Muscles of the Forearm

The muscles of the forearm are responsible for a wide range of movements involving the wrist, hand, and fingers, as well as pronation and supination of the forearm itself. These muscles are anatomically divided into the anterior (flexor–pronator) compartment and the posterior (extensor–supinator) compartment. Each compartment is further subdivided into superficial and deep layers. Together, the forearm muscles provide precision, strength, and dexterity for fine motor tasks and powerful grip functions.

Location

The forearm extends from the elbow to the wrist. Muscles are organized into two main compartments:

• Anterior Compartment: Located on the ventral (palmar) side; primarily flexors and pronators; innervated mostly by the median nerve.
• Posterior Compartment: Located on the dorsal side; primarily extensors and supinators; innervated mostly by the radial nerve.

Anterior Compartment Muscles

Superficial Layer (5 Muscles)

• Pronator teres: Pronates the forearm and assists in elbow flexion.
• Flexor carpi radialis: Flexes and abducts the wrist.
• Palmaris longus: Weak wrist flexor; absent in ~15% of people.
• Flexor carpi ulnaris: Flexes and adducts the wrist; innervated by the ulnar nerve.
• Flexor digitorum superficialis: Flexes the proximal interphalangeal joints (PIP) of fingers 2–5.

Deep Layer (3 Muscles)

• Flexor digitorum profundus: Flexes distal interphalangeal joints (DIP) of fingers 2–5; lateral half by median nerve, medial half by ulnar nerve.
• Flexor pollicis longus: Flexes the thumb.
• Pronator quadratus: Primary pronator of the forearm; deepest muscle near the wrist.

Posterior Compartment Muscles

Superficial Layer (6 Muscles)

• Brachioradialis: Flexes the elbow (despite being in the extensor compartment); most lateral forearm muscle.
• Extensor carpi radialis longus: Extends and abducts the wrist.
• Extensor carpi radialis brevis: Works with longus to extend/abduct wrist.
• Extensor digitorum: Extends fingers 2–5 at all joints.
• Extensor digiti minimi: Extends the 5th digit (little finger).
• Extensor carpi ulnaris: Extends and adducts the wrist.

Deep Layer (5 Muscles)

• Supinator: Supinates the forearm; wraps around the upper radius.
• Abductor pollicis longus: Abducts and extends the thumb at the carpometacarpal joint.
• Extensor pollicis brevis: Extends the thumb at the metacarpophalangeal joint.
• Extensor pollicis longus: Extends the thumb at the interphalangeal joint.
• Extensor indicis: Extends the index finger independently.

Function

• Wrist and finger movements: Flexion, extension, abduction, and adduction of the wrist and fingers.
• Forearm rotation: Pronation by pronator teres and quadratus; supination by supinator and biceps brachii.
• Grip strength: Flexor digitorum muscles play a major role in grip and grasping objects.
• Precision movements: Extensor muscles help coordinate finger extension during typing, writing, and instrument playing.

Physiological Role(s)

• Enable complex tasks such as writing, gripping, cutting, and manipulating tools.
• Maintain wrist stability during upper limb movements.
• Support forearm posturing during fine and gross motor activities.

Development

The muscles of the forearm develop from the mesoderm of the upper limb bud during embryogenesis. The anterior compartment arises from the ventral muscle mass and is innervated primarily by the median nerve (and ulnar nerve). The posterior compartment originates from the dorsal mass and is innervated by the radial nerve. Differentiation and segmentation occur by the 6th–8th week of development. Full muscular function continues to mature postnatally through motor learning and activity.

Clinical Significance

• Compartment syndrome: Increased pressure in the forearm compartments may impair perfusion and require emergency fasciotomy.
• Medial epicondylitis (golfer’s elbow): Overuse of flexor-pronator group causing pain at the medial epicondyle.
• Lateral epicondylitis (tennis elbow): Overuse of extensor-supinator group causing pain at the lateral epicondyle.
• Pronator teres syndrome: Median nerve entrapment leading to forearm and hand weakness.
• Radial nerve injury: May result in wrist drop due to paralysis of wrist extensors.
• Ulnar nerve compression: Can affect flexor carpi ulnaris and half of flexor digitorum profundus, leading to hand weakness.

Surface Anatomy and Palpation

• The tendons of flexor carpi radialis and palmaris longus are visible on wrist flexion.
• The brachioradialis is palpable on the lateral forearm when the elbow is flexed against resistance.
• Extensor tendons form the "anatomical snuffbox" near the thumb during extension and abduction.

Imaging

• Ultrasound: Useful for dynamic tendon assessment and guiding injections.
• MRI: Gold standard for evaluating muscle tears, tendinopathies, and nerve entrapment syndromes.

Anatomical Variations

• The palmaris longus may be absent unilaterally or bilaterally (in ~15% of the population).
• The flexor digitorum superficialis to the fifth digit may be absent.
• Accessory muscles like the extensor medii proprius or an accessory abductor pollicis may be present.