Before you go about actually performing the chest fly, it is important to understand why this exercise is good and what muscles it strengthens. The chest fly is a great exercise for toning and strengthening the pectoral muscles, as well as the triceps and shoulders. This exercise adds definition to these muscles, and increases the metabolic rate of the body due to an increase in the amount of muscle mass that it provides. Finally, this exercise helps to strengthen and support the collarbone, making it a great exercise for people who have recently suffered an injury to this part of their body.

To perform the chest fly, you must first find a great bench on which to do the exercise. Ideally, the bench should come up to approximately the midpoint between your knees and thighs. Lay down on this bench, with your body positioned towards one end. Your head, neck, torso and buttocks should be resting comfortably, with your legs off the end of the bench. Your knees should be bent with your feet flat on the floor. Hold a dumbbell in each hand. To start, your elbows should be bent and the dumbbells should be resting at or near your chest.

Take a few deep breaths, relaxing your body before you actually perform the exercise. Look directly up at the ceiling, keeping your spine in great alignment. As you inhale, slowly extend your elbows out to the side. You should be making a "T" with your upper body. Exhale, as you carefully bring the dumbbells together over the center of your chest, making a "rainbow." Once the dumbbells come together, slowly bring them back to their starting position. Perform 10 repetitions of this exercise before taking a short break. Do two more sets of 10 repetitions.

Once this exercise begins to be too easy, it is important to increase the intensity in order to see results. You can do this through a number of methods. First, try alternating the weights. Bring both dumbbells to their upright positions over the center of your chest. Carefully lower the dumbbell in the right hand out to the extended position, and then slowly lift it back up over the center of your chest. Repeat the same process with the dumbbell in your left hand. Do 10 repetitions with each hand. This exercise is more difficult because it forces you to hold the dumbbells for a longer period of time, thereby producing greater muscle fatigue.