15.15
Muscles that Move the Arm
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Q1: What are the two main categories of arm muscles?
Arm muscles are divided into axial and scapular muscles. Axial muscles, the pectoralis major and latissimus dorsi, originate from the axial skeleton. Scapular muscles, numbering seven, originate from the scapula. Together, these nine muscles enable all arm movements and are classified as axial and appendicular muscles based on their skeletal origins.
Q2: How does the pectoralis major function in arm movement?
The pectoralis major has two heads with distinct functions. Its clavicular head flexes the arm, while the sternocostal head extends a flexed arm back to anatomical position. When both heads work together, they adduct and medially rotate the humerus, enabling powerful pushing and pulling movements.
Q3: What is the rotator cuff and which muscles compose it?
The rotator cuff consists of four scapular muscles and their tendons: supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles stabilize the shoulder joint and enable precise arm rotation. The supraspinatus is particularly prone to strain and degenerative changes due to its positioning between the humerus head and acromion.
Q4: What movements does the deltoid muscle enable?
The deltoid is the most superficial shoulder muscle with three fiber groups. Anterior fibers flex and medially rotate the arm, lateral fibers abduct it, and posterior fibers extend and laterally rotate it. This multi-directional arrangement allows the deltoid to produce the shoulder's rounded shape and enable diverse arm movements.
Q5: How do the infraspinatus and teres minor work together?
The infraspinatus and teres minor both insert at the greater tubercle of the humerus and function synergistically. While infraspinatus primarily causes lateral arm rotation, teres minor assists with both lateral rotation and arm extension. Their coordinated action enables precise rotational control and stability of the shoulder joint.
Q6: What role does the subscapularis play in arm movement?
The subscapularis originates from the subscapular fossa on the anterior scapula and inserts on the lesser tubercle of the humerus. It is the primary medial rotator of the arm and is part of the rotator cuff. This muscle works with other rotator cuff muscles to stabilize and control arm rotation.
Q7: How do the latissimus dorsi and teres major coordinate arm movements?
The latissimus dorsi originates from the thoracic and sacral regions, while teres major originates from the inferior scapular angle. Both muscles insert on the humerus and work together to extend, adduct, and medially rotate the arm. This coordination enables powerful pulling and climbing movements essential for upper body function.