Dominic Stone
The humerus is a long bone that forms the upper arm, connecting the shoulder to the elbow. It is the longest bone in the upper extremity and is crucial for the overall movement and stability of the upper limb. The humerus serves as an attachment point for various muscles, including the supraspinatus, infraspinatus, and teres minor, which contribute to the coordinated actions of the upper limb. The shoulder joint, or glenohumeral joint, is a ball-and-socket joint that permits a vast range of motion. The shoulder's main flexors are the anterior deltoid, coracobrachialis, and pectoralis major, with the biceps brachii assisting weakly. The subscapularis muscle, one of the four rotator cuff muscles, also plays a role in internally rotating the humerus.
| Characteristics | Values |
|---|---|
| Muscles that flex the humerus | Anterior deltoid, coracobrachialis, pectoralis major, biceps brachii, subscapularis, latissimus dorsi, teres major, and anterior deltoid |
| Humerus joint | Glenohumeral joint |
| Humerus function | Plays a crucial role in transmitting forces generated by the muscles, enabling forearm movements, and contributing to the overall dexterity of the upper limb |
| Humerus attachments | Supraspinatus, infraspinatus, and teres minor |
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What You'll Learn
The coracobrachialis, biceps brachii, and pectoralis major muscles
The biceps brachii muscle is involved in flexion and adduction of the arm at the shoulder joint. It originates with its long head from the supraglenoid tubercle of the scapula and with its short head from the coracoid process of the scapula. Both heads of the biceps brachii join at the level of the deltoid tuberosity of the humerus. The biceps brachii muscle is one of the flexors of the arm, along with the coracobrachialis and brachialis muscles.
The pectoralis major muscle is located anteriorly in the thorax and forms much of the anterior wall of the axillary cavity. It has three parts: a clavicular head, a sternum-costal head, and an abdominal head. The bundles of these three parts converge outwards and insert at the lateral lip of the bicipital groove of the humerus. The pectoralis major muscle is one of the shoulder's main flexors, along with the anterior deltoid and coracobrachialis. It also contributes to internal rotation and adduction on a horizontal plane.
These three muscles work together to facilitate the movement of the humerus, with the coracobrachialis and biceps brachii muscles providing flexion and adduction of the arm, and the pectoralis major muscle contributing to flexion, internal rotation, and adduction. The biceps brachii weakly assists in the flexion action, while the pectoralis major is one of the primary flexors of the shoulder.
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The latissimus dorsi, teres major, and long head of triceps brachii muscles
The teres major is a small muscle that runs along the lateral border of the scapula. It is one of the seven scapulohumeral muscles that facilitate shoulder movement. Embryologically, the teres major is closely related to the latissimus dorsi, as they both originate from the pre-muscle sheath of the arm. The teres major is also involved in the extension, adduction, and internal rotation of the shoulder. It is active in the extension of the humerus, particularly when initiated from a position of partial flexion or abduction.
The triceps brachii is a large muscle on the back of the upper limb, consisting of three parts: the medial, lateral, and long head. The long head crosses both the elbow and shoulder joints, allowing for elbow extension and shoulder movements. The triceps brachii is classically believed to be innervated by the radial nerve, but recent studies have found that in some individuals, the long head is innervated by the axillary nerve. The triceps brachii is an antagonist of the biceps and brachialis muscles, and it can fixate the elbow joint during fine movements.
These three muscles work together to facilitate various movements of the humerus, including extension, adduction, and internal rotation of the shoulder, as well as elbow extension.
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The supraspinatus, infraspinatus, and teres minor muscles
The supraspinatus is one of the prime abductors of the arm, believed to be important for movement initiation and early abduction. The deltoid muscle, the other prime abductor, is engaged from approximately 20° of abduction and carries the arm through to the full 180° of abduction. The supraspinatus complements the infraspinatus and teres minor in their role of opposing the upward pull of the deltoid muscle and stabilizing the humeral head against the glenoid fossa, preventing the upward displacement of the humeral head and impingement on the coracoid process.
The infraspinatus muscle is found on the dorsal surface of the scapula, arising lateral and inferior from the supraspinatus muscle and medial and superior to the teres major and minor. Due to their close anatomical relation and being separated only by the spine of the scapula, the infraspinatus and supraspinatus are considered as one functional unit. The infraspinatus is also a rotator cuff muscle that externally rotates the humerus and protects the shoulder joint.
The teres minor is a posterior muscle of the shoulder that extends between the scapula and the head of the humerus. It is the smaller of the two "teres muscles", the other being the "teres major". The teres minor is also a rotator cuff muscle that externally rotates and adducts the arm. Working together with the infraspinatus, the teres minor pulls the greater tubercle posteriorly, resulting in external (lateral) rotation of the arm. This action supports the shoulder and at the same time loads the force in the preload phase of overhead movements (e.g. throwing).
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The subscapularis muscle
The primary blood supply to the subscapularis muscle is the subscapular artery, a branch of the axillary artery. The subscapularis is innervated by both the upper and lower subscapular nerves (C5-C6), which originate from the posterior cord of the brachial plexus. The upper subscapular nerve supplies the upper part of the subscapularis, while the lower subscapular nerve branches into two, with one branch supplying the lower part.
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The deltoid muscle
The anterior deltoid is responsible for flexion, internal rotation, and horizontal adduction. To stretch this muscle, the actions of flexion, internal rotation, and horizontal adduction are reversed, resulting in extension, external rotation, and horizontal abduction. The posterior deltoid, on the other hand, is primarily involved in extension, external rotation, and horizontal abduction. Stretching this muscle involves reversing its actions, leading to flexion, internal rotation, and horizontal adduction.
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Frequently asked questions
The humerus is a long bone that forms the upper arm. It is the longest bone in the upper extremity.
The muscles that flex the humerus include the anterior deltoid, coracobrachialis, pectoralis major, and biceps brachii.
The humerus serves as an attachment point for 13 muscles, which contribute to the movement of the hand, elbow, and upper limb.
The shoulder muscles support and stabilise the shoulder joint, allowing for a wide range of movements. They also help protect and maintain the joint.
