shoulder muscle

# The shoulder girdle: muscles and their organization The shoulder girdle is comprised of four distinct muscle groups that act as primary motors for its movement and stabilization [2](#page=2). ## 1. The shoulder girdle: muscles and their organization The muscles surrounding the shoulder girdle are systematically organized into four distinct groups: anterior, posterior, lateral, and medial. These groups collectively form the axillary hollow (armpit) [2](#page=2). ### 1.1 The anterior muscle group The anterior muscle group is organized by depth, progressing from a deep plane to a superficial plane [3](#page=3). #### 1.1.1 Deep plane: foundational stabilizers * **Subclavius muscle** * Origin: Underside of the clavicle [4](#page=4). * Insertion: 1st rib [4](#page=4). * Action: Lowers the collarbone or raises the 1st rib [4](#page=4). * Innervation: Subclavian nerve [4](#page=4). * **Pectoralis minor muscle** * Origin: 3rd, 4th, and 5th ribs [4](#page=4). * Insertion: Coracoid process of the scapula [4](#page=4). * Action: Lowers the coracoid process when the costal area is fixed; acts as Levatores costarum longi when the scapula is fixed [4](#page=4). * Innervation: Medial pectoral nerve [4](#page=4). #### 1.1.2 Superficial plane: the pectoralis major * **Pectoralis major muscle** * Origin: Three heads: * Clavicular head: Medial two-thirds of the anterior edge of the clavicle [5](#page=5). * Sternocostal head: Anterior side of the sternum and the first six costal cartilages [5](#page=5). * Abdominal head: Sheath of the right abdominal muscle [5](#page=5). * Insertion: Inter-tuber furrow (intertubercular groove) [5](#page=5). * Action: Adducts and medially rotates the arm when the costal area is fixed; elevates the trunk and acts as an accessory inspirator when the humerus is fixed [5](#page=5). * Innervation: Lateral and medial pectoral nerves [5](#page=5). ### 1.2 The posterior muscle group This is a complex group responsible for rotation, stabilization, and powerful extension of the arm. It includes the subscapularis, supraspinatus muscle, infraspinatus muscle, teres minor, teres major, and latissimus dorsi [6](#page=6). #### 1.2.1 The rotator cuff muscles The rotator cuff is comprised of four muscles that contribute to shoulder stability and movement. * **Subscapularis muscle** * Origin: Subscapular fossa [7](#page=7). * Insertion: Minor tubercle of the humerus [7](#page=7). * Action: Medial rotator of the arm [7](#page=7). * Innervation: Subscapular nerve [7](#page=7). * **Supraspinatus muscle** * Origin: Supraspinous fossa [7](#page=7). * Insertion: Major tubercle of the humerus [7](#page=7). * Action: Abducts the arm [7](#page=7). * Innervation: Subscapular nerve [7](#page=7). * **Infraspinatus muscle** * Origin: Infraspinous fossa [8](#page=8). * Insertion: Major tubercle of the humerus [8](#page=8). * Action: Lateral rotator and abductor of the arm [8](#page=8). * Innervation: Subscapular nerve [8](#page=8). * **Teres minor muscle** * Origin: The upper third of the lateral edge (border) of the scapula [8](#page=8). * Insertion: Major tubercle of the humerus [8](#page=8). * Action: Lateral rotator and abductor of the arm [8](#page=8). * Innervation: Axillary nerve [8](#page=8). #### 1.2.2 Posterior power movers * **Teres major muscle** * Origin: Lateral edge and lower angle of the scapula [9](#page=9). * Insertion: Inter-tubercular groove [9](#page=9). * Action: Adducts and medially rotates the arm [9](#page=9). * Innervation: Round nerve (likely Teres Major nerve) [9](#page=9). * **Latissimus dorsi muscle** * Origin: Median sacral crest; spinous processes and interspinous ligaments from T7 to L5 [9](#page=9). * Insertion: Minor tubercle of the humerus [9](#page=9). * Action: Adducts and medially rotates the arm [9](#page=9). * Innervation: Thoracodorsal nerve [9](#page=9). ### 1.3 The lateral and medial groups These groups include the deltoid and serratus anterior muscles, respectively. #### 1.3.1 Lateral muscle group: deltoid * **Deltoid muscle** * Origin: Lateral one-third of the collarbone, acromion, and scapular spine [10](#page=10). * Insertion: Deltoid tuberosity on the lateral side of the humerus [10](#page=10). * Action: Abducts the arm; accessorily acts as an ante-pulser (flexor) and retro-pulser (extensor) [10](#page=10). * Innervation: Axillary nerve [10](#page=10). #### 1.3.2 Medial muscle group: serratus anterior * **Serratus anterior muscle** * Origin: First 10 ribs [10](#page=10). * Insertion: Medial edge of the scapula [10](#page=10). * Action: Pulls the scapula against the thorax by sliding it over the chest wall [10](#page=10). * Innervation: Long thoracic nerve [10](#page=10). --- # The muscular architecture of the arm compartments The arm is anatomically divided into two principal muscle compartments: the anterior and posterior compartments, delineated by the medial and lateral brachial intermuscular septa and enclosed by the brachial fascia [11](#page=11). ### 2.1 The anterior compartment: flexors and supinators The anterior compartment of the arm houses muscles primarily responsible for flexing the forearm and supinating the forearm. This compartment can be further divided into deep and superficial planes [12](#page=12). #### 2.1.1 Deep plane muscles * **Coracobrachialis** * **Origin:** Coracoid process [12](#page=12). * **Insertion:** Medial surface of the humerus [12](#page=12). * **Action:** Flexion and adduction of the arm [12](#page=12). * **Innervation:** Musculocutaneous nerve [12](#page=12). * **Brachialis** * **Origin:** Inferior half of the medial and lateral surfaces of the humerus [12](#page=12). * **Insertion:** Ulnar tuberosity at the coronoid process [12](#page=12). * **Action:** Flexion of the forearm [12](#page=12). * **Innervation:** Casserio's nerve (Musculocutaneous nerve) [12](#page=12). #### 2.1.2 Superficial plane muscles * **Biceps brachii** * **Origin:** * Long head: Supraglenoid tubercle [12](#page=12). * Short head: Coracoid process [12](#page=12). * **Insertion:** Radial tuberosity [12](#page=12). * **Action:** Flexion of the forearm and supination of the forearm [12](#page=12). * **Innervation:** Musculocutaneous nerve [12](#page=12). ### 2.2 The posterior compartment: the triceps brachii The posterior compartment of the arm is dominated by a single, large muscle. * **Triceps brachii** * **Origin:** * Long head: Infraglenoid tubercle [13](#page=13). * Lateral and medial heads: Posterior surface of the humerus [13](#page=13). * **Insertion:** Olecranon [13](#page=13). * **Action:** Extension of the elbow [13](#page=13). * **Innervation:** Radial nerve [13](#page=13). > **Tip:** Understanding the origins and insertions of these muscles is crucial for predicting their actions. Pay close attention to the specific bony landmarks mentioned for each muscle. --- # Principles of muscle function and integration This section explores how muscles in the shoulder and arm function as an integrated system, emphasizing the interplay between mobility and stability, and the direct relationship between muscle structure and its functional role [14](#page=14). ### 3.1 Integrated movement system The musculoskeletal system operates as a coordinated network where different muscle groups work synergistically to produce movement [14](#page=14). #### 3.1.1 Shoulder complex The shoulder joint exemplifies a balance between the need for extensive mobility and the requirement for joint stability [14](#page=14). * **Mobility muscles:** Muscles such as the deltoid and pectoralis major are primarily responsible for generating large ranges of motion at the shoulder [14](#page=14). * **Stability muscles:** The rotator cuff muscles and the serratus anterior play crucial roles in stabilizing the shoulder joint, preventing excessive movement and maintaining proper alignment during activity [14](#page=14). #### 3.1.2 Arm musculature The arm's musculature is organized into distinct functional compartments, often operating in an agonist-antagonist relationship [14](#page=14). * **Agonist/Antagonist Pairing:** A prime example is the anterior flexor compartment (e.g., biceps brachii) acting as agonists to flex the elbow, while the posterior extensor compartment (e.g., triceps brachii) acts as antagonists, opposing flexion and facilitating extension [14](#page=14). ### 3.2 Form dictates function A fundamental principle in muscle physiology is that the anatomical structure of a muscle directly determines its functional capabilities [14](#page=14). * **Origin and Insertion:** The specific origin (proximal attachment) and insertion (distal attachment) points of each muscle dictate the direction and type of force it can generate, thereby defining its precise role in the biomechanics of human movement [14](#page=14). * **Symphony of Movement:** This precise relationship between anatomical form and functional role allows for the intricate and coordinated execution of complex human movements [14](#page=14). