Forearm Muscles Anatomy & Function

The forearm muscles can be subdivided into 19 intrinsic muscles and 24 extrinsic muscles arranged in anterior, posterior and mobile wad compartments.

• The intrinsic forearm muscles are located entirely within the hand; they arise and insert within the hand.
• The extrinsic forearm muscles, whose muscle bellies lie proximal to the wrist, originate in the forearm and insert within the hand.

The flexors, which are located in the anterior compartment, flex the wrist and digits while the extensors, located in the posterior compartment, extend the wrist and the digits.

The design of the extrinsic and intrinsic muscle groups provides for a large number of muscles to act on the hand without excessive bulkiness. The extrinsic tendons enhance wrist stability by balancing flexor and extensor forces and compressing the carpals.

The amount of tendon excursion determines the available range of motion at a joint. To calculate the amount of tendon excursion needed to produce a certain number of degrees of joint motion involves an appreciation of geometry. A circle’s radius equals approximately 1 radian (57.29 degrees). The mathematical radius, which is equivalent to the moment arm, represents the amount of tendon excursion required to move the joint through 1 radian.32 For example, if a joint’s moment arm is 10 mm, the tendon must glide 10 mm to move the joint 60 degrees (approximately 1 radian) or 5 mm to move the joint 30 degrees (1/2 radian).

See Also: Elbow Anatomy

Anterior Compartment of the Forearm

Anterior Compartment of the Forearm consists of three leyers: the superficial, Intermediate and deep.

Palmer Superficial Forearm Muscle

1. Pronator Teres Muscle:

The pronator teres has two heads of origin: a humeral head and an ulnar head.

The humeral head arises from the medial epicondylar ridge of the humerus and common flexor tendon, whereas the ulnar head arises from the medial aspect of the coronoid process of the ulna.

The pronator teres inserts on the anterolateral surface of the midpoint of the radius. The muscle functions predominantly to pronate the forearm, but can also assist with elbow flexion.

2. Flexor Carpi Radialis:

The FCR originates from the medial humeral epicondyle as part of the common flexor tendon.

It inserts on the anterior surface and base of the second metacarpal, possibly providing a slip to the third metacarpal.

The FCR is innervated by the median nerve and functions to flex and radially deviate the wrist.

3. Palmaris Longus:

The inconsistent palmaris longus arises from the medial humeral epicondyle as part of the common flexor tendon and inserts on the transverse carpal ligament and anterior (palmar) aponeurosis.

It receives its innervation from the median nerve.

The function of the palmaris longus is to flex the wrist, and it may play a role in thumb abduction in some people.

4. Flexor Carpi Ulnaris:

The FCU arises from two heads:

• The humeral head arises from the medial humeral epicondyle as part of the common flexor tendon,
• while the ulnar head arises from the proximal portion of the subcutaneous border of the ulna.

The FCU inserts directly onto the pisiform, the hamate via the pisohamate ligament, and onto the anterior surface of the base of the fifth metacarpal, via the pisometacarpal ligament.

The FCU is innervated by the ulnar nerve and functions to flex and ulnarly deviate the wrist.

Palmer Intermediate Forearm Muscles

1. Flexor Digitorum Superficialis:

The FDS has a three-headed origin:

• The humeral head arises from the medial humeral epicondyle as part of the common flexor tendon.
• The ulnar head arises from the coronoid process of the ulna.
• The radial head arises from the oblique line of the radius.

The FDS inserts on the middle phalanx of the medial four digits via a split “sling” tendon.

This muscle is innervated by the median nerve and serves to flex the proximal and middle IP joints of the medial four digits and assist with elbow flexion and wrist flexion. The FDS possesses tendons that are capable of relatively independent action at each finger.

Palmer Deep Forearm Muscles

1. Flexor Pollicis Longus:

The FPL has its origin on the ventral surface of the radius, medial border of the coronoid process of the ulna, and the adjacent interosseous membrane.

It inserts on the distal phalanx of the thumb.

The FPL is innervated by the anterior interosseous branch of the median nerve, and it functions to flex the thumb.

2. Flexor Digitorum Profundus:

The FDP arises from the medial and anterior (ventral) surfaces of the proximal ulna, the adjacent interosseous membrane, and the deep fascia of the forearm.

The FDP inserts on the base of the distal phalanges of the medial four digits.

The FDP has a dual nerve supply: the medial two heads are supplied by the ulnar nerve, while the lateral two heads are supplied by the anterior interosseous branch of the median nerve.

The FDP functions to flex the DIP joints, after the FDS flexes the second phalanges, and assists with flexion of the wrist.

The tendons of the FDS and FDP are held against the phalanges by a fibrous sheath. At strategic locations along the sheath, the previously mentioned five dense annular pulleys (designated A1, A2, A3, A4, and A5) and three thinner cruciform pulleys (designated C1, C2, and C3) prevent tendon bowstringing.

Unlike the FDS tendons, the FDP tendons cannot act independently. To isolate the PIP joint flexor function of these two muscles, a clinician holds the adjoining finger(s) in extension while the patient attempts to flex the finger being tested. This anchors the profundus muscle of the finger being tested distally and allows the superficialis muscle to act alone at the PIP joint.

Tendinous connections between the FDP and the FPL are a common anatomic anomaly, which have been linked to a condition causing chronic forearm pain, called Linburg syndrome, although the association is by no means conclusive.

3. Pronator Quadratus:

The pronator quadratus arises from the anterior (ventral) surface and distal quarter of the ulna and inserts on the anterior (ventral) surface and distal quarter of the radius.

