When it comes to resistance training, the angles matter! They matter because the angle you’re moving your body or how you have your joints positioned determines what muscles are being worked during which exercises. This is especially true for one particular exercise that many “gym bro’s” swear by, the Pec Fly.
The Pec Fly is an isolation exercise for the chest. The chest muscles attach along the sternum in the middle of the rib cage and attach along the collarbone and onto the top of the humerus bone in the arm. Its primary functions are to adduct the arm, or bring it in towards the center of the body, and rotate the arm inward. Another movement the chest aids in is flexing the shoulder, and in turn raising the arm. However, this movement is limited to the fibers of the chest that attach along the collarbone, which are commonly referred to as the upper chest. The middle and lower chest fibers do not aid in this movement at all.
It is very common to see people in the gym performing a Pec Fly only by bringing their arms from the bottom to the top, or vice versa. However, if the arms do not come to or cross the center of the body, then there isn’t much activation of the whole chest, only the upper chest. This is because the middle and lower chest aren’t doing their job of bringing the arm inward to the middle. Denying these two portions of the chest to drive the movement robs the exerciser of valuable work and development in the chest and makes the Pec Fly more of a Front Raise with an underhand grip.
In order to keep the Pec Fly specifically an isolation exercise for the chest, it’s important that you emphasize bringing your arms either to the center or even across the center of your body. This will activate the most muscle fibers inside the chest and allow you to get more work out of your workouts, and therefore burn more calories and build more lean muscle tissue.
Remember, you don’t want to invent the wheel! Be absolutely sure that your movement in any exercise is to make the muscles do the job they were built to do, but with resistance and control. To force movement of the body without proper activation of the muscles leads to improper training, lagging results, and an open invitation for injury.