The flat bench dumbbell fly is a great chest exercise that works on your pectoral muscle. Although this exercise is considered an isolation exercise, it also hits the shoulders , triceps, and some muscles on your back.
The function of your pectoral muscle (the main chest muscle) is to draw your arms across your chest – exactly the same movement that the dumbbell fly accomplishes.
While press movements like the bench press build thickness in the chest, a fly movement will build chest width, This exercise mostly develops the outer portion of the pectoralis major.
The flye movement can be done with dumbbells or with cable, while both work on the same muscles in a similar way, there are some differences. Doing the exercise with free weights, will strengthen more stabilizing muscles in the shoulders and arms that aren't worked when using a cable.
Keep your shoulders safe. Due the arms position, fly exercises of all types have a large potential to damage the shoulder joint. Make sure to start with a low weight, and increment gradually as long as you don't feel shoulder pain after or during the exercise.
Keep your arm straight throughout the movement, don't bend your elbows, this will make the biceps do part of the lifting, so the exercise will not be as effective on your chest. If you find keeping your arms straight too hard, you are using too much weight, The flyes exercises usually allows you use much less weight than bench press movements.
Do not clang the dumbbells together at the top. It might feel nice, but the striking of the weights together momentarily releases tension from the working muscles, which you don’t want.
Ensure you maintain some tension in your abs and don’t allow your lower back to arch excessively.