What Muscles Do Butterfly Exercises Work?

While a butterfly flaps its wings to fly around, you can flap your wings -- or in this case, arms -- to develop stronger chest and arm muscles. The butterfly exercise has many names in weightlifting, including the dumbbell pec fly or pec deck fly. Whatever the name, practicing proper form for the butterfly exercise helps you tone your chest muscles. Always speak to your physician before beginning any exercise program to ensure you are in good enough health to get started.

About the Exercise

The butterfly exercise can be performed using no weights, dumbbells, a cable machine or a pec deck, an exercise machine that has two handles that move in an arc toward your chest. You can complete the exercise while sitting or lying on a inclined or flat weight bench. Hold a dumbbell in each hand with your arms extended to your sides, even with your shoulders. Your elbows should have a slight bend to them as if you are hugging a large, round object. Your palms should face the wall in front of you. Maintain your arm form as you bring the weights in toward the center of your body, stopping just before your weights touch each other. Slowly open your arms to return to your starting position. Perform 10 to 12 repetitions, rest for 30 seconds and repeat for one to two additional sets.

Main Muscle Worked

The chief muscle worked during the butterfly exercise is the sternal portion of the pectoralis major, according to ExRx.net, a physical therapy and training website. This muscle makes up most of your chest wall and not only adds definition to your chest, it also works with the shoulder muscles and joint to rotate the arm inward and outward.

Synergist Muscles

The synergist muscles in an exercise help to support the major working muscle group. Three synergist muscles work with your sternal pectoralis major to complete the butterfly exercise: the clavicular pectoralis major, anterior deltoid and the short head of the biceps brachii or simply biceps. These muscles are found on the front portion of your shoulder and arm. If you sweep your hand from your chest below your collarbone, to your shoulder and then to the inner crook of your elbow, you have just touched the synergist muscles in the butterfly.

Stabilizers

Stabilizer muscles are those that contract without moving significantly during the exercise. Instead, they provide added stability. Stabilizers in the butterfly exercise are the long head of the biceps brachii, brachialis, triceps brachii and the wrist flexors. The long head of the biceps brachii is located on the outer portion of your upper arm while your brachialis muscle is underneath your biceps, close to your elbow. Your triceps are located on the back of your upper arm and wrist flexors are on the inner part of the lower arm.