While most people consider the bench press to be the very best
exercises to build the chest, the chest fly is superior in many
ways. This is not to say that you should do only one or the other but based
surely on kinesiology you can see why the chest fly is superior. The bench
press relies on the synergist muscles (triceps and anterior deltoids) to
perform much of the work during the bench
press while the pecs are completely isolated during chest flies.
The stability ball dumbbell fly is a great alternative to doing them on a
weight bench. The advantage of doing the chest fly on the stability ball is
you are able to retract (squeeze together) your shoulders more easily which
will let you isolate your pecs more efficiently provided you keep the
scapula (shoulder blades) retracted.