IV. MUSCLES OF THE HAND

A. EXTRINSIC MUSCLES - term used to define muscles which originate in the forearm and insert into the hand.

1. Muscles which act only on the wrist:
   • extensor carpi radialis longus
   • extensor carpi radialis brevis
   • extensor carpi ulnaris
   • flexor carpi radialis
   • flexor carpi ulnaris
   • palmaris longis
2. Muscles Which Act on the Wrist and Digits
   • extensor digitorum
   • extensor indicis
   • extensor digiti minimi
   • extensor pollicis longus
   • abductor pollicis longus
   • flexor digitorum superficialis
   • flexor digitorum profundus
   • flexor pollicis longus

B. INTRINSICS - refer to muscles which originate and insert into the hand.

1. FOUR LUMBRICALS (4):
   • origin: arise from the tendons of FDP in the palm.
   • inserts: on lateral side of corresponding extension expansion.
   • innervation: medial two lumbricales innervated by median nerve. Lateral two lumbricals innervated by ulnar nerve.
   • function: assisted by the interossei, they flex the MCP joints and extend the IP joints.
2. PALMAR INTEROSSEI (four) *Some authors describe only three palmar interossei and describe a second head of the FPB.
   • origin: the first arises from the medial side of the base of the 1^st metacarpal bone. The second, third, and fourth arise from the anterior surfaces of the respective metacarpal bones.
   • inserts: first one inserts into the medial side of the base of the proximal phalanx. Second one is inserted into the medial side of the base of the proximal phalanx of the index finger. The third and fourth are located on the lateral aspects of the ring/little fingers.
   • innervation: deep branch of ulnar nerve.
   • action: adduction of digits, flex MCP joints and extend IP joints. (PAD)
3. DORSAL INTEROSSEI (four)
   • origin: from sides of metacarpal bones.
   • inserts: first one inserts on lateral side of base of the index finger proximal phalanx. Second one inserts on lateral side of base of the middle finger proximal phalanx. Third one inserts on medial side of base of ring finger proximal phalanx.
   • innervation: deep branch of ulnar nerve.
   • function: abduction of digits away from the middle finger. Also flexes MCP joints and extends IP joints. (DAB)
4. ABDUCTOR POLLICIS BREVIS
   • origin: scaphoid, trapezium, and flexor retinaculum.
   • inserts: lateral aspect of proximal phalanx of thumb.
   • innervation: median nerve.
   • action: abduction of thumb at CMC and MCP joints.
5. FLEXOR POLLICIS BREVIS
   • origin: anterior surface of flexor retinaculum.
   • insertion: lateral aspect of thumbs' proximal phalanx, usually a small sigmoid bone is present.
   • innervation: median nerve.
   • action: flexes thumb, MCP joint.
6. OPPONENS POLLICIS
   • origin: flexor retinaculum.
   • inserts: shaft of thumb metacarpal.
   • innervation: median nerve.
   • action: pulls thumb medially and forward across the palm.
7. ADDUCTOR POLLICIS
   • origin: oblique head - second and third metacarpals; transverse head - third metacarpal.
   • inserts: base of proximal phalanx of thumb.
   • innervation: deep branch of ulnar nerve.
   • action: adduction of thumb.
8. ABDUCTOR DIGIT MINIMI
   • origin: pisiform bone.
   • inserts: base of little finger proximal phalanx.
   • innervation: deep branch of ulnar nerve.
   • action: abducts little finger.
9. FLEXOR DIGIT MINIMI
   • origin: flexor retinaculum.
   • inserts: base of little finger proximal phalanx.
   • innervation: deep branch of ulnar nerve.
   • action: flexes little finger.
10. OPPONENS DIGITI MINIMI
    • origin: flexor retinaculum.
    • insert: medial border of little finger metacarpal.
    • innervation: deep branch of ulnar nerve.
    • action: pulls fifth metacarpal forward as in cupping the hand.
11. PALMARIS BREVIS
    • origin: flexor retinaculum, palmar aponeurosis.
    • insert: skin of palm.
    • innervation: superficial branch of ulnar nerve.
    • action: corrugates skin to improve grip.

C. MUSCLE BALANCE: When there is normal resting tone in the intrinsic and extrinsic muscle groups, the wrist and digits will be maintained in a balanced position. With the forearm in neutral, the wrist extended and the digits in moderate flexion, the hand is in the optimum position to be functional.

To a large extent, the wrist is the key joint and has a strong influence on the long extrinsic muscle performance on the digits. Maximum digital flexion strength is facilitated by wrist extension. Conversely, a position of wrist flexion will markedly weaken grasping power.