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The chest press and chest fly have several things in common: They are both chest exercises, they both require outside resistance and should both be performed with the assistance of a spotter. However, there are also several differences that distinguish the two exercises.
While both the chest press and chest fly work the chest, different muscles are targeted with each exercise. The chest fly exclusively works the pectoralis major and the anterior deltoid muscles. The chest press also works the pectoralis major and anterior deltoids but also utilizes the triceps brachii muscles on the back of the upper arm.
Each exercise is performed for a different reason. The chest press can be performed using heavy weights and therefore be used to build and strengthen the working muscles. Chest flys, on the other hand, should never be performed using heavy weights or injury is likely to occur. Chest flys are a fundamental exercise for developing thoracic expansion and are used to stretch the chest muscles rather than for muscle gain.
Primary Muscle Actions
Concentric muscle actions occur when the muscle fibers shorten as they work against the resistance. On the other end of the spectrum are eccentric muscle actions, which occur when the muscle lengthens under resistance. The chest press primarily relies on a concentric muscle action to build muscles as you extend your elbows and adduct your shoulders during the upward phase of the exercise. An eccentric contraction is used to a lesser extent as you lower the weight to your chest. On the contrary, the eccentric muscle action is most important during chest flys to stretch and lengthen the chest muscles as you slowly lower the weights toward the floor. A concentric contraction is then used to return the weights to the upright position.
The chest fly can only be performed using dumbbells or cables to get the full effect of the eccentric contraction used in the downward phase of the exercise. The chest press can be performed with dumbbells, cables or a barbell.