keeping the upper back pressed tightly into the ball. The palms may face towards each other (as pictured above) or with the palms out like using a barbell. The palms in position is favored as it may place less stress on the shoulder joint.
Mensinger Performance and Fitness Systems 