Scapulohumeral Muscles

Scapulohumeral Muscles

The scapulohumeral muscles originate from the scapula and attach to the humerus. They are formed by the supraspinatus, infraspinatus, round minor, subscapularis and round major. The supraspinatus, infraspinatus, teres minor and subscapularis comprise the rotator cuff muscles of the shoulder. The rotator cuff muscles center the humeral head in the glenoid fossa and provide the "fine adjustment" or "direction" needed to perform various upper limb tasks.

Subscapularis

When contracted concentrically, the subscapularis muscle is a medial rotator of the humerus. This muscle also works to center the humeral head in the glenoid fossa and acts as a humeral head depressor with the other rotator cuff muscles during air activities. To assess the length of the subscapularis muscle, the subject is positioned supine with the elbow resting on the trunk while the humerus is rotated laterally. Performing this movement bilaterally allows the examiner to quickly compare the two extremities. Additionally, the tension of the subscapularis muscle prevents dissociation of the humerus from the scapula during the last 40 degrees of elevation. Failure to dissociate the humerus from the scapula results in abduction of the scapula or protrusion of the lower corner beyond the lateral trunk wall.

Infraspinatus and Teres Minor

Concentric contraction of the infraspinatus and teres minor muscles produces lateral rotation of the humerus. Cocontraction of the external rotators and subscapularis muscle results in depression and centering of the humeral head in the glenoid fossa during overhead activities. Shortness of the infraspinatus and teres minor muscles results in a decrease in medial rotation of the humerus. Muscle length assessment for these muscles can be performed with a single motion. The subject is positioned supine with the humerus abducted 90 and the elbow positioned at 90 of flexion.

The examiner stabilizes the scapula by pushing posteriorly on the head of the humerus with one hand while the other hand rotates the subject’s arm into medial rotation. When the examiner feels the scapula elevate off the table or feels tissue tension increase during medial rotation, end range has been reached.

Normal medial rotation is approximately 70° when the arm is abducted to 90. Restriction in medial

rotation can also be caused by capsular tightness of the glenohumeral joint.

Teres Major

Concentric contraction of the teres major can produce medial rotation, adduction, or extension of the shoulder. To assess the length of the teres major muscle, the subject is positioned supine so the table can assist with stabilization of the scapula. The subject performs shoulder flexion, as is also performed

with the latissimus dorsi muscle length test. The examiner observes the amount of shoulder flexion

achieved and the position of the inferior angle of the scapula. If the inferior angle of the scapula protrudes more than half an inch beyond the lateral wall of the trunk (excessive scapular abduction), a short teres major muscle is suspected. To verify, the examiner has the subject return his or her arm to the starting

position and repeat the shoulder flexion motion while the examiner stabilizes the inferior angle of the scapula at the lateral chest wall to prevent excessive scapular abduction. If the subject has less shoulder flexion compared with the previous attempt, the teres major muscle is further implicated. To confirm the shortening of the teres major muscle more definitively, the examiner instructs the subject to rotate

the shoulder medially and maintain the position of shoulder flexion with the scapula stabilized. If the subject is able to gain additional shoulder flexion, the teres major is most likely shortened.

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