The fly movement has been a popular choice for bodybuilders who wish to isolate the muscles of the chest. The dumbbell variation has been around for decades, with the exercise a favourite of bodybuilding legend Arnold Schwarzenegger. Dumbbell flies isolate the chest effectively, although the muscles of the arms, and also notably of the shoulders, are also somewhat recruited during the exercise.
The form of the exercise is of importance to ensure not only maximum effectiveness, but also safety. A slight bend should be maintained in the elbow joint, with the upper arms brought down so there is a notable stretch of the chest at the bottom of the descend, but not excessively far due to the potential for injury. Always use a weight which enables you to perform the exercise with sound form. Excessive weight can result in potential injury to the shoulder joint, or can turn the fly into more of a pressing movement due to the weight being too heavy to maintain form.
Using the dumbbell for the fly movement does have some negative points when comparing the exercise to its cable variation. The resistance from the dumbbells obviously comes from the gravitational pull of the dumbbell towards the floor, so the relative resistance to the chest weakens as the dumbbells are brought upwards above the torso. At the top of the exercise the resistance is mainly being transferred down the arms, with little stress to the chest fibres. This is not the case with the cable variation, with the resistance being continual throughout the exercise.
The dumbbell fly may be a suitable choice for pre-exhausting the chest prior to bench pressing, with the movement isolating the chest to tire it before the pressing. The aim is for the chest to fail before the assisting muscles during the press exercises, so to prevent the assisting muscles overpowering the chest.
