Butterfly exercises, or flys, involve drawing your arm toward the midline of your body when it is raised to your side. It is an isolation movement with motion at the shoulder only. This exercise is typically performed with dumbbells while you lie on a bench. It can also be done seated on a machine that is specifically designed for the purpose. Flys work the muscles of your chest.
Pecs
The pectoralis major, or pecs, are the primary muscle groups of your chest. Your pecs originate on different bones of your torso including the clavicle, or collarbone; the sternum, or breastbone; and the upper six ribs. Fibers from all of these locations converge to narrow tendons that attach to the front portion of your upper arms. When your pecs develop tension and shorten, they pull your arms across your body toward the points of origination. This is the motion that occurs when you perform flys.
Dumbbell Flys
Dumbbell flys are done while lying supine because the opposition to movement is gravity, which is directed down. Dumbbell flys can be performed on a flat bench or on one that is angled. When you change the angle of the bench, it alters the direction that your arms move. This shifts emphasis to a different section of your pecs. For example, when you lie on a bench that is inclined, it targets your upper pecs, while flys on a decline bench work the lower section.
Machine Flys
Many gyms have machines with weight stacks that you can sit on to perform flys. There are two versions of these machines. One that is called a pec deck has perpendicular pads attached to movement arms that transfer the weight of the stack to your pecs. To perform this exercise, you position your forearms against the pads and squeeze them together in front of you. Your elbows are bent at right angles as you use this machine. The other fly machine has handles that you grasp that are perpendicular to the floor. This means that when you perform flys on this machine, your arms are straight. Machine flys provide more stimulation to the inner portion of your pecs because tension is retained throughout.
Chest Workout
A chest workout can include both flys and bench presses. Bench presses involve a similar motion at the shoulder, but also require movement at the elbow because you are pushing the weights away from your chest. For this reason, you are also working your triceps when you perform bench presses. Conversely, flys isolate your pecs.
References
- "Kinesiology: Scientific Basis of Human Motion Twelfth Edition"; N. Hamilton, W. Weimar and K. Luttgens 2008
- "ACE Personal Trainer Manual 4th Edition"; American Council on Exercise; 2010
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