Supraspinatus

Origin:  Supraspinous fossa (immediately above spine of scapula)

Insertion: Greater tuberosity of the humerus

Innervation: Suprascapular nerve (C4-6)

Action: Abduction and external rotation

Why do I care:  This is the muscle that most often is the culprit of shoulder pain.  It is the rotator cuff muscle that typically ends up with tendonitis and/or becomes torn.  This muscle, when irritated, will radiate pain into the lateral shoulder.  The reason this muscle is so likely to give you trouble is because of how it traverses underneath the acromion to get to its insertion point on the humerus. If the scapula is not moving in sync with the humerus when you lift your arm up, or put your arm down, you risk irritating the supraspinatus tendon with the underside of the acromion.  This can lead to fraying and/or tearing of the supraspinatus tendon.