Arm and Posterior Forearm Muscles – Study Notes
Introduction
Focus of the presentation: muscles of the arm and the posterior forearm (extensor forearm).
Biceps brachii
Name implies two muscle bellies: long head (more lateral) and short head (more medial).
Long head tendon path:
Travels within the intertubercular groove (between the lesser and greater tubercles of the humerus).
Passes within the glenohumeral joint capsule to reach the scapula.
In the right view, the tendon arches over the head of the humerus in the intertubercular groove and inserts at the supraglenoid tubercle (upper aspect of the glenoid fossa).
Also attaches to the glenoid labrum.
Short head origin: coracoid process of the scapula.
The two heads form a common tendon distally that inserts into the radial tuberosity on the proximal radius.
Actions: elbow flexion and forearm supination.
Biceps aponeurosis:
A thin layer of connective tissue that peels off the tendon and inserts into the deep fascia of the forearm.
Not the same as the actual tendon of the biceps brachii.
Deep to biceps brachii (after removal): two muscles
Coracobrachialis: origin from coracoid process; inserts on the humerus midshaft; action includes shoulder flexion.
Brachialis: origin from the humerus (midshaft); tendinous insertion at the ulnar tuberosity; action: elbow flexion.
Note: the coracoid process also serves as an attachment for pectoralis minor and the short head of biceps (three muscles with coracoid attachments so far: pectoralis minor, short head of biceps, coracobrachialis).
Brachioradialis:
Origin: distal humerus.
Insertion: radius.
Function: elbow flexion.
Extensor Carpi Radialis Longus (ECRL) and Extensor Carpi Radialis Brevis (ECRB)
ECRL
Origin: lateral surface of the distal humerus.
Insertion: base of the distal end of the second metacarpal (radial side).
Function: wrist extension and radial (lateral) deviation of the wrist.
Note from transcript: ECRL is described as capable of producing elbow flexion in the image/dialog; anatomically this is not its primary function.
ECRB
Origin: lateral epicondyle of the humerus.
Insertion: base of the third metacarpal (radial side, near the middle finger).
Relationship: ECRL is superficial to ECRB; ECRB lies deep to ECRL.
Function: wrist extension and radial deviation.
Common origin region:
Both originate from the lateral epicondyle of the humerus and are palpated as a bony elevation.
both: Extend the wrist and contribute to radial deviation (on the radial side of the forearm).
Extensor Digitorum, Extensor Digiti Minimi, and Extensor Carpi Ulnaris (ECU)
Extensor Digitorum (ED)
Origin: lateral epicondyle of the humerus.
Insertion: tendons insert into the distal phalanges of digits 2–5 (not the thumb).
Actions: extend the wrist (crosses the wrist) and extend digits 2–5.
Extensor Digiti Minimi (EDM)
Functionally part of ED; specifically extends the little finger (fifth digit).
Extensor Carpi Ulnaris (ECU)
Origin: lateral epicondyle of the humerus.
Insertion: base of the fifth metacarpal (ulnar side).
Actions: extend the wrist and abduct/ulnar deviation (ulnar deviation).
Visual recap: these muscles form a superficial posterior layer originating from the lateral epicondyle and extending tendons to the metacarpals and digits.
Chapter transition: next focus is a second, deeper layer of the posterior forearm muscles.
Deep layer intro and leading muscles
Deep/deeper posterior forearm layer includes two prominent groups:
Abductor Pollicis Longus (APL) and Extensor Pollicis Brevis (EPB) as part of the second layer.
Extensor Pollicis Longus (EPL) and Extensor Indicis as additional deep layer muscles.
Supinator also belongs in the deeper layer.
A note on descriptive naming:
Names are descriptive of attachment and action (e.g., pollicis = thumb; longus/brevis = long/short tendon ranges). This helps predict actions.
Abductor Pollicis Longus, Extensor Pollicis Brevis, and Extensor Pollicis Longus
Abductor Pollicis Longus (APL)
Origin: from the ulna (and possibly radius as part of the shared origin area).
