What Muscle Does a Dumbbell Fly Work?

The dumbbell fly is a weight training exercise that recruits a number of muscles in the upper body. From a lying position on a flat bench, hold the dumbbells over your chest with your arms fully extended. While allowing a slight bend in your elbow, open up your arms out to your sides until they’re nearly parallel to the floor and then squeeze your arms back together.

The muscle that handles most of the work during the dumbbell fly is the pectoralis major, which is the main chest muscle. The pectoralis major has two heads: The clavicular head at the upper section of your chest, and the larger sternal head. The sternal head originates at your sternum and spreads out to each of your shoulders where it inserts at the top of your upper arm bone.

The major muscle in the shoulders is the deltoid, which features three heads: The anterior, lateral and posterior heads. The anterior head of the deltoid, lat the front of your shoulders, is recruited during the dumbbell fly to assist the chest muscle in squeezing your arms together.

When you’re performing the dumbbell fly exercise, your biceps brachii muscles isometrically contract to hold your elbows in a slightly bent position. The biceps run down the front of your upper arm, all the way from your shoulders down to the top of your lower forearm bone. They’re responsible for bending your elbows. When they isometrically contract, they hold your elbow joint in a static position.

The dumbbell fly can also be performed with the bench set to an incline position. Instead of the bench lying flat, set it so that it's tilted about 60 degrees. You'll be performing the exercise from a reclined position instead of lying flat. During this version of the exercise, the same collection of muscles is recruited. However, because you’re in an inclined position, a greater percentage of work is done by the anterior deltoid. In addition, the clavicular head of the pectoralis major at your upper chest becomes the primary muscle instead of the sternal head.