The Deltoid muscle makes up the bulk of our shoulder muscle, attaching to the edge of our collarbone where it meets the shoulder -almost wrapping around to our shoulderblade! It then attaches down to a small point about half way down our upper arm called the deltoid tuberosity! (how fitting that the point where the muscle attaches is named after it!)
and more technically speaking: the deltoid muscle attaches from the the lateral 1/3 of the clavicle (collarbone), acromion process (bump at the top of shoulder) & the spine of the scapula (that 'wrapping around shoulder effect) and ATTACHES to the deltoid tuberosity of the humerus (upper arm bone)
The entire deltoid muscle is like a big teardrop , an upside down triangle, or get this: a DELTOID shape if you're into geometry! Try feeling your own deltoid!
Let's go a little bit deeper into the anatomy of the DELTOID MUSCLE!
This muscle is divided up into 3 sections
Anterior (aka - front)
Middle (aka middle - hah)
Posterior (aka back)
Anterior is going to assist with arm flexion AND internal rotation
Middle helps to bring arm up into ABDUCTION
Posterior assists with arm extension AND external rotation
If you wanted to STRENGTHEN deltoids you could simply perform the above actions with a little weight in your hands (try a CAN of soup if you have no weights!)
So what ACTION does the Deltoid muscle do? A few actually! Because it wraps around the shoulder it can assist other muscles with all kinds of movements of the arm at shoulder joint - BUT! The main action of deltoid is ABDUCTION OF THE ARM AT THE shoulder joint.
Abduction meaning to raise your arm away from your body. Every time we lift our arms even to TYPE - deltoids are hard at work!
All these actions make stretching not so simple suddenly! Here is a link
to a great video I found showing how to isolate & stretch each section of the deltoid.