An exercise which has been around for decades, the dumbbell fly is an exercise which isolates the chest through transverse flexion of the shoulder joint. The anterior deltoid head (front muscle head of the shoulder) plays a role in the movement, but being a single-joint exercise the majority of the load is placed on the chest and not shared with additional muscle groups like during the bench press and chest dip.
The dumbbell fly is considered a great chest exercise, but athletes may also want to consider trying the lying cable fly as an alternative. Indeed, cables get around the disadvantage faced with using dumbbells; namely the reduced chest tension experienced through the latter stages of the fly movement. The cables allow the tension to remain on the chest throughout the entire movement.
There is a common myth surrounding the fly exercise – it is deemed a shaping/toning exercise and not a mass builder. This belief likely started from the common practice of bodybuilders incorporating multi-joint compound exercises, such as the bench press, during the off-season, and using more isolation exercises, such as the fly, as they approach competition. Of course, the chiselled physique of an in-season bodybuilder is mostly down to the lower body fat levels they possess during than this period, rather than the switch from compound to isolation exercises.