MUSCLES OF THE FOREARM
· Flexor muscles of the forearm are in the anterior compartment of the forearm and are separated from the extensor muscles of the forearm by the radius and ulna bone.
· The tendons of most flexor muscles are located on the anterior surface of the wrist and are held in place by the palmar carpal ligament and the flexor retinaculum ligament.
MUSCLES OF ANTERIOR COMPARTMENT OF FOREARM
- The anterior forearm muscles are divided into 3 muscular layers; deep, superficial and intermediate
- A septum of deep fascia separates the deep layer of flexor muscles from the superficial and intermediate layers.
- Most of the flexor tendons are held in place by the flexor retinaculum, a thickening of the deep fascia of the forearm.
- This retinaculum prevents bow stringing of the tendons when the flexor muscles contract and also help improve the effective of the muscles by changing the direction of force of the tendons.
The Superficial Layer of Flexors
- This is an elongated and narrow muscle and is a pronator of the forearm and a flexor of the elbow joint.
- It forms the medial boundary of the cubital fossa and has two heads of proximal attachment.
- Proximal attachments are: medial epicondyle of humerus and coronoid process of ulna.
- Distal attachments are: middle of lateral surface of radius.
- Innervation: median nerve (C6 and C7).
- The pronator teres pronates the forearm and flexes it.
- It also assists the pronator quadratus during pronation and is a weak flexor of the forearm.
- This is a long, fusiform muscle that is located medial to the pronator teres.
- Proximal attachments are: medial epicondyle of the humerus.
- Distal attachments are: base of 2nd and 3rd metacarpal bone.
- Innervation: median nerve (C6 and C7).
- The flexor carpi radialis flexes the hand and abducts it.
- This is a small fusiform muscle that is absent on one or both sides (usually the left) in about 14% of people, but its actions are not missed.
- Proximal attachments are: medial epicondyle of humerus.
- Distal attachments are: distal half of flexor retinaculum and the palmar aponeurosis.
- Innervation: median nerve (C7 and C8).
- The palmaris longus flexes the hand and tightens the palmar aponeurosis.
- This is the most medial of the superficial flexor muscles in the forearm.
- It has 2 heads of proximal attachment, between which the ulnar nerve passes distally in the forearm.
- Proximal attachments are: humeral head-medial epicondyle of humerus, ulnar head-olecranon and posterior border of ulna.
- Distal attachments are: pisiform bone, hook of hamate bone and the 5th metacarpal bone.
- Innervation: ulnar nerve (C7 and C8).
- The flexor carpi ulnaris flexes and adducts the hand.
Intermediate Layer of Flexors
The Flexor Digitorum Superficialis
- This is the superficial muscle in the forearm and it forms an intermediate layer between the superficial and deep groups of forearm muscles.
- It has two heads.
- The median nerve and ulnar artery passes deep to the heads of this muscle.
- Near the wrist, the flexor digitorum superficialis gives rise to four tendons, which pass deep to the flexor retinaculum.
- Proximal attachments are: humeroulnar head-medial epicondyle of humerus, ulnar collateral ligament, and coronoid process of the ulna, radial head-superior half of anterior border of radius.
- Distal attachments are: bodies of the middle phalanges of the medial four digits.
- Innervation: median nerve (C7, C8, and T1).
- This muscle flexes the middle phalanges of the medial four digits by flexing the proximal interphalangeal joints.
- It also flexes the proximal phalanges (i.e., metacarpophalangeal joints) and the wrist joint.
The Flexor Digitorum Profundus
- This is a long, thick muscle.
- It is the only one that can flex the distal interphalangeal joints of the digits
- Proximal attachments are: proximal 3/4 of medial and anterior surfaces of the ulna and interosseous membrane.
- Distal attachments are: bases of the distal phalanges of medial four digits.
- Innervation: medial part-ulnar nerve (C8 and T1), lateral part-median nerve (C8 and T1).
- The flexor digitorum profundus flexes the 2nd to 5th digits after the flexor digitorum superficialis muscle has flexed the middle phalanges.
- Proximal attachments are: anterior surface of the radius and adjacent interosseous membrane.
- Distal attachments are: base of the distal phalanx of the thumb.
- Innervation: anterior interosseous nerve from the median nerve (C8 and T1).
- It flexes the distal phalanx of the thumb and, secondarily, the proximal phalanx and the first metacarpal bone.
- The flexor pollicis longus is the only muscle that flexes the interphalangeal joints of the thumb.
