Kinesiology of the Shoulder in Sports

The shoulder complex is very important in most athletic competition.

Shoulder movement is described as combined motions of the glenohumeral and the scapula thoracic joint.

The joints of the shoulder complex function as a series of links, all cooperating to maximize the range of motion available to the upper limb. A weakened, painful or unstable link anywhere along the chain ultimately decreases the effectiveness of the entire complex.

There are four joints within the shoulder complex. They are as follows:

1)  sternoclavicular

The sternoclavicular joint is enclosed by a capsule and reinforced by anterior and posterior sternoclavicular ligaments. There is a connecting ligament between the medial end of the right and left clavicles called the interclavicular ligament. The tissues that stabilize the sternoclavicular joint include these three ligaments in addition a costoclavicular ligament as well as the sternocleidomastoid, sternothyroid and sternohyoid muscles.

2)  acromioclavicular

The acromioclavicular joint is susceptible to dislocation when landing on the tip of the shoulder abruptly against the ground tearing the supporting ligaments. The acromioclavicular joint is the articulation between the lateral end of the clavicle and the acromium of the scapula. The acromioclavicular joint is surrounded by a capsule that is reinforced by superior and inferior ligaments.  These ligaments are further reinforced by the attachments of the deltoid and trapezius muscles.  The tissues that stabilize the acromioclavicular joint are the superior inferior capsular ligaments as well as the coracoclavicular ligament.  The articular disc is  held in place by the overlying deltoid and upper trapezial muscles.

3) scapulothoracic

The scapulothoracic joint is not a true anatomical joint but an interfacing of two bones. The point of contact between the anterior surface of the scapula and posterior lateral surface of the thorax is the area of the scapulothoracic joint.

4) glenohumeral

The glenohumeral joint is the articulation formed between the head of the humerus and the concavity of the glenoid fossa.  The glenohumeral joint is surrounded by a fibrous capsule, which isolates the internal joint from the surrounding tissue.  The rotator cuff muscle (subscapularis, supraspinatus, infraspinatus and teres minor) and the capsular ligaments run into the fibrous capsule providing stabilization of this joint.  The long head of the biceps also contributes to stabilizing the glenohumeral joint as well as the glenoid labrum.  The glenohumeral joint capsules receive additional reinforcement from the coracohumeral ligament.

EMG work by Dr. Basmajian revealed that vertically running muscles, such as the biceps, triceps and middle deltoids, are not actively involved in providing stability to the glenohumeral joint even when significant downward traction is applied to the arm. The supraspinatus and, to a lesser extent, the posterior deltoid provide the secondary source of static stability.

Additionally, the normally negative intraarticular pressure within the glenohumeral joint offers a secondary source of static stability. Experimental release of the pressure by piercing the capsule with a needle shows to cause inferior subluxation of the humeral head, when this vacuum pressure is disrupted.

Eight separate bursa sacs are located in the shoulder providing a buffer to reduce frictional forces between tendons, capsule, bone, muscle, ligaments or two muscles acting on each other. 

Chronic impingement syndrome of the shoulder is common to athletes and laborers who repeatedly abduct the shoulders over 90 degrees. Many factors predispose the shoulder impingement syndrome. One factor is the inability of muscles, especially the rotator cuff or the serratus anterior, to optimally coordinate the glenohumeral joint in abduction. Additional factors include the posture, degeneration of the rotator cuff muscles, instability of the glenohumeral joint, and adhesions within the joint capsule.  Finally, physical barriers such as osteophytes or bone spurs around the acromioclavicular joint, can lead to impingement syndromes.

A study showed that individuals with chronic impingement syndrome have a reduced upward movement of the scapula with reduced muscle activity from the serratus anterior during abduction. Slight weakness of the serratus anterior can disrupt the normal dynamics of the shoulder. Without the normal range of upward rotation, the acromium is more likely to interfere with the abducting humeral head.  Similarly, should the serratus anterior be shortened, the latissimuss dorsi is usually also shortened not allowing for elevation of the acronioclavicular joint and resulting impingement.

The supraspinatus muscle is usually the most utilized muscle of the entire shoulder complex. In addition to assisting the deltoid during abduction, the muscle provides dynamic and static stability to the glenohumeral joint. The supraspinatus muscle can generate the force 20 times greater than the load. Unfortunately, with overuse, these loads increase the rate of tendon tears, making them more susceptible to injury.

Should you have any further questions regarding this article, please direct your questions or comments to "Ask the Doctor" section.

Copyright © 2004 - 2010 Taras V. Kochno, M.D.  All Rights Reserved
Board Certified in Physical Medicine and Rehabilitation

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