Section 1: Posterior Extensor Muscles

Question 1

Brachioradialis

Answer 1

1. Superficial layer
   Or: proximal 2/3 of supra-epicondylar ridge of humerus
   Ins: Lateral surface of distal end of radius proximal to styloid process
   Inv: Radial Nerve (C5, C6*, C7)
   BS:
   Action: Relatively weak flexion of forearm; maximal when forearm is in mid-prodated position

Question 2

Extensor Carpi Radialis Longus (ECRL)

Answer 2

2. Superficial layer
   Or: Lateral supra-epicondylar ridge of humerus
   Ins: Dorsal aspect of vase of 2nd metacarpal
   Inv: Radial Nerve (C6, C7)
   BS:
   Action: Extend and abduct hand at the wrist joint; ECRL active during fist clenching

Question 3

Extensor Carpi radialis brevis (ECRB)

Answer 3

3. Superficial layer
   Or: Lateral epicondyle of humerus
   Ins: Dorsal aspect of base of 3rd metacarpal
   Inv: Deep branch of radial nerve (C7*, C8)
   Action: Extend and abduct hand at the wrist joint; ECRL active during fist clenching

Question 4

Extensor digitorum

Answer 4

4: superficial layer
Or: Lateral epicondyle of humerus
Ins: Extensor expansions of medial four digits
Inv: Deep branch of radial nerve (C7*, C8)
BS:
Action: Extends medial four digits primarily at metacarpophalangeal joints, secondarily at interphalangeal joints

Question 5

Extensor digiti minimi (ECU)

Answer 5

5: Superficial layer
Or: Lateral epicondyle of humerus
Ins: Extensor expansion of 5th digit
Inv: Deep branch of radial nerve (C7*, C8)
BS:
Action: Extends 5th digit primarily at metacarpophalangeal joint, secondarily at interphalangeal joint

Question 6

Extensor carpi ulnaris (ECU)

Answer 6

6: superficial layer
Or: Lateral epicondyle of humerus; posterior border of ulna via a shared aponeurosis
Inv: Dorsal aspect of base of 5th metacarpal
BS:
Action Extends and adducts hand at wrist joint (also active during fist clenching)

Question 7

Supinator

Answer 7

1. Deep layer
   Or: Lateral epicondyle of humerus; radial collateral and anular ligaments; supinator fossa; crest of ulna
   Ins: Lateral, posterior, and anterior surfaces of proximal third of radius
   Inv: Deep branch of radial nerve (C7, C8*)
   BS:
   Action: Supinates forearm; rotates radius to turn palm anteriorly or superiorly (if elbow is flexed)

Question 8

Extensor indicis

Answer 8

2 Deep layer
Or: Posterior surface of distal third of ulna and interosseous membrane
Ins: Extensor expansion of 2nd digit
Inv: Posterior interosseous nerve (C7*, C8), coninuation of deep branch of Radial nerve
BS:
Action: Extends 2nd digit (enabling its independent extension); helps extend hand at wrist

Question 9

Abductor pollicis longus (APL)

Answer 9

1: outcropping muscle of deep layer
Or: Posterior surface of proximal halves of ulna, radius, and interosseous membrane
Ins: Base of 1st metacarpal
Inv: Posterior interosseous nerve (C7, C8*), continuation of deep branch or radial nerve
BS:
Action: Abducts thumb and extends it at carpometacarpal joint

Question 10

Extensor pollicis longus (EPL)

Answer 10

2: outcropping muscle of deep layer
Or: Posterior surface of middle third of ulna and interosseous membrane
Ins: Dorsal aspect of base of distal phalanx of thumb
Inv: Posterior interosseous nerve (C7, C8*), continuation of deep branch of radial nerve
BS:
Action: Extends distal phalanx of thumb at interphalangeal joint; extends metacarpophalangeal and carpometacarpal joint

Question 11

Extensor pollicis brevis (EPB)

Answer 11

3: outcropping muscle of deep layer
Or: Posterior surface of distal third of radius and interosseous membrane
Ins: Dorsal aspect of base of proximal phalanx of thumb
Inv: Posterior interosseous nerve (C7, C8*), continuation of deep radial nerve
BS:
Action: Extends proximal phalanx of thumb at metacarpophalangeal joint; extends carpometacarpal joint