Both the bench press and chest fly primarily target the major muscles in your chest. However, they feature slightly different techniques, with different assisting muscles contributing to the movements. With bench presses, you can use a greater variety of weighted implements, whereas you’re more limited in the types of weights that can be used for chest flyes.
Both bench press and chest flyes are performed while lying on your back on a flat bench. For the bench press, hold a pair of dumbbells in your hands and position them so that they’re just outside your torso at the line of your chest. Your palms face your feet. Exhale and press the dumbbells up towards the ceiling, extending your elbows fully and bringing the weights together at the very top. Inhale and bend your elbows to lower them back down to your torso and repeat. For chest flyes, begin by holding the dumbbells out to your sides with your arms nearly fully extended and positioned parallel with the floor. Palms should face upward. Exhale and pull your arms together so that the weights meet together up over your torso with your arms fully extended. Inhale and spread your arms out to the side again to return to starting position, and then repeat.
According to ExRx.net, the pectoralis major, which is the largest muscle in the chest, is the primary mover for both bench press and chest fly. In addition, in both exercises, the anterior deltoid, which is the major muscle in the shoulder, contributes to the movement. However, because the bench press features elbow extension as you push the weights upward, the triceps brachii muscle, located at the back of your upper arms, is recruited for the exercise. During chest flyes, your arms are forced to hold your elbows in a slightly bent position, which is completed with assistance from the biceps brachii, located at the front of your upper arms.
Because your arms separate from each other during chest flyes, the exercise must be completed while using weighted implements that can be held and controlled with one hand. Dumbbells or kettlebells are appropriate. The bench press, however, can be performed with dumbbells and kettlebells, as well as a barbell. When performing the exercise with a barbell, grip the bar with your hands positioned slightly outside the width of your shoulders with palms facing your feet.
The bench press is a compound exercise, meaning that it requires movement at multiple joints. As you push the weights toward the ceiling, your shoulder joints are performing transverse adduction, which means your arms are moving toward the midline of your body; your elbow joints are performing extension. The chest fly is an isolated exercise, which means that movement occurs at only one joint. In this case, the only joint movement occurring is at your shoulders, which are performing transverse adduction.
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