Dumbbell pullovers are a bit of an exercise anomaly in that they work two opposing muscles simultaneously: the chest and the back muscles. The chest muscles are the primary movers, but several muscles of the back assist during the movement. Because you hold the weight directly over your face, consider having a spotter available when performing this exercise.
Both muscle heads that make up the pectoralis major, or main chest muscle, are involved in the pullover exercise, but not to the same extent. The sternal head, the largest and most pronounced chest muscle, is the primary mover during the dumbbell pullover exercise. It originates on the sternum, extends across your upper torso and attaches to the upper arm bone. The clavicular head, a much smaller chest muscle that lies above the sternal head, is only minimally involved in the pullover exercise as a stabilizer.
The latissimus dorsi, which is a large back muscle that fans out from your upper arm bone to your spine and then extends down to your hip bone, assists during the pullover exercise. Several other muscles, including the rhomboids, rear delts and triceps, are also involved as secondary movers. The serratus anterior, which originates on the ribs, wraps around your side and attaches to the shoulder blade, helps stabilize the scapula during the pullover exercise.
When performing the dumbbell pullover, lock your elbows into a slight bend and do not move them during the exercise. If you allow your elbows to straighten as you lift the dumbbell overhead, some of the load will transfer away from the chest and back muscles to the triceps, increasing their involvement in the movement.