Introduction

In this lab, you will review the elbow and wrist joints and the muscles of the posterior forearm. The muscles of the posterior compartment of the forearm generally contain muscles that extend the wrist. Most of these extensors originate from the area of the lateral epicondyle of the humerus. Palpate your lateral epicondyle and move your fingers to the mass of muscle just distal to it. Extend your wrist, and you should feel this extensor mass contract. The posterior compartment also contains a muscle that abducts the thumb, as well as the supinator muscle. The supinator muscle works with the biceps brachii to supinate the hand. Supination makes the palm face forward in anatomical position. You can remember the position of a supinated hand by thinking that, in the supinated position, you can hold a cup of soup.

Lab Activity 1: Muscles of the Posterior Forearm – Donor Tissue

Observe the muscles of the posterior forearm. The names of these muscles tell a lot about their location and/or function. They tend to extend the wrist or digits, and most originate on the lateral epicondyle of the humerus. This compartment can be divided into superficial and deep layers, each with five muscles. We won’t ask you which layer a muscle is located in. However, sometimes breaking the muscles into smaller groups can be helpful for learning. As you are exploring the muscles of the posterior forearm, especially those crossing the wrist and hand joints, you can GENTLY pull on these tendons to watch how they move the wrist and digits.

Be sure to observe these structures on each of the upper extremity tissues we have in the lab.

Superficial Layer:

• On the radial side of the forearm, observe the extensor carpi radialis longus and extensor carpi radialis brevis muscles. Don’t confuse these with the brachioradialis, which sits partially superficial to these. The brachioradialis has an origin that is proximal to the lateral epicondyle, while these muscles originate on or near the lateral epicondyle. Both extensor carpi radialis longus and brevis cross the wrist and act to extend it. The difference between the longus and brevis muscles is in the tendon length and position. Extensor carpi radialis longus has a longer tendon and is just superficial to the extensor carpi radialis brevis. The extensor carpi ulnaris (only one on this side) extends the wrist and inserts on the ulnar side.
• Extensor digitorum extends to the distal phalanx of digits 2-5 and extends the MCP, DIP, and PIP of these digits. There is only one extensor digitorum muscle, so there is no other descriptor in the name (e.g., superficialis, profundus, brevis, or longus. Observe the small angled dense connective tissue connecting the tendons of the extensor digitorum in the hand. This is called the juncturae tendinae. It functions to coordinate and distribute force across the extensor tendons. Try extending your ring finger (4^th digit) while keeping the other digits in a fist. You are not able to extend that finger without moving the others because of the juncturae tendinae. However, you can extend your 5^th digit much farther while the others are flexed. That is because of the extensor digiti minimi muscle. This muscle acts to extend the fifth digit. In this tissue, it appears that this muscle has blended with the extensor digitorum muscle. Be sure to observe this muscle in atlas images.

Deep Layer:

• You may also notice that you can extend your 2^nd digit (pointer finger) while flexing your other digits. That is because another muscle extends this digit: extensor indices.
• The supinator acts to supinate the forearm. This muscle is deep and more difficult to find, but it can be observed running from the lateral epicondyle to the shaft of the radius.
• The other muscles of the deep posterior forearm act on the thumb. Abductor pollicis longus acts to abduct the thumb, while extensor pollicis brevis and longus act to extend the joints of the thumb. The extensor pollicis brevis tendon sits between the abductor pollicis longus and extensor pollicis longus tendons.

Observe the dense connective tissue running over the extensor tendons. This is called the extensor retinaculum. This structure holds the extensor tendons in place and provides a mechanical advantage with extension.

Lab Activity 2:  Joints of the Upper Extremity – Visible Body Digital Atlas

Explore the elbow and wrist joints using the Visible Body Digital Atlas. Some of the joints and joint structures will not be specifically listed in the app.

• Begin by examining the elbow joint. The elbow is constructed of three separate joints: the humeroulnar joint between the humerus and ulna, the humeroradial joint between the humerus and radius, and the proximal radioulnar joint between the radius and ulna proximally. The ligaments of these joints are best seen on the iPads, but you may be able to appreciate them on one of the upper extremity dissections. Often these ligaments are blended into the surrounding capsule, so they can be challenging to define. However, you may be able to appreciate the ulnar or radial collateral ligaments on either side of the joint or the annular ligament surrounding the head of the radius.

• Click on the Regions tab and scroll to 9. Cubital Fossa. Click on Systems at the bottom left if the systems column is not already present on the left side of the screen. Click on the arterial, venous, nervous, and lymphatic systems twice to remove them since we will not be looking at these systems in this lab. At the very top of the systems column, you have the option to select a region. Scroll to the left and for shoulder/arm, click the muscle twice, so you only have the bones on the left side of the body and muscles on the right side of the body.
• Explore the muscles of the forearm and hand. Some muscles are deep to others. Thus you will have to remove the more superficial muscles to see the deep muscles.
• You will be asked to identify the origins, insertions, actions, and innervations of these muscles (this will be listed in the app after highlighting the structure and clicking the book). However, you may not need to know particular specifics for each muscle. Follow what is in the muscle tables for the specifics of what you need to know for each muscle.
• You can view the actions of these muscles by clicking on the Muscle Actions tab from the home screen and then selecting the movement you want to view.