The deltoid muscle consists of 3 fiber regions. The anterior portion flexes and medially rotates the humerus. The posterior portion extends and laterally rotates the humerus. The middle portion abducts the humerus. From the rest position, the deltoid generates a superiorly directed vector that pulls the head of the humerus toward the acromion. The humeral head should not glide superiorly. If the posterior head of the deltoid has become the dominant lateral rotator, the result is anterior glide of the humeral head during motion. The humeral head should not glide anteriorly.
If the anterior head if the deltoid has become a dominant medial rotator and/or the subscapularis, a medial rotator, is dominating the lateral rotators, the resting glenohumeral position is one of medial rotation. The combination of abduction and medial rotation predisposes the subject to an impingement syndrome.
It is essential that the depressors of the humeral head primarily the supraspinatus, infraspinatus, teres minor, and subscapularis adequately offset the proximal pull of the deltoid muscle. Continue reading
The supraspinatus muscle abducts and laterally rotates the shoulder, depresses, and stabilizes the humeral head in the glenoid. Because this muscle passes under the acromion, it is particularly vulnerable to injury when the shoulder is depressed. If the humeral head glides superiorly, does not glide inferiorly during shoulder flexion and abduction, or does not rotate laterally enough to prevent impingement of the greater tuberosity against the coracoacromial ligament, then the supraspinatus muscle and tendon are exposed to compression forces. Continue reading
The infraspinatus muscle laterally rotates and depresses the head of the humerus. The infraspinatus and teres minor muscles are the primary lateral rotator muscles that depresss the head of the humerus, with some assistance from the supraspinatus. Continue reading
The teres minor muscle has the same role as the infraspinatus, laterally rotating and depressing the head of the humerus. Shortness or greater stiffness of the lateral rotators relative to the stiffness of the axioscapular muscle (serratus anterior) is common and can contribute to excessive anterior and superior glide of the humeral head. Restricted posterior glide and excessive anterior glide are factors in shoulder impingement syndromes.
The lateral rotators often become short or stiff and can interfere with the posterior glide of the humeral head. It is therefore extremely important to monitor their length and strength characteristics. Continue reading
The subscapularis muscle medially rotates and depresses the head of the humerus. The subscap not only depresses the head of the humerus but also pulls it posteriorly, thus offsetting the muscles acting to cause anterior/superior glide of the humerus. The subscap and the joint capsule together provide anterior glenohumeral stability. Continue reading
The teres major muscle medially rotates, adducts, and extends the shoulder joint. Shortness of this muscle limits shoulder flexion and can impede depression and lateral rotation of the humeral head. Continue reading