Rotator Cuff Functionality Inside the Shoulder Muscle Structure

by Jsantos, October 28, 2013

Rotator Cuff Functionality Inside the Shoulder Muscle Structure

Rotator cuff injuries are among the most common cases attended by orthopedic doctors like Doctor Mead. Being a flexible joint, the shoulder is very vulnerable and requires full attention when performing activities that require some stress. Athletes of all disciplines are constantly exposed to this injuries. Learning the functuanilty of the shoulder in its muscular structure can help you understand its capabilities and limitations, preventing further trauma.

The Muscle Structure

The musculature surrounding the shoulder joint is arranged so that it produces large stabilizing components especially by the four rotator cuff muscles. Regardless of the position of the arm, the anterior, posterior and superior deltoids have large stabilization components because of their small angle of pull. Further stability is provided by the long heads of the bicep on the anterior shoulder and the triceps on the posterior side. As with the deltoid muscle, the upward pull of these muscles is counteracted by the downward pull of the rotator cuff muscles.

Most of the other muscles surrounding the shoulder joint also exert a stabilizing force, but their main function is to move the arm. In addition, as the arm moves into motion, the muscles involved change their angles of pull considerably. Thus they may not always be major stabilizers of the shoulder joint.

The muscles that serve as the primary movers of the arm at the shoulder joint are the deltoid, coracobrachialis, pectorals major, latissimus dorsi and teres major, the long and the short heads of the biceps and the long head off the triceps on the posterior side. The muscles located on the front of the chest and shoulder are involved mainly in flexion and horizontal adduction while those on the posterior side are involved mainly in extension and horizontal abduction.

The latissimus dorsi and trees major on the posterior side rotate to the arm medially at the shoulder joint. In addition, the infraspinatus and the trees minor, also located on the posterior side of the humerus, act in a wheel-axle like mechanism to laterally rotate the arm around the longitudinal axis of the humerus. The subscapularis, located on the anterior side of the humeral head, also functions in a wheel-axle like mechanism to medially rotate the arm. The supraspinatus is attached to the top of the humeral head and functions as a first class lever to pull the top of the humeral head inward. As a result the humerus moves into abduction.

Movement Limitations

When your arms are all the way to the rear of the body and the scapulae are retracted, the initial contraction of the muscles the front of the body move the head of the humerus more to the front in order to produce horizontal flexion in the shoulder joint. The posterior rotator cuff muscles counteract this forward force component. Problems arise in the shoulder joint if the stabilizing forces are not effective in counteracting the dislocating forces of the muscles involved in the movement.

The action of arm abduction is complex. The supraspinatus initiates the first few degrees of shoulder abduction. It is a first-class lever arrangement, which gives a better angle of pull than the deltoid muscle. The deltoid does not come into play until the arm is approximately 45 degrees out to the side and up. As shoulder abduction takes place the scapula upwardly rotates in coordination with the arm movement. In this way, the acromion process of the scapula is moved out of the way as the greater tuberosity of the humerus gets close to it.

Shoulder impingement usually occurs in activities that require the arm to be abducted or flexed and medially rotated, as for example in baseball pitching. Also susceptible are tennis players and swimmers. In addition, this combination of actions occurs when you do lateral arm raises to shoulder level with the arm medially rotated.

Sources: The Complete Guide. Frederick C. Hatfield, PhD


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