
To raise the arm further, the head of the humerus must glide downward, still in contact with the acromion, and the shoulder blade must rotate upwards as if the arm was a lever, prying it up. At the same time, the inner end of the clavicle drops down against the sternum and the acromion rises up against the outer end of the clavicle.Ĭomplex motion also takes place in horizontal adduction. At that point, the head of the humerus comes in contact with the bottom of the acromion. The humerus fits easily in the glenoid fossa up til about 80 degrees of abduction. In the picture above, “Scapular motion”, you can see how the shoulder blade moves in relation to arm motion. However, once the arm is raised above 80 degrees, the other bones of the complex must give way a little. All of this movement happens with the humerus below approximately 80 degrees of abduction. Mechanics of Shoulder Range of MotionĪbout half of the shoulder joint movement takes place at the glenohumeral joint without involving the other bones of the complex. Both attach at the joint by ligament, the names of which are the same as the joint (acromioclavicular ligament and sternoclavicular ligament).īoth ends of the clavicle are slightly mobile to accommodate movement at the shoulder joint. The two attachment points of the collarbone are at the acromion (acromioclavicular joint) at the outer end, and the breastbone (sternoclavicular joint) at the inner end. The clavicle is also mobile, but nowhere near as much as the shoulder blade. The shoulder blade itself is mobile and, through muscles with various attachments on the humerus, spine, clavicle, and ribs, is able to rotate upwards and downwards, lifted up and down, and can slide around the ribcage forward and backward.

Not only does movement occur at the glenohumeral joint, where the head of the humerus sits in the glenoid fossa.

Scapular motionĪs mentioned earlier, shoulder movement is possible because the shoulder complex itself is so mobile. All of these muscles are subject to dysfunction of different kinds: inhibition, facilitation, tearing, weakness, atrophy, hypertonicity, repetitive motion trauma, and inflammation. Both dysfunctional amounts of tension result in range of motion problems.Īltogether, eighteen muscles control the movement and stability of the shoulder, with the primary stabilizers being the four rotator cuff muscles, supraspinatus, infraspinatus, teres minor, and subscapularis. Too lax and the joint becomes hypermobile.

The distinction works out to be a fine line. However, the extraordinary range of motion in arcs of movement require that the very structures which give the joint stability also give it flexibility as well. Shoulder joint capsule Rotator cuff musclesīecause of this lack of axial strength, stability of the shoulder joint and shoulder range of motion relies on the muscles of the rotator cuff and shoulder girdle, ligaments, fascia, and the joint capsule. The single attachment point is at the acromioclavicular joint (ACJ), where a small ligament connects the acromion to the outer end of the collarbone, known as the clavicle. Rather, most of the bones just float on muscles and fascia covering and attaching to the spine, ribcage and humerus. The most fascinating thing about the shoulder is that the entire complex has no direct, stable, connection to the axial skeleton. Because the glenoid is considerably shallower than a true ball-and-socket joint, a small cup made of cartilage gives the joint a slightly larger socket, called the labrum (not shown). The humerus is held in place by tiny, loosely fitted ligaments attached to the edge of the fossa. The head of the humerus sits in a hollowed indentation in the edge of the shoulder blade called the glenoid fossa. The bony architecture of the shoulder consists of the shoulder blade and acromion in a single unit, the clavicle, and the humerus. To see just how involved shoulder movement is first requires a look at the anatomy of the shoulder complex of bones and joints. Movement at the shoulder joint, however, is much more complicated than simple movement in the one joint. Due to the uniqueness of the joint design, a healthy individual should be able to freely move their shoulder in eleven planes - flexion, extension, neutral and horizontal abduction, neutral and horizontal adduction, neutral and horizontal internal rotation, neutral and horizontal external rotation, and circumduction. The shoulder joint has the greatest range of motion of any other joint in the body. Medicare, Medicaid, Medicare Supplemental & Advantage Plans.
