Arm and Forearm Muscles

Arm and Forearm Muscles

Arm Compartments

The arm has anterior and posterior compartments, separated by medial and lateral intermuscular septa arising from the medial and lateral lips of the intertubercular groove and inserting into the humerus.

Anterior arm (musculocutaneous nerve, C5–C7):

• Biceps brachii: long head from supraglenoid tubercle; short head from coracoid process; both insert onto the radial tuberosity and bicipital aponeurosis. Actions: flexion of forearm at elbow and supination (most powerful supinator). Rupture of long head tendon produces "Popeye" deformity.
• Coracobrachialis: coracoid → medial humerus shaft; flexion and adduction of arm.
• Brachialis: anterior humerus shaft → ulnar tuberosity/coronoid process; the primary forearm flexor (works regardless of forearm position).

Posterior arm (radial nerve):

• Triceps brachii: three heads — long head (infraglenoid tubercle), medial head (posterior humerus, medial to radial groove), lateral head (posterior humerus, lateral to radial groove); all insert onto the olecranon. Action: elbow extension. The radial nerve lies in the radial (spiral) groove of the humerus; mid-shaft humeral fractures damage the radial nerve and profunda brachii artery here.

Forearm Compartments

Anterior compartment (median and ulnar nerves):

• Superficial: pronator teres, flexor carpi radialis (FCR), palmaris longus (PL), flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU). Most arise from the medial epicondyle (common flexor origin); medial epicondyle tenderness = golfer's elbow.
• Deep: flexor digitorum profundus (FDP, medial half by ulnar nerve / lateral half by anterior interosseous nerve), flexor pollicis longus (FPL), pronator quadratus.

Posterior compartment (radial nerve and branches):

• Superficial: extensor carpi radialis longus (ECRL) and brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), brachioradialis. Most arise from the lateral epicondyle (common extensor origin); lateral epicondyle tenderness = tennis elbow.
• Deep: supinator, abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis (EI).

Anatomical Snuff Box

Bounded anteriorly by APL and EPB, posteriorly by EPL; floor is scaphoid and trapezium; roof is superficial radial nerve. The radial artery passes through the floor. Tenderness in the snuff box after a fall on outstretched hand suggests scaphoid fracture.