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Shoulder girdle | A complex of muscles and bones that connect the arm to the torso, allowing for a wide range of arm movements and providing stability. | | Axillary hollow | The armpit region, formed by the collective muscles surrounding the shoulder. | | Anterior muscle group | One of the four main muscle groups of the shoulder girdle, organized into deep and superficial planes, including muscles like the subclavius, pectoralis minor, and pectoralis major. | | Deep plane | The layer of muscles situated beneath superficial muscles, often serving foundational stabilizing roles, such as the subclavius and pectoralis minor. | | Superficial plane | The outermost layer of muscles, typically responsible for more powerful or gross motor movements, like the pectoralis major. | | Subclavius muscle | A small muscle located beneath the clavicle, which helps to depress the collarbone or elevate the first rib. | | Pectoralis minor muscle | A muscle situated beneath the pectoralis major, originating from the ribs and inserting onto the scapula, involved in moving the scapula. | | Pectoralis major muscle | A large, fan-shaped chest muscle with three heads (clavicular, sternocostal, and abdominal), responsible for adduction and medial rotation of the arm, and also acting as an accessory inspirator. | | Clavicular head | The portion of the pectoralis major muscle originating from the clavicle. | | Sternocostal head | The portion of the pectoralis major muscle originating from the sternum and costal cartilages. | | Abdominal head | The portion of the pectoralis major muscle originating from the sheath of abdominal muscles. | | Inter-tubercular furrow | A groove on the humerus where the pectoralis major and latissimus dorsi muscles insert. | | Posterior muscle group | One of the four main muscle groups of the shoulder girdle, including muscles involved in rotation, stabilization, and extension of the arm, such as the rotator cuff muscles and the latissimus dorsi. | | Subscapularis | A muscle located in the subscapular fossa of the scapula, acting as a medial rotator of the arm. | | Supraspinatus muscle | A muscle of the rotator cuff, originating from the supraspinous fossa, responsible for arm abduction. | | Infraspinatus muscle | A muscle of the rotator cuff, originating from the infraspinous fossa, acting as a lateral rotator and abductor of the arm. | | Teres minor | A small muscle of the rotator cuff, originating from the scapula, also acting as a lateral rotator and abductor of the arm. | | Teres major | A muscle originating from the scapula, acting as an adductor and medial rotator of the arm. | | Latissimus dorsi | A large back muscle originating from the spine and ribs, inserting on the humerus, responsible for adduction and medial rotation of the arm. | | Rotator cuff | A group of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) that surround the shoulder joint, providing stability and enabling rotation. | | Lateral muscle group | A muscle group of the shoulder girdle, prominently featuring the deltoid muscle, responsible for arm abduction and assisting in flexion and extension. | | Deltoid muscle | The large, triangular muscle covering the shoulder joint, responsible for arm abduction and aiding in flexion and extension. | | Medial muscle group | A muscle group of the shoulder girdle, including the serratus anterior, which is crucial for scapular movement and stability against the thorax. | | Serratus anterior | A muscle originating from the ribs and inserting onto the medial border of the scapula, responsible for protracting and rotating the scapula. | | Brachial fascia | The connective tissue sheath that encases the muscles of the arm. | | Intermuscular septa | Fibrous sheets of connective tissue that divide the arm into anterior and posterior compartments. | | Anterior compartment of the arm | The section of the arm containing muscles primarily responsible for flexing the forearm and supinating the hand, including the biceps brachii, brachialis, and coracobrachialis. | | Posterior compartment of the arm | The section of the arm containing the triceps brachii muscle, responsible for extending the elbow. | | Coracobrachialis | A muscle in the anterior compartment of the arm that flexes and adducts the arm. | | Brachialis | A muscle in the anterior compartment of the arm that is a primary flexor of the forearm. | | Biceps brachii | A two-headed muscle in the anterior compartment of the arm, responsible for flexing the forearm and supinating the hand. | | Triceps brachii | A three-headed muscle in the posterior compartment of the arm, responsible for extending the elbow. | | Olecranon | The bony prominence at the proximal end of the ulna, forming the point of the elbow, serving as the insertion point for the triceps brachii. | | Agonist/antagonist pairing | A functional relationship between muscles where one muscle (agonist) performs a primary action, and another muscle (antagonist) opposes it, allowing for controlled movement. |