DEEP LAYER OF POSTERIOR COMPARTMENT

GENERAL INFORMATION

The deep layer of the posterior forearm compartment contains muscles that are responsible for extending the fingers and thumb, as well as abducting the thumb and supinating the forearm. Additionally, the deep layer of the posterior forearm compartment contributes to precise hand movements, wrist stability, and the ability to rotate the forearm. Moreover, they play a crucial role in activities such as writing, typing, lifting, and gripping.

ORIGIN

• Extensor Indicis: Posterior surface of the ulna and interosseous membrane.
• Supinator: Lateral epicondyle of the humerus, radial collateral and annular ligaments, and the supinator crest of the ulna.
• Abductor Pollicis Longus: Posterior surface of the ulna, radius, and interosseous membrane.
• Extensor Pollicis Longus: Posterior surface of the middle third of the ulna and interosseous membrane.
• Extensor Pollicis Brevis: Posterior surface of the radius and interosseous membrane.

INSERTION POINT

• Extensor Indicis: Extensor expansion of the index finger.
• Supinator: Lateral, posterior, and anterior surfaces of the proximal radius.
• Abductor Pollicis Longus: Base of the first metacarpal (thumb).
• Extensor Pollicis Longus: Base of the distal phalanx of the thumb.
• Extensor Pollicis Brevis: Base of the proximal phalanx of the thumb.

MAJOR ARTERIES

• Posterior Interosseous Artery: Supplies blood to the deep muscles of the posterior forearm, including the extensors of the thumb and index finger.

NEURAL INNERVATION

• Extensor Indicis: Radial nerve (posterior interosseous nerve, C7, C8).
• Supinator: Radial nerve (posterior interosseous nerve, C6, C7).
• Abductor Pollicis Longus: Radial nerve (posterior interosseous nerve, C7, C8).
• Extensor Pollicis Longus: Radial nerve (posterior interosseous nerve, C7, C8).
• Extensor Pollicis Brevis: Radial nerve (posterior interosseous nerve, C7, C8).

TRIGGER POINT

Trigger points in the deep layer of the posterior compartment of the forearm muscles, particularly in the extensor indicis and supinator, can cause referred pain in the back of the hand and wrist. These trigger points often result from repetitive use of the wrist and hand, such as in typing or heavy gripping, leading to stiffness and discomfort during finger extension.

CONCENTRIC FUNCTION

• Extensor Indicis: Extends the index finger, contributing to precise hand movements like typing and pointing.
• Supinator: Supinates the forearm, turning the palm upwards for activities like opening doors or using a screwdriver.
• Abductor Pollicis Longus: Abducts and extends the thumb at the carpometacarpal joint, essential for thumb movement.
• Extensor Pollicis Longus: Extends the thumb at the distal phalanx, facilitating actions like hitchhiking or grasping objects.
• Extensor Pollicis Brevis: Extends the thumb at the proximal phalanx, aiding in grasping and manipulating objects.

ECCENTRIC FUNCTION

• Extensor Indicis: Controls the flexion of the index finger, providing smooth release after gripping.
• Supinator: Controls pronation of the forearm, preventing excessive inward rotation.
• Abductor Pollicis Longus: Controls the movement of the thumb as it returns to a resting position after abduction.
• Extensor Pollicis Longus: Regulates the movement of the thumb as it returns from extension to a neutral position.
• Extensor Pollicis Brevis: Controls the flexion of the thumb at the proximal phalanx.

ISOMETRIC FUNCTION

The deep layer of the posterior forearm compartment stabilises the thumb, index finger, and forearm during activities requiring static positioning, such as holding objects, grasping, or maintaining wrist stability during precise movements. The supinator helps stabilise the forearm during static hand rotations.

RELATED MUSCLE – SCIENTIFIC NAMES

• Extensor Digitorum: Works with extensor indicis to extend the fingers.
• Brachioradialis: Assists in forearm flexion and stabilisation.
• Extensor Carpi Radialis Longus: Aids in wrist extension and radial deviation.
• Flexor Pollicis Longus: Opposes the extensor pollicis muscles by flexing the thumb.
• Pronator Teres: Works in opposition to the supinator, pronating the forearm.

ANTAGONIST

The antagonists of the deep layer of the posterior forearm compartment are as follows:

• Flexor Digitorum Profundus: Opposes the extensor indicis by flexing the fingers.
• Pronator Teres: Opposes the supinator by pronating the forearm.
• Flexor Pollicis Longus: Opposes the extensor pollicis longus and brevis by flexing the thumb.
• Flexor Carpi Ulnaris: Works against wrist extension by flexing the wrist.
• Palmaris Longus: Opposes the extensor muscles by assisting in wrist flexion and grip control.

COMMON INJURIES

• Tennis Elbow (Lateral Epicondylitis): Inflammation of the extensor tendons at the lateral epicondyle, causing pain in wrist extension and gripping.
• De Quervain’s Tenosynovitis: Inflammation of the abductor pollicis longus and extensor pollicis brevis tendons, leading to thumb and wrist pain.
• Intersection Syndrome: Pain and inflammation where the extensor pollicis longus crosses over the extensor carpi radialis tendons, common in rowers.
• Extensor Tendonitis: Inflammation of the extensor tendons due to overuse, causing pain during wrist and finger extension.
• Supinator Strain: Overuse or injury to the supinator muscle, resulting in pain during supination and resistance to forearm rotation.

EXERCISES FOR THE DEEP LAYER OF THE POSTERIOR COMPARTMENT

• Wrist Extensions: To perform this exercise, hold a dumbbell with the palm facing down, and then lift the wrist upwards in order to strengthen the wrist extensors and stabilise the forearm.
• Thumb Abduction With Resistance Band: Use a resistance band to pull the thumb away from the hand, strengthening the abductor pollicis longus.
• Supination With Dumbbell: For this exercise, hold a light dumbbell and rotate the forearm from a palm-down to a palm-up position, specifically targeting the supinator.
• Reverse Finger Extensions: In this exercise, place a rubber band around the fingers and then extend them outward to effectively strengthen the extensor muscles.
• Thumb Extension: Using a small weight or resistance band, extend the thumb upwards in order to engage the extensor pollicis longus as well as the brevis.

STRETCHES FOR DEEP LAYER OF POSTERIOR COMPARTMENT

• Wrist Extensor Stretch: Extend the arm with the palm down, and then gently press the hand downward to stretch the extensor muscles of the forearm.
• Thumb Stretch: Pull the thumb back gently with the opposite hand to stretch the extensor pollicis muscles and improve flexibility.
• Supinator Stretch: Extend the arm and rotate the palm downward, applying gentle pressure to stretch the supinator and pronator muscles.
• Finger Extension Stretch: Spread the fingers wide and hold the position to stretch the extensor indicis and related finger extensors.
• Forearm Stretch: Hold the arm out, palm facing down, and use the opposite hand to gently press the fingers towards the body for an overall forearm stretch.