The muscle functions to pronate the forearm, and it is innervated by the anterior interosseous branch of the median nerve.

The pronator quadratus is well-designed biomechanically as an effective torque producer and a stabilizer of the DRUJ, its line of force is oriented almost perpendicular to the forearm’s axis of rotation.

Posterior Compartment of the Forearm

Posterior Compartment of the Forearm consists of two layers: Superficial and deep.

Dorsal Superficial Forearm Muscles

1. Extensor Carpi Radialis Longus:

The ECRL takes its origin at the supracondylar ridge of the humerus about 4–5 cm proximal to the epicondyle, and the thickest part of the muscle is proximal to the elbow joint.

The ECRL inserts on the base of the second metacarpal and functions to extend and radially deviate the wrist. It also plays a role in elbow flexion, losing a part of its wrist action when the elbow is flexed.

2. Extensor Carpi Radialis Brevis:

The ECRB arises from the common extensor tendon on the lateral epicondyle of the humerus and from the radial collateral ligament.

It inserts on the posterior surface of the base of the third metacarpal bone and receives its nerve supply from the posterior interosseous branch of the radial nerve.

The muscle stretches across the radial head during forearm pronation, resulting in increased tensile stress when the forearm is pronated, the wrist is flexed, and the elbow is extended.

The more medial location of the ECRB compared to the ECRL makes it the primary wrist extensor, but it has also a slight action of radial deviation.

3. ED and EDM:

The extensor digitorum (ED) arises from the lateral humeral epicondyle, part of the common extensor tendon, while the extensor digiti minimi (EDM) arises from a muscular slip from the ulnar aspect of the ED muscle.

The ED inserts on the lateral and posterior (dorsal) aspect of the medial four digits, while the EDM inserts on the proximal phalanx of the fifth digit.

Both muscles are innervated by the posterior interosseous branch of the radial nerve. While the ED functions to extend the medial four digits, the EDM extends the fifth digit.

4. Extensor Carpi Ulnaris:

The ECU arises from the common extensor tendon on the lateral epicondyle of the humerus and the posterior border of the ulna.

It inserts on the medial side of the base of the fifth metacarpal bone.

It is innervated by the posterior interosseous branch of the radial nerve.

The ECU is an extensor of the wrist in supination and primarily causes ulnar deviation of the wrist in pronation, working in synergy with the FCU to prevent radial deviation during pronation.

Extension of the wrist is dependent on three muscles:

1. ECRL,
2. ECRB,
3. ECU.

The ECRB and ECRL are commonly considered to be similar muscles, but in fact they differ in many respects and have very different moment arms of extension.

The ECRB, because of its origin on the epicondyle, is not affected by the position of the elbow, so that all of its action is on the wrist, making it the most effective extensor of the wrist (because it has the greatest tension and the most favorable moment arm).

The ECRL has longer muscular fibers, mostly at the level of the elbow. The ECRL only becomes a wrist extensor after radial deviation is balanced against the ulnar forces of the ECU.

The ECU, the antagonist of EPL, has the weakest moment of extension, which becomes zero when the wrist is in complete pronation.

Dorsal Deep Forearm Muscles

1. Abductor Pollicis Longus:

The APL arises from the posterior (dorsal) surface of the proximal portion of the radius, ulna, and interosseous membrane and inserts on the ventral surface of the base of the first metacarpal.

The APL is innervated by the posterior interosseous branch of the radial nerve and functions in abduction, extension, and external rotation of the first metacarpal.

2. Extensor Pollicis Brevis:

The EPB arises from the posterior (dorsal) surface of the radius and interosseous membrane, just distal to the origin of the APL.

It inserts on the posterior (dorsal) surface of the proximal phalanx of the thumb via the extensor expansion.

The EPB is innervated by the posterior interosseous branch of the radial nerve and functions to extend the proximal phalanx of the thumb.

3. Extensor Pollicis Longus:

The EPL arises from the posterior (dorsal) surface of the midportion of the ulna and interosseous membrane.

It inserts on the posterior (dorsal) surface of the distal phalanx of the thumb via the extensor expansion.

The EPL is innervated by the posterior interosseous branch of the radial nerve. It functions in extension of the distal phalanx of the thumb and is thus involved in extension of the middle phalanx and the MCP joint of the thumb.

4. Extensor Indicis:

The EI arises from the posterior (dorsal) surface of the ulna, distal to the other deep muscles, and inserts on the extensor expansion of the index finger.

It is innervated by the posterior interosseous branch of the radial nerve and is involved in extension of the proximal phalanx of the index finger.

Wrist, Hand & Forearm Muscles

References

1. Mitchell B, Whited L. Anatomy, Shoulder and Upper Limb, Forearm Muscles. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK536975/
2. Wadsworth C: Wrist and hand. In: Wadsworth C, ed. Current Concepts of Orthopedic Physical Therapy – Home Study Course. La Crosse, WI: Orthopaedic Section, APTA, 2001.
3. Kaplan EB: Anatomy and kinesiology of the hand. In: Flynn JE, ed. Hand Surgery, 2nd ed. Baltimore, MD: Williams and Wilkins, 1975.
4. Brand PW, Hollister AM, Agee JM: Transmission. In: Brand PW, Hollister AM, eds. Clinical Mechanics of the Hand. St Louis, MO: Mosby Inc, 1999:61–99.
5. Linburg RM, Comstock BE. Anomalous tendon slips from the flexor pollicis longus to the flexor digitorum profundus. J Hand Surg Am. 1979 Jan;4(1):79-83. doi: 10.1016/s0363-5023(79)80110-0. PMID: 759509.