Tendon path: travels to the proximal base of the first metacarpal.
Unique path detail: tendon travels to the anterior aspect of the first metacarpal to abduct the thumb (posterior forearm muscle with a tendon crossing to the ventral/anterior side of the first metacarpal).
Function: abducts the thumb.
Extensor Pollicis Brevis (EPB)
Origin: from the radius.
Insertion: proximal phalanx of the thumb.
Function: extends the proximal phalanx of the thumb.
Extensor Pollicis Longus (EPL)
Origin: from the ulna.
Insertion: distal phalanx of the thumb.
Function: extends the thumb (both joints involved, with EPL longer than EPB).
Composite view in this layer:
APL and EPB converge over the tendons of ECRL and ECRB.
EPL extends to the distal phalanx of the thumb.
Other second-layer muscle: Extensor Indicis (EI)
Origin: ulna.
Insertion: distal phalanx of the index finger (digit 2).
Function: extends the index finger.
Supinator (deep layer)
Origin: posterior ulna.
Path: wraps around the proximal radius to insert on the anterior surface of the radius.
Function: supination of the forearm (forearm rotated so palm faces anteriorly).
Mechanics note: when the forearm is pronated, the supinator lengthens; after pronation, it shortens to bring us back to supination.
Concept check on forearm positions:
In anatomical position, the forearm is in supination.
Supination can be produced by the supinator and the biceps brachii.
Recap: layers and key muscles discussed include APL, EPB, EPL, EI, and Supinator in the second/deeper layer.
Tendon Mechanics, Anatomic Snuff Box, Retinaculum, and Clinical Correlates
Recap of the first/deeper layer tendons in the posterior forearm:
Abductor Pollicis Longus (APL) inserting at the base of the first metacarpal (anterior to the first metacarpal).
Extensor Pollicis Brevis (EPB) inserting at the proximal phalanx of the thumb.
Extensor Pollicis Longus (EPL) inserting at the distal phalanx of the thumb.
Extensor Indicis (EI) inserting at the distal phalanx of the index finger.
Anatomic snuff box:
A triangular area on the radial aspect of the wrist bordered by tendons of EPL, EPB, and APL.
Contents: radial artery traverses the center of this triangle.
Clinical relevance: palpate the scaphoid bone through the snuff box; the scaphoid is the most commonly fractured wrist bone.
Extensor retinaculum:
A connective tissue band around the back of the wrist.
Function: hold the posterior forearm tendons down as they cross the wrist joint to prevent bowstringing.
Bowstringing: without the retinaculum (or around the joint), shortening of the muscles could cause the tendons to rise away from the wrist, reducing efficiency of contraction.
Clinical correlates and illustrations:
Example of extensor retinaculum damage leading to bowstringing as tendons rise under the skin when the wrist is extended.
Illustrations show bowstringing at finger joints where tendons could move away from bones without connective tissue hold-downs.
Olecranon bursa:
Location: over the olecranon process of the ulna, under the skin.
Function: reduce friction between the skin and the bony prominence.
Olecranon bursitis: repetitive irritation or trauma can inflame the bursa, causing swelling and pain.
Common origin of superficial posterior forearm muscles:
The superficial layer (first layer) of the posterior forearm muscles converges on the lateral epicondyle of the humerus (common tendon origin).
Real-world relevance:
Repetitive stress injuries can inflame the common tendinous origin at the lateral epicondyle (lateral epicondylitis), commonly known as tennis elbow.
Lateral epicondylitis (tennis elbow):
Pathophysiology: inflammation of the common tendinous attachment at the lateral epicondyle due to repetitive stress or overuse.
Clinical impact: pain and difficulty using the posterior forearm extensor muscles until inflammation subsides.
Final takeaway:
Understanding the attachments, tendon paths, and the role of the extensor retinaculum and snuff box helps explain both normal function and clinical conditions affecting the elbow and wrist.