Forearm muscles : Tricks to remember

Anterior Forearm (Compartment) Muscles

Total muscles: 8 (4 superficial + 1 intermediate + 3 deep)

Mnemonic: Do it yourself as shown in the figure below! Place your thenar/hypothenar eminence over medial epicondyle and fan out 5 fingers with thumb resting below the 4 fingers. The 4 fingers represent superficial flexors while the thumb represents intermediate/middle flexor. Starting from the index finger count – “Pass, Fail, Pass, Fail, and Fail (at last)” i.e. Pronator teres (PT), Flexor carpi radialis (FCR), Palmaris longus (PL), Flexor carpi ulnaris (FCU) and Flexor digitorum superficialis (FDS) resectively.

Superficial and Middle flexors

Common flexor origin: Medial epicondyle

But remember that the 3 muscles containing “U” have more attachments:

• Ulnar (deep) head of pronator teres (may be absent): Ulna (coronoid process)
• Flexor Carpi Ulnaris: Ulna (olecranon) and Ulnar shaft
• Flexor digitorum sUperficialis: Ulna (coronoid) and Radial shaft

Nerve supply: Median nerve except FCU which is supplied by ulnar nerve

Deep flexors

Mnemonic: All 3 deep flexors have a word starting with “P“.
1. flexor digitorum Profundus
2. flexor Pollicis longus
3. Pronator quadratus

All these 3 are supplied by AIN and can be tested with “OK” sign.

Nerve supply: Anterior interosseous branch of median nerve (AIN) except Flexor digitorum profondus of ring and little fingers which is also supplied by ulnar nerve.

Remember few things:

• FPL is for thumb, hence originates at the side of thumb (radius and interosseous membrane)
• FDS is for digits, hence originates at the side of the digits (ulna and interosseous membrane)
• Pronators, be it “teres” or “quadratus” goes from ulna to radius.
• “Longus” goes to distal phalanx base.
• “Superficialis” goes to proximal phalanx base.

Remember the Rule of three (3):

3 wrist flexors: FCR, PL, FCU
3 finger flexors: FDS, FDP, FPL

Other 2 are pronators: PT, PQ

Posterior Forearm (Compartment) Muscles

Total muscles: 12 (4 Superficial + 3 Mobile wad + 5 Deep)

Superficial Externsors

Mnemonic: Do it yourself as shown in the picture! Hold your elbow with thumbs up and other 4 fingers curling behind the lateral epicondyle. Then go in the sequence of thumb (B-C), little finger (C-D), ring finger (D), middle finger (C) and index finger (A). The thumb is above and represents the origin point as lateral condyle for Brachioradialis and extensor Carpi radialis longus (B-C). All other fingers represent origin point as lateral epicondyle: Little finger – extensor Carpi radialis brevis, extensor Digitorum (C-D); Ring finger – extensor Digitorum minimi (D); Middle finger – extensor Carpi ulnaris; Index finger – Anconeus (A).

Common extensor origin: Lateral epicondyle

• Brachioradialis and ECRL originates from: Lateral supracondylar ridge

Nerve supply: Radial nerve or one of it’s branches

• ECRB, EDC, EDM, ECU (lateral epicondyle group) are supplied by raidal nerve branche – PIN (posterior interosseous nerve).

Remember:

• ECU is a large muscle like FCU and has origin on posterior ulna too.
• ECRL and ECRB like FCR insert into 2nd and 3rd metacarpal base respectively.
• ECU like FCU inserts on 5th metacarpal base.

Brachioradiailis, ECRL and ECRB are the components of Mobile wad of Henry. These structures can be grasped between thumb and index finger just distal to lateral epicondyle. Brachioradialis is a paradoxical muscle. Its origin and innervation are characteristic of a extensor muscle, but it is actually a strong flexor of forearm.

Deep Extensors

Mnemonic: Like the deep flexor muscles, all the extensor muscles that have “P” in them are deep extensors –
1. abductor Pollicis longus (APL)
2. extensor Pollicis brevis (EPB)
3. extensor Pollicis longus (EPL)
4. extensor indicis Proprius (EIP)
5. PIN – supply all these muscles
6. SuPinator

Nerve supply: Radial nerve – PIN

Remember few things:

1. Supinator like Pronator teres has also origin from the epicondyle (lateral epicondyle in contrast to medial epicondyle in pronator teres)
2. Remember the 6 extensor compartments of wrist. The 1st compartment muscle (APL and EPB), they originate from radius and interosseous membrane.
3. The 3nd and 4th compartment muscle (EPL and EIP respectively) originate from ulna and interosseous membrane.

From superior to inferior, origin of muscles are:
a. APL (radius and ulna) – remember A for Above
b. EPL (ulna) and EPB (radius)
d. EIP (ulna)

As in flexors, we have rule of threes (3) for extensors as well:

3 muscles for wrist extension: ECRB, ECRL, ECU
3 muscles for finger extension: EDC, EIP, EDM
3 muscles for thumb: EPL, EPB, APL

Another handy relation to keep in the back of head is: longus, brevis, longus, brevis (longus is lateral to brevis)

In the distal forearm, APL and EBP crosses from medial to lateral over ECRL and ECRB. APL and EPB enter 1st extensor compartment at wrist while the ECRL and ECRB enter the 2nd.

Recommended reading: Netter’s atlas for images

Dr. Sulabh Kumar Shrestha, MS Orthopedics

He is the section editor of Orthopedics in Epomedicine. He searches for and share simpler ways to make complicated medical topics simple. He also loves writing poetry, listening and playing music.