Shoulder Musculature

I will be outlining the main muscles of the shoulder including their anatomy and actions.  There are many other muscles that may attach near or around the shoulder joint which will be covered in other posts. Here we will cover the rotator cuff muscles, deltoid muscles, bicep/tricep, and upper trapezius.

1. Rotator Cuff muscles- As you can see in the picture above, the 4 rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) surround the shoulder blade and attach out at the humerus (arm bone). It helps with abduction, external rotation, and internal rotation.  This means they lift the arm out to the side, behind your back, and behind your head. They are the most important muscles of the shoulder.
2. Deltoid muscle- The deltoid originates at the clavicle (collarbone) and acromion (shoulder bone) and inserts out at the lateral humerus (upper arm bone).  There are three different parts to the deltoid: anterior fibers, medial fibers, and posterior fibers. They all perform Abduction of the arm which is raising it to the side (to a “T”).  The anterior fibers assist with flexion & internal rotation of the arm as well.  The posterior fibers assist with extension & external rotation of the arm as well.
3. Bicep muscle- The bicep has two heads: long and short. They both run from the top of the scapula down to the radius (forearm). Its main action is flexing or bending the arm. It also helps with supination which is bringing the palm facing up. In addition, it helps raise your arm up into flexion.
4. Upper Trapezius muscle- The upper trap runs from the base of your skull down and out to your collar bone. It helps tilt your head to the side and shrug your shoulder up.
5. Tricep muscle- The tricep has three heads: long, medial, and lateral. They run from the back of your humerus/arm and down to your elbow bone. Their main action is shoulder and elbow extension, which is straightening the arm behind you.

So now that you know the anatomy and how each muscle works, you can better work them out.  In a future post, I will discuss the best strengthening exercises for each muscle group.  Until then, here are some of the best stretches for the shoulder, tricep, bicep, and upper trap: