Dumbbell pullovers work both the chest and back.
Due to confusion regarding which major muscle group the dumbbell pullover primarily works, the exercise is often incorporated into both chest and back workouts. While both the chest and back muscles are recruited during the exercise, the chest takes on the greatest percentage of the load. Therefore, if you're interested in building the chest, the dumbbell pullover is a quality exercise to include in your training.
Get the Technique Right
The pullover is performed with a single dumbbell. Grip the dumbbell's stem with both hands and lie perpendicular on a flat bench so that your scapulas are resting on the bench. With your feet flat on the floor, lift your hips so that you create a straight line through your thighs and torso. Begin with the dumbbell held over your chest with your arms fully extended. Keep your elbows primarily straight as you lower the dumbbell behind your head. Continue until your upper arms are parallel to the floor and then extend your shoulders to bring the dumbbell back to the starting position.
The largest muscle in the chest is the pectoralis major and it's primarily responsible for shoulder transverse adduction, which means it squeezes your arms together towards the center line of your body, like it does during the bench press. However, the pectoralis major also performs shoulder extension, which means it moves your upper arms down and back behind you. During a dumbbell pullover, as you lift the weight from behind your head to back over your chest, your shoulders are extending due to the contribution from your chest muscle.
The reason that lifters are often confused about whether the chest or back is the primary muscle recruited during the dumbbell pullover is because the latissimus dorsi, which is the largest muscle in the back, also performs shoulder extension. While not the primary muscles involved, the lats are recruited during the dumbbell pullover. However, a 2001 study published by the "Journal of Applied Biomechanics" analyzed muscular activity of the pectoralis major and latissimus dorsi during the pullover and found that the chest muscle is activated significantly more than the back.
A Great Chest Builder
In order for dumbbell pullovers to help you build mass in your chest, you've got to perform them at an appropriate frequency, volume and intensity. Incorporate the exercise into your chest workout, consisting of two to three additional chest exercises. Complete that workout two days per week while allowing two days of rest in between. Perform four to six sets of six to 12 reps, with about three minutes in between sets. Select a dumbbell that makes completing six to 12 reps challenging. Warm up beforehand and recruit a friend to act as a spotter while you're performing the exercise.