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HUMAN KINETICS

Excerpts

Anatomy of the shoulder

This is an excerpt from Kinetic Anatomy, Second Edition, by Robert S. Behnke.


Figure 3.33
Figure 3.33

Posterior Muscles of the Shoulder Joint

The following muscles are found on the posterior aspect of the shoulder joint.

Infraspinatus: The infraspinatus muscle (figure 3.33) gets its name from the anatomical structure where it originates: the infraspinous fossa beneath the inferior surface of the spine of the scapula. The infraspinatus muscle inserts on the middle facet of the greater tuberosity of the humerus. Contraction of the infraspinatus muscle produces external rotation and extension of the shoulder joint. The infraspinatus muscle is also part of the rotator cuff.

Hands on...

Place your partner’s shoulder joint in abduction, external rotation, and extension and then locate the infraspinatus muscle (figure 3.34).


Figure 3.33 Posterior view of the suprapinatus, infraspinatus, teres minor, and teres major muscles.
Figure 3.34

Teres minor: The teres minor muscle is often considered together with the infraspinatus muscle because they share the same function. The teres minor (figure 3.33) originates on the upper and middle portions of the lateral border of the scapula and inserts on the distal facet of the greater tuberosity of the humerus. Contraction of the teres minor muscle, like the infraspinatus muscle, produces external rotation and extension of the shoulder joint. This muscle is also one of the shoulder joint muscles of the rotator cuff.

Hands on...

Place your partner’s shoulder joint in abduction, external rotation, and extension and then locate the teres minor muscle (figure 3.34).

Rotator Cuff

Four of the muscles of the shoulder joint insert on a musculotendinous structure running between the facets located on the lesser and greater tuberosities of the humerus. This structure is commonly referred to as the rotator cuff (figure 3.35). The motions produced at the shoulder joint by these four muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) have been presented, but these four muscles are also responsible for maintaining the stability of the shoulder joint, which is particularly necessary because the socket of this ball-and-socket joint is so shallow and thus provides little stability. In an action such as throwing, not only do the muscles of the rotator cuff generate the force necessary to throw, but they also decelerate the force generated. In other words, the action of the rotator cuff muscles actually prevents the entire upper extremity from following the object thrown by keeping the humeral head in the glenoid fossa.

 

Focus on...The Rotator Cuff

Understanding the action of the rotator cuff makes it easier to understand why some individuals involved in repetitive throwing activities (e.g., pitchers, quarterbacks) develop problems of the rotator cuff. The muscles not only generate the force needed by contracting concentrically (shortening) but also apply a braking action through an eccentric (lengthening) contraction to prevent the upper extremity from leaving the body.

Figure 3.33 Posterior view of the suprapinatus, infraspinatus, teres minor, and teres major muscles.



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