HOSQ002 Pectoral Fly The posterior fasciculus of the deltoid muscle has always been a challenge in exercise, and the reverse flying bird is a great training method for exercising the posterior fasciculus of the deltoid muscle. Of course, it also involves practicing back muscles. There are many different postures for flying birds in fitness, but using dumbbells for standing and sitting positions is very difficult to master. The main reason is to overcome the gravity of the dumbbells themselves and choosing to use instruments (butterfly machines/tensioners) is much better.