Leif's Anatomy Tips: Muscles
Muscles 4: Tour of forearm and hand muscles, with origins and insertions explained
Last revised: 30 Sep 2011
The information on this page is organized according to the order in which we typically examine the muscles on the cadaver. The idea is to explain all the origins and insertions of these muscles based on the muscle positions, their functions, and/or mnemonics.
Forearm - Anterior
- At rest, the anterior muscles face medially, so it makes sense that many of them have an origin on medial epicondyle of humerus:
- Layer 1: palmaris longus, flexor carpi ulnaris, flexor carpi radialis. Also include pronator teres with this group (if you look carefullly it's right next to flexor carpi radialis).
- Layer 2: flexor digitorum superficialis
- In addition, two of these (with a prominent "u" in the name) are large and have additional origins:
- Flexor carpi ulnaris: shaft of ulna (because of "ulnaris"), olecranon process (because it's superficial and on the ulnar edge, so it can reach along medial side of forearm).
- F. digitorum superficialis: wide muscle, so makes sense it has origins on three bones. One is shaft of radius. Another is a process on ulna, but must be an anterior process due to muscle position, so it's the coronoid process.
- Layer 3: deeper muscles, so they are blocked by other muscles and can't reach the humerus, so they only connect to radius, ulna, interosseous membrane.
- F. pollicis longus origins = radius (makes sense, for thumb muscle) + interosseous membrane.
- F. digit. profundus origins = ulna (because not a thumb muscle) + interosseous membrane.
- Layer 4: pronator quadratus. Since ulna forms most of elbow joint with humerus, in pronation the radius is considered the more mobile bone, so I = radius, O = ulna. Actually it's "distal anterior ulna" and "distal anterior radius", but these are obvious from observing the position of this muscle.
- Both pronators: insert on the radius. For pronator teres, it's "lateral" -- can see on cadaver (thumb side).
- Palmaris longus: inserts on palmar aponeurosis, as the name (partly) implies.
- F. carpi radialis: inserts on metacarpals 2 and 3. Matches the two extensor carpi radialis muscles on back side (see posterior forearm diagram on general muscle OIA tips page).
- F. carpi ulnaris: again, this is a large muscle, so makes sense it inserts on 3 bones along ulnar edge: pisiform, hamate, and 5th metacarpal.
- Flexor digitorum muscles: can see in the fingers; the superficialis splits and inserts at middle phalanges, so profundus can pass through it to insert on distal phalanges.
- Flexor pollicis longus: as usual, "longus" muscles goes to distal phalanx.
- The muscles in the table are all thumb muscles. By default, most of these have O = trapezium (makes sense, next to thumb) plus flexor retinaculum.
- Two more details for abductor and adductor, as follows:
- Abductor pollicis brevis pulls straight out (toward anterior direction in anatomical position) so it makes sense that it also is able to uses the other thumb-side carpal (scaphoid) for this.
- The only one that doesn't use trapezium is adductor pollicis: pulls thumb in opposite direction from abductor, so its origins are found in roughly the other end of the hand. Capitate plus 2nd-4th metacarpals (mnemonic: "if you hold up your middle finger [which is adjacent to metacarpals 2-4] at someone, you may get decapitated").
- All go to proximal phalanx of thumb by default (note: typically, "brevis" muscles insert on "proximal" phalanx).
- Exception: Opponens pollicis needs to rotate thumb medially in order to orient the thumb so as to touch the fingertips in opposition; this action requires a more transverse fiber direction, so it inserts on 1st metacarpal.
Forearm - Posterior
- Many of the superficial muscles originate on lateral epicondyle of humerus. Makes sense because when arm and forearm are relaxed, posterior forearm faces laterally.
- However, the first two muscles along radial side of forearm extend farther up the humerus, and originate on lateral supracondylar ridge: brachioradialis, ext. carpi radialis longus. You can see this on cadaver.
- Muscles that insert on the lateral epicondyle are the other superficial muscles: ext. carpi radialis brevis, ext. digitorum, ext. digiti minimi, ext. carpi ulnaris.
- In addition, supinator has origin on lateral epicondyle, which makes sense because its antagonist, the pronator teres, inserts on the medial epicondyle on the other side.
- In addition to the lateral epicondyle, two of these muscles (with a prominent "u" in the name) already mentioned have additional origins:
- extensor carpi ulnaris: a large muscle (like the flexor carpi ulnaris), so "posterior ulna" too.
- supinator: deeper than the pronator teres, so maybe not surprising that unlike pronator teres, it also originates on "proximal ulna".
- Deep muscles (four):
- The three thumb muscles (abductor pollicis longus, ext. pollicis brevis, ext. pollicis longus) all have origins on radius (makes sense), ulna (that's a surprise; try to picture all three of these deep muscles spreading wide to both bones), and interosseous membrane (not surprising, once you know they go to both bones).
- Extensor indicis: ulna and interosseous membrane. Makes sense, since it lies deep, along the ulna. Note: origins are same as flexor digit. profundus on the anterior side except it's "posterior" ulna, which makes sense for this posterior muscle.
- See diagram of posterior forearm on general muscle OIA tips page.
- Brachioradialis: occupies most radial position in forearm, and has "radialis" in the name, so inserts on radius (styloid process, close to hand where other muscles insert).
- Ext. carpi rad. longus: occupies the next position over, so goes to metacarpal 2.
- Ext. carpi rad. brevis: since metacarpal 2 is "taken", this one goes to metacarpal 3.
- Ext. digitorum: distal phalanges of digits 2-5, makes sense (note: there is no longus or brevis, so this muscle needs to do the work of both, so must go to distal phalanges).
- Ext. digiti minimi: distal phalanx digit 5 (makes sense for pinky muscle).
- Ext. carpi ulnaris: metacarpal 5 (makes sense for muscle on ulnar side).
- Supinator: proximal radius (you can see this on cadaver)
- Deep muscles:
- Thumb muscle insertions are arranged in same relative position (proximal to distal) as we observe the muscle bellies on the forearm (proximal to distal and radial to ulnar):
- Abductor pollicis longus goes to 1st metacarpal (also to trapezium, which is adjacent bone)
- Ext. pollicis brevis goes to proximal phalanx
- Ext. pollicis longus goes to distal phalanx
- Extensor indicis goes to distal phalanx of 2nd digit (same insertion as branch of extensor digitorum).