If you want to build muscles in your upper body, butterfly exercises can help. Designed specifically to target muscles in your chest, flyes engage several muscle groups in your arms and shoulders as well, getting you ready for tank-top weather. There are several ways to do fly exercises, including using special machines at the gym or using dumbbells on a bench or stability ball.
Butterfly exercises work your chest muscles as you open and close your arms at shoulder height, often using resistance such as dumbbells. Your pectoralis major muscle group pulls your arms inward and stabilizes them as you lower them slowly. Keeping your elbows slightly bent allows you to drop your elbows lower than your body as you lie on your back on a weight bench, or to push them behind you if you're sitting up in a fly machine.
Your shoulders are the only joints moving as you perform flyes, so some of the muscles in that area get a workout. The deltoid muscles help you move in a slow, fluid motion. Secondarily, your rotator cuff helps stabilize your movements.
Although you don't engage your upper arms as a major muscle group when you get your flyes on, you need those muscles to move the weights. Your biceps and triceps hold your elbow at a steady angle and contract along with your shoulder muscles to make the weights move.
Your wrist flexors are the closest arm muscles to the weights if you're using dumbbells. These muscles contract and keep the weight steady as you move your arms in the flying motion.
Using your abdominal muscles to support your core gets those muscles working as you do your flyes. These muscles help you balance your body on a weight bench, but you can get deeper into the ab workout by using a stability ball for your butterfly exercises. Rest your upper back on top of the ball with your knees bent and your feet flat on the floor. Your lower back has no support from the stability ball, which means you must use your abs to keep your back straight.