- It also flexes the metacarpophalangeal and carpometacarpal joints of the thumb, and it may assist with the flexion of the wrist.
- This small muscle is quadrangular and pronates the forearm.
- It cannot be palpated or observed, except in dissections, because it is the deepest muscle in the anterior aspect of the forearm.
- Proximal attachments are: distal fourth of anterior surface of ulna.
- Distal attachments are: distal fourth of anterior surface of radius.
- Innervation: anterior interosseous nerve from median (C8 and T1).
- It pronates the forearm at the superior and inferior radioulnar joints and is the prime mover in pronation of the forearm.
- The pronator quadratus initiates pronation and is assisted by the pronator teres when more speed and power are needed.
- The pronator quadratus also helps the interosseous membrane to hold the radius and ulna together, particularly when upward thrusts are transmitted through the wrist.
Muscles of the posterior compartment of forearm
Superficial layer
The Brachioradialis
- This is a fusiform muscle that forms the lateral boundary of the cubital fossa and is the most superficial muscle on the radial side of the forearm.
- Proximal attachments are: proximal two-thirds of lateral supracondylar ridge of humerus.
- Distal attachments are: lateral surface of distal of end of radius.
- Innervation: radial n. (C5, C6, C7).
- It is used to give power and speed and acts to its best advantage when the forearm is in the midprone position. It is therefore capable of initiating both pronation and supination.
The Extensor Carpi Radialis Longus
- This is a fusiform muscle that is partly overlapped by the brachioradialis with which it is often blended.
- Proximal attachments are: lateral supracondylar ridge of humerus.
- Distal attachments are: base of 2nd metacarpal bone.
- Innervation: radial nerve (C6 and C7).
- It extends and abducts the hand at the wrist joint.
The Extensor Carpi Radialis Brevis
- This is a fusiform muscle and it is shorter than the extensor carpi radialis longus, which covers it.
- Proximal attachments are: lateral epicondyle of humerus.
- Distal attachments are: base of 3rd metacarpal bone.
- Innervation: deep branch of radial nerve (C7 and C8).
- Proximal attachments are: lateral epicondyle of the humerus.
- Distal attachments are: extensor expansions of the medial four digits.
- Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.
- It extends the proximal phalanges and through its collateral reinforcements, the middle and distal phalanges as well.
- It also helps to extend the hand at the wrist joint after exerting its traction primarily on the digits.
- Proximal attachments are: lateral epicondyle of humerus.
- Distal attachments are: extensor expansion of the 5th digit.
- Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.
- The extensor digiti minimi extends the proximal phalanx of the 5th digit at the metacarpophalangeal joint and assists in the extension of the interphalangeal joints.
- It also assists with extension of the hand after exerting its traction primarily on the 5th digit.
- This is a long fusiform muscle, located on the medial border of the forearm.
- It has 2 heads.
- Its tendon runs in a groove between the head and styloid process of the ulna, within a special compartment of the extensor retinaculum.
- Proximal attachments are: lateral epicondyle of humerus and posterior border of the ulna.
- Distal attachments are: base of 5th metacarpal bone.
- Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.
- The extensor carpi ulnaris extends and adducts the hand at the wrist joint.
- Proximal attachments are: posterior surfaces of the ulna and radius, and the interosseous membrane.
- Distal attachments are: base of the 1st metacarpal bone.
- Innervation: posterior interosseous nerve (C7 and C8).
- It abducts and extends the thumb at the carpometacarpal joint.
- Proximal attachments are: posterior surface of radius and interosseous membrane.
- Distal attachments are: base of proximal phalanx of thumb.
- Innervation: posterior interosseous nerve (C7 and C8).
- It extends the proximal phalanx of the thumb at the metacarpophalangeal joint.
- It also helps to extend the metacarpal bone of the thumb and to extend and abduct the hand.
- Proximal attachments are: posterior surface of middle 1/3 of ulna and interosseous membrane.
- Distal attachments are: base of distal phalanx of thumb.
- Innervation: posterior interosseous nerve (C7 and C8).
- It extends the distal phalanx of the thumb, and in continued action, it extends the metacarpophalangeal and interphalangeal joints of the thumb.
- This is a narrow, elongated muscle that lies medial to and alongside the extensor pollicis longus.
- Proximal attachments are: posterior surface of ulna and interosseous membrane.
- Distal attachments are: extensor expansion of 2nd digit (index finger).
- Innervation: posterior interosseous nerve (C7 and C8).
- Extends 2nd digit (enabling its independent extension) helps extend at wrist
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