Shoulders (Part 2): Extrinsic Shoulder Muscles
This is a two part post. Read Part 1 here.
When yoga teachers and therapists talk about the shoulders, we often mean the geography of the shoulder girdle. Included in the shoulder area are the upper back, proximal arm bone, lower neck and the bones of the shoulder girdle, which consist of the scapula, clavicles and the upper portion of the sternum (manubrium). The placement of the shoulder girdle on the thorax directly affects our head position and posture, which can make standing up straight a daunting task. My Last blog discussed the placement of the scapula on the thorax and the basic corresponding muscular actions that move them. This blog will talk about those muscular actions in more detail and provide some exercises.
The shoulder (gleno-humeral) joint and is held in place by the extrinsic and intrinsic shoulder muscles. These muscle groups are called the posterior axioappendicular muscles and attach the bones of the arm to the trunk. If you think about it, the shoulder complex, without such strong muscular attachments, would dramatically collapse forward off the trunk (a disaster) and the arm bone would dangle down (subluxation) from the shoulder. The arm bone is held in the socket by the intrinsic (rotator cuff muscles) and the scapula is positioned on the back by the larger, extrinsic shoulder/back muscles. Besides the shoulder joint itself, the only other joints are the fairly unmovable sternoclavicalar joint which attaches the collarbone to sternum (and is the only joint which attaches the upper extremity complex to the axial skeleton) and the more distal acromiaoclavicular joint, which absorb kinetic forces between the arm and the axial skeleton.
The two large superficial (extrinsic) shoulder muscles are the fan shaped trapezius (upper, middle and lower) and the huge latissimus dorsi. The trapezius directly attaches the shoulder girdle to the trunk by way of the posterior neck and upper- mid back. The job of the middle and lower trapezius is to depress and retract, actions that fix the scapula on the thoracic wall. Weakness of this muscle would cause the shoulders to droop. The latissimus dorsi is a large fan shaped back muscle that attaches to the proximal humerus bone and retracts and abducts the humerus – so if your LD were very tight, you would not be able to raise your arm over your head. Also the LD pulls the trunk towards the arms, like spider man.
The two deep posterior (extrinsic) shoulder muscles include the levator scapula and the rhomboids. The levator scapula are deep strap like muscles that run from the inner border of the scapula to the side of the first 4 vertebrae. The job of the levator scapula is to lift the scapula up and tilt it forward (the rhomboids help) towards the shoulder socket. They also work to extend and tilt the neck and head. The rhomboids lie underneath the middle trapezius and attach the scapula to the sides of the spine. These are my favorite shoulder muscles because they retract and downwardly rotate the scapula and help depress the shoulder socket. They also work with the serratus anterior to hold the scapula on the back and fix them down when the arm moves up. The rhomboid minor is about ½ the size of the major, and superior to it (towards the head).