The deltoid muscle is the round muscle that encapsulates your shoulder. There are three heads, or three origins points, of the deltoid muscle: anterior, middle and posterior. Each of these heads plays a different in moving your shoulder.
Anterior (front): Responsible for raising your arms straight out in front of you (flexion)
Middle: allows you to bring your arms out directly to the side (abduction)
Posterior: when hands are out in front of you with arms parallel to the ground, this muscle is responsible for spreading you arms apart (horizontal abduction)
Exercises To Grow Middle Delt’s:
https://www.youtube.com/watch?v=XGqH9EAMmUc Skip to 20:45