Dumbbell fly

Dumbbells and bench

Opens the arms in an arc with the elbows nearly fixed, isolating the chest's role of bringing the arms together.

What each muscle does in the dumbbell fly

Muscle	Role	Action
Pectoralis major	Primary	Flexes, adducts and medially rotates the arm
Anterior deltoid	Secondary	Flexes and medially rotates the arm at the shoulder
Biceps brachii	Secondary	Flexes the elbow and supinates the forearm

Primary muscles are the prime movers; secondary muscles assist or stabilise. The balance shifts with grip, stance, range of motion and technique.