Shoulder joint

Shoulder joint

Shoulder joint is the most flexible joint in the body, but pays the price of instability, several muscles cross each shoulder joint to insert on the humerus. All muscles acting on the humerus originate from the pectoral girdle; however two of these- the latissimus dorsi (back muscle)and pectoralis major-primarily originate on the axial skeleton.
Of the nine muscles covered here, only the pectoralis major,latissimus dorsi and deltoid muscle are prime movers of the arm movements. The remaining six are the synergists(helper) and fixators. Four of these, the supraspinatus, teres minor, infraspinatus and subscapularis are rotator cuff muscles. They originate on the scapula, and their tendons blend woth the fibrous capsule of the shoulder joint en route to the humerus. Although the rotator cuff muscles act as synergists in the angular and rorational movements of the arm, their main function is to reinforce the capsule of the shoulder joint to prevent dislocation of the humerus. The remaining two muscles, teres major and coracobrachialis,cross the shoulder joint but do not contribute to its reinforcement.
Generally speaking,muscles that originate anterior to the shoulder joint(pectoralis,coracobrachialis and anterior fibers of the deltoid) flex the arm, i.e., lift it anteriorly. The prime mover of arm flexion is the pectoralis major. The biceps brachii of the arm also assists in this action. Muscles originating posterior to the shoulder joint extend the arm. These include the latissumus dorsi and posterior fibers of the deltoid muscles and teres major. Note that the latissimus dorsi and pectoralis muscles are antagonists of one another in the flexion extension movements of the arm.
The middle region of the fleshy deltoid muscle of the shoulder,which extends over superolateral side of the humerus,is the prime mover of the arm abduction. The main arm abductors are the pectoralis major anteriorly and latissimus dorsi posteriorly. The small muscle acting on the humerus promote lateral and medial rotation of the arm. The interactions among these nine muscle are complex and each contributes to several movements.

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