CHAPTER 32: Forearm

MUSCLES OF THE FOREARM

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. Forearm muscles are organized into two fascial compartments, similar to those of the arm muscles. The anterior compartment contains flexor muscles (innervated by the median and ulnar nerves). The posterior compartment contains extensor muscles (innervated by the radial nerve).

ACTIONS OF THE WRIST

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

• Flexion. Wrist bends anteriorly.

• Extension. Wrist bends posteriorly.

• Radial deviation (abduction). Wrist abducts toward the radius.

• Ulnar deviation (adduction). Wrist adducts toward the ulna.

FOREARM MUSCLES OF THE ANTERIOR COMPARTMENT

The muscles in the anterior compartment of the forearm have the following in common:

• Common attachment. Medial epicondyle of the humerus.

• Common innervation. Median nerve with some contribution from the ulnar nerve.

• Common action. Flexion.

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 (PT). Possesses two heads and crosses anteriorly to the elbow complex.

    • Attachments. Two heads (medial epicondyle and medial supracondylar ridge of the humerus as well as the coronoid process of the ulna); inserts laterally on the midshaft of the radius.

    • Actions. Pronates forearm.

    • Innervation. Median nerve (C6–C7); the median nerve enters the forearm by coursing between the two heads.

  • Flexor carpi radialis muscle (FCR).

    • Attachments. Medial epicondyle (humerus); base of second and third metacarpals.

    • Actions. Wrist flexion and radial deviation.

    • Innervation. Median nerve (C6–C7).

  • Palmaris longus muscle (PL). This muscle may be absent on one or both sides in some individuals.

    • Attachments. Medial epicondyle (humerus); palmar aponeurosis.

    • Actions. Weak wrist flexion; resists shearing forces of the palmar aponeurosis.

    • Innervation. Median nerve (C7–C8).

  • Flexor carpi ulnaris muscle (FCU). Possesses two heads.

    • Attachments. Two heads (medial epicondyle and olecranon process); pisiform bone and base of the fifth metacarpal.

    • Actions. Wrist flexion and ulnar deviation.

    • Innervation. Ulnar nerve (C7–T1); the ulnar nerve enters the forearm by coursing deep to the medial epicondyle and traverses the two heads of the flexor carpi ulnaris muscle.

• Intermediate group (Figure 32-1C).

  • Flexor digitorum superficialis muscle (FDS). Arises from two heads and gives rise to four tendons, which traverse the carpal tunnel to insert on digits 2 to 5.

    • Attachments. Two heads (medial epicondyle and radius); gives rise to four tendons, which traverse the carpal tunnel and insert ...