Big Picture

The forearm (antebrachium) consists of the radius and ulna. Proximally, the forearm articulates with the humerus through the elbow complex (humeroulnar and humeroradial joints). Distally, the forearm articulates with the carpal bones through the wrist complex, enabling a wide array of actions. The muscles of the forearm that act upon the elbow, wrist complex, and the digital joints are organized into two fascial compartments, similar to those of the arm muscles. The anterior compartment contains flexor muscles and the posterior compartment contains extensor muscles.

Actions of the Wrist

The configuration of the wrist complex allows for motion in two planes (Figure 32-1A):

• Flexion
  
• Extension
  
• Radial deviation (abduction)
  
• Ulnar deviation (adduction)

Forearm Muscles of the Anterior Compartment

The actions produced by the muscles in the anterior compartment of the forearm depend upon which joints the muscles cross. Some muscles cross the elbow, wrist, digits, and perhaps a combination of each. The muscles in the anterior compartment of the forearm have the following similar features:

• Common attachment. Medial epicondyle of the humerus.
  
• Common innervation. Median nerve with minimal contribution from the ulnar nerve.
  
• Common action. Flexion.

The vascular supply to the anterior forearm muscles is from branches of the ulnar and radial arteries.

The muscles in the anterior compartment of the forearm are divided into three groups: superficial, intermediate, and deep.

• Superficial group (Figure 32-1B)
  
  
  
  • Pronator teres muscle. Possesses two heads and crosses the elbow complex. The humeral head of the pronator teres muscle attaches to the medial epicondyle and the supraepicondylar ridge of the humerus, and the ulnar head attaches to the coronoid process. Distally, the pronator teres muscle attaches to the midshaft of the radius. The pronator teres muscle primarily produces pronation at the forearm. The median nerve provides innervation (C6–C7) to the pronator teres muscle.
    
  • Flexor carpi radialis muscle. Attaches to the medial epicondyle and the base of metacarpals 2 and 3. The primary action of the flexor carpi radialis muscle is wrist flexion and radial deviation. The median nerve (C6–C7) supplies innervation to this muscle.
    
  • Palmaris longus muscle. Attaches to the medial epicondyle of the humerus and courses superficially over the flexor retinaculum to the palmar aponeurosis in the hand. The primary action of the palmaris longus muscle is to resist shearing forces of the palmar aponeurosis; it is also considered a wrist flexor. Innervation is provided by the median nerve (C7–C8). It is important to note that the palmaris longus muscle may be absent on one or both sides in some individuals.
    
  • Flexor carpi ulnaris muscle. Possesses two heads. A humeral head attaches to the medial epicondyle of the humerus, and an ulnar head attaches to the olecranon process. Both heads come together and attach to the pisiform, hamate and base of metacarpal 5. The flexor carpi ulnaris muscle crosses both the elbow and the wrist complex, producing weak elbow flexion, wrist flexion, and ulnar deviation. It is innervated by the ulnar nerve (C7–T1).
  
  
• Intermediate group (Figure 32-1C)
  
  
  
  • Flexor digitorum superficialis muscle. Possesses two heads. A humeral head attaches proximally to the medial epicondyle and a radial head attaches to the radius. The flexor digitorum superficialis muscle attaches distally to the middle phalanges of digits 2 to 5. The muscle primarily produces flexion at the wrist and at the metacarpophalangeal and proximal interphalangeal joints. Innervation is provided by the median nerve (C8–T1). The four tendons of the flexor digitorum superficialis muscle cross under the flexor retinaculum at the wrist and enter the hand through the carpal tunnel.
  
  
• Deep group (Figure 32-1D)
  
  
  
  • Flexor pollicis longus muscle. Attaches proximally to the radius and the interosseous membrane and to the distal phalanx of the thumb. The flexor pollicis longus muscle produces flexion at the metacarpophalangeal and interphalangeal joints of digit 1 and is innervated by the anterior interosseous nerve from the median nerve (C7–C8).
    
  • Flexor digitorum profundus muscle. Attaches proximally to the ulna and interosseous membrane and travels across the wrist complex and attaches distally to the distal phalanges of digits 2 to 5. The flexor digitorum profundus muscle produces flexion at the wrist as well as flexion of the metacarpophalangeal and proximal and distal interphalangeal joints of digits 2 to 5. The lateral half of the flexor digitorum profundus muscle is innervated by the anterior interosseous nerve from the median nerve (C8–T1), and the medial half of the muscle is innervated by the ulnar nerve (C8–T1).
    
  • Pronator quadratus muscle. Courses horizontally from the distal anterior surface of the ulna to the distal anterior surface of the radius. The pronator quadratus muscle produces pronation, and is innervated by the anterior interosseous nerve from the median nerve (C7–C8).

Forearm Muscles of the Posterior Compartment

The actions produced by the muscles in the posterior compartment of the forearm depend upon which joints the muscles cross. Some muscles cross the elbow, wrist, and digits, and perhaps a combination of each. The muscles in the posterior compartment of the forearm have the following similar features (Table 32-1):

• Common attachment. Lateral epicondyle of the humerus.
  
• Common innervation. Radial nerve.
  
• Common action. Extension.