Introduction

In this lab, you will review the muscles of the anterior forearm and hand and the joints of the hand.

Lab Activities

Activity 1: Muscles of the Anterior Forearm 

The muscles of the anterior compartment, or anterior aspect of the forearm, are generally flexors of the wrist or fingers. The flexors originate from a flexor mass with its proximal attachment point on the medial epicondyle of the humerus. Palpate your medial epicondyle and then move your fingers to the mass just distal to the medial epicondyle. Flex your wrist, and you should feel this flexor mass contract. The anterior compartment also contains pronators of the arm. From anatomical position, this motion would be turning your hand so that your thumb points medially and your hand faces backward. If you imagine holding a glass of water, pronation would also be the motion of turning your wrist and hand to pour the water out of the glass.

The anterior compartment can be divided into three layers.  We won’t ask you specifically which layer a muscle is located in. However, sometimes breaking up these muscles into smaller groups can make it easier to learn them.

Before we observe the muscles of the forearm in these layers, you should observe the brachioradialis. This muscle originates in the brachium, or arm, and inserts onto the styloid process of the radius. It looks like the muscles of the forearm, but it doesn’t actually cross the wrist. It assists with flexion of the elbow. 

Superficial layer:

• The pronator teres inserts onto the mid-shaft of the radius. It is the shortest muscle originating from the medial epicondyle.
• Flexor carpi radialis and flexor carpi ulnaris cross only the wrist joints and flex the wrist (flexor = flex, and carpi = wrist). Flexor carpi ulnaris inserts on the ulnar side of the wrist and assists with ulnar deviation (wrist adduction). Flexor carpi radialis inserts on the radial side of the wrist and also assists with radial deviation (wrist abduction).
• The palmaris longus is a very small muscle and weak flexor of the wrist. It is not present in 10-20% of the population.

Intermediate Layer:

• Observe flexor digitorum superficialis (FDS). It crosses the wrist, MCP, and PIP joints of digits 2-5 in the hand. Notice how this muscle splits and inserts on the middle phalanx of these digits.

Deep Layer:

• Observe flexor digitorum profundus (FDP). Profundus means “deep,” and this muscle is deep to the flexor digitorum superficialis. Both of these muscles act to flex MCP and PIP joints of the digits. However, the FDP runs deep to the tendon of FDS through its split at the middle phalanx and continues distally to insert on the distal phalanx of digits 2-5. Therefore, only FDP will flex the DIP of the digits.
• The flexor pollicis longus muscle sends its tendon across the IP joint of the first digit to insert on the distal phalanx of digit 1, the thumb. Therefore, it flexes the MCP and IP joints of the thumb.
• Finally, very deep on the anterior aspect of the distal forearm, observe the pronator quadratus. This muscle sits just anterior to the interosseus membrane, which you can observe proximal to this muscle.

Observe how the tendons of these muscles run underneath a thick sheet of connective tissue. This is called the flexor retinaculum. This helps to hold these flexor tendons in place and create a mechanical advantage, increasing the amount of force these muscles can produce. The median nerve also runs deep to the retinaculum. The space that these tendons and the median nerve runs through is commonly called the carpal tunnel. Irritation of the median nerve often occurs in this place, and this is called carpal tunnel syndrome. Note that the palmaris longus is the only muscle whose tendon does not travel through the carpal tunnel but, instead, travels superficial to the flexor retinaculum. If you forcibly squeeze your thumb and fingertips together, tensing your palm while flexing your wrist slightly, you can see this tendon clearly in the wrist area if you have this muscle.

Activity 2: Muscles of the hand

Observe the intrinsic muscles of the hand.  All of these muscles are completely located within the hand. The muscles of the hand can be divided into three groups. The thenar muscles are those that act on the thumb, and the hypothenar muscles are those that act on the 5^th digit. Both the thenar and hypothenar muscles contain an abductor, flexor, and opponens muscle. The opponens muscles act on the thumb or 5^th digit to make these digits touch. All muscles containing the word “pollicis” act on the thumb, and all muscles that contain the words “digiti minimi” act on the 5^th digit. The deep layer of the hand contains three muscles, including one that adducts the thumb. The fifth digit does not have a counterpart to this muscle.  

• Observe the muscles of the thenar eminence: flexor pollicis brevis and abductor pollicis brevis are superficial to opponens pollicis muscle.
• Observe the muscles of the hypothenar eminence. Opponens digiti minimi is deep to the flexor digiti minimi and abductor digiti minimi.
• Observe the deep muscles of the hand.
  • The adductor pollicis muscle is part of the deep muscles of the hand. It has two heads: one runs from the thumb horizontally across the hand, while the other runs from the same origin obliquely across the palm. These two heads create a “7” shape. This muscle acts to adduct the thumb.
  • Observe the lumbricals, which attach to the flexor tendons within the hand. These thin muscles help you form an “L” with your 2^nd-4^th digits (if you turn your palm upwards). They flex the MCP joints while extending the PIP and DIP joints.
  • Finally, on the dorsal aspect of the hand, you can observe the dorsal and palmar interossei. Though their names specify directionality, you can actually see both from the dorsal aspect of the hand. You will need to observe the fiber direction for these muscles. You can remember their actions by remembering DAB and PAD: Dorsal interossei ABduct the digits, and Palmer interossei ADduct the digits. Therefore, by looking at the fiber directions, you can understand if those interossei would be pulling the fingers medially or laterally to the midline of the hand. The middle (3^rd) digit can only abduct, so it has only dorsal interossei on either side of it. For the other digits, the interosseus muscle on the medial aspect of the digit with its fibers running proximally towards the midline are palmar interossei. The interosseous muscle on the lateral aspect of the digit with its fibers running distally towards the midline are dorsal interossei.