The primary muscle worked during the straight-arm pullover is the pectoralis major, which is the largest muscle in the chest. Your lats are also heavily involved. Both are responsible for performing shoulder extension during the pullover. The posterior head of the deltoid also assists shoulder extension. Your triceps brachii, at the back of your arms, work to keep your elbows extended as you lift and lower the dumbbell.
Sit on the stability ball holding the dumbbell with both hands. Slowly begin walking your feet forward as you tuck your tail under, lowering your spine onto the ball as you walk your feet away from the ball until your head, shoulders and upper back are resting on the ball. Feet should be parallel and shoulder width apart. Knees are bent to about 90 degrees with your thighs and torso at or nearly parallel to the floor. Distribute your weight evenly through your feet. Finally, lift the dumbbell straight up over your chest as the palms rest on the underside of one of the sides of the dumbbell.
Inhale and slowly lower the weight down in an arc behind your head, while keeping your arms straight, stretching out your chest.
Exhale as you return the dumbbell back to the starting position.