Elbow and forearm

The forearm contains numerous muscles, which are best organized using the classical anatomical classification, dividing them into anterior and posterior compartments.

The anterior compartment includes the pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, and pronator quadratus muscles.

The posterior compartment consists of the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, anconeus, supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis muscles.

Remembering all the muscles of the forearm and their actions can be challenging. One trick to make it easier is to remember the "rule of 3s":

• 3 wrist flexors (flexor carpi radialis, flexor carpi ulnaris, palmaris longus)
• 3 finger flexors (flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus)
• 3 wrist extensors (extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris)
• 3 finger extensors (extensor digitorum, extensor indicis, extensor digiti minimi)
• 3 thumb extensors (abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus)

The term "arm" is commonly used by most people to refer to the entire upper limb between the shoulder and wrist joints. However, in anatomy, the term "arm" refers specifically to the region between the shoulder and elbow joints, while the area between the elbow and wrist joints is called the forearm.

The forearm comprises numerous muscles. An effective way to categorize them is by organizing them into the traditional anterior and posterior groups along with their respective layers:

• Anterior Compartment:

  • Superficial Layer: 4 muscles (pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris)
  • Intermediate Layer: 1 muscle (flexor digitorum superficialis)
  • Deep Layer: 2 muscles (flexor digitorum profundus and flexor pollicis longus), 1 muscle only for pronation (pronator quadratus)

• Posterior Compartment:

  • Superficial Layer: 7 muscles (brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus)
  • Deep Extensors: 5 muscles (supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis)

This results in a total of 20 muscles.

The primary extensor of the forearm at the elbow joint is the triceps brachii muscle, located in the posterior compartment of the arm. The anconeus muscle, a small triangular muscle located at the back of the elbow, assists the triceps brachii in extension of the forearm and stabilizes the elbow joint.

Supination is the movement of rotating the forearm so that the palm of the hand faces upward. The primary supinators of the forearm are the biceps brachii muscle, a muscle located in the anterior compartment of the arm, and the supinator muscle, located in the posterior compartment of the forearm. The biceps brachii is the main supinator when the elbow is flexed and also during active movement, while the supinator works better when the elbow is extended. An accessory supinator of the forearm is the brachioradialis muscle, which can return the forearm to a neutral position when the forearm is fully pronated.

The epidermis of the skin has five layers. From superficial to deep, these layers are:

• Stratum corneum: Formed by dead, keratinized cells, that act as a protective layer
• Stratum granulosum: Consisting mainly of keratinocytes
• Stratum spinosum: Consisting of keratinocytes joined by desmosomes
• Stratum basale (germinativum): A single layer of basal cells that are mitotically active and maintain the epidermis