Arm and forearm muscles

The arm and forearm of the human body have numerous muscles that work together to move these limbs and allow them to perform all the functions that are characteristic of it, such as writing, playing musical instruments, holding different objects and moving them, lifting their own weight, etc..

Human beings have two arms and two forearms, one pair on each side of the body, which are structurally composed of part of the appendicular skeleton and its associated muscles. The arms are formed by the humerus bone and the forearms by the bones called the ulna and radius..

The arm of a human being extends from the shoulder to the elbow and the forearm spans the space between the elbow and the wrist. The shoulder corresponds to the joint between the humerus bone and the clavicle, one of the bones of the shoulder girdle, and the elbow is the joint between the humerus, ulna and radius; the wrist, meanwhile, is the joint between the ulna, radius and the first bones of the hand.

What movements do the muscles of the arm allow?

Thanks to the association of these bones with their respective muscles, the arms are responsible for movements such as:

• Flexion: which consists of the approach of two parts of the body to each other, such as the movement we make when we bring the arm and forearm closer when flexing the elbow.
• Extension: that has to do with the distancing of two parts of the body from each other, as when we extend the arm and straighten the elbow.
• Abduction: which is the movement of a part of the body out of the body center, as well as when we raise the arm out and away from the body.
• Adduction: the movement of a part of the body towards the body center, as well as when the arm approaches the body.

Likewise, the function of these muscles is controlled directly through the central nervous system, specifically by three nerves (medial, ulnar and radial), belonging to the portion related to the peripheral nervous system known as brachial plexus.

The blood supply, on the other hand, depends on the brachial artery, which runs along the arm, passes through the elbow and is divided into two branches: one radial and the other ulnar, which supply the entire forearm and hand..

Arm muscles

The arm, which corresponds to the region of the upper extremities corresponding to the structures located between the shoulder and the elbow, has 4 muscles, divided into two regions, one anterior and one posterior..

Anterior face or region

Where are the muscles that correspond to the region of the palm of the hand, that is, the one that is exposed when we extend the limb with the elbow and the nails facing the ground.

• Biceps brachii muscle: it is as long as the distance between the clavicle and the elbow; it is very bulky, has two portions -one short and the other long- and participates in the rotation and flexion movement of the forearm with respect to the elbow.
• Anterior brachialis muscle: it is smaller than the biceps brachii, it is wide and flat, and it is the most important in the movement of flexion of the forearm on the arm.
• Coracobrachialis muscle: it is located between the scapula (second bone of the shoulder girdle) and the biceps; facilitates quick movements and in the elevation of the arm with the immobile shoulder.

Face or posterior region

Where are the muscles that correspond to the region of the back of the hand, where the knuckles are; the one that is exposed when we extend our arms in front of us with the palms of the hands pointing towards the ground.

• Triceps brachii muscle: it is the main anterior muscle of the arm and has three portions, one long, one internal and one external; works on the extension of the forearm with respect to the elbow.

Forearm muscles

The forearm contains 20 muscles, which are anatomically located in three regions: one anterior, one posterior, and one lateral. The muscles in the anterior and posterior regions are organized in layers; the anterior region has 4 layers and the posterior region has two, one superficial and one deep.

The anterior region of the forearm is characterized by being a "compartment" that participates in flexion movements, while the posterior region participates in extension movements..

Anterior region or face (flexion)

This muscular region of the forearm is made up of 8 muscles in total, distributed in four “layers”: first, second, third and fourth. In this the muscles that allow the flexion of the fingers of the hand can be appreciated.

First layer

These muscles originate-insert in the humerus, in the middle region of the epicondyle of this bone.

• Pronator round muscle: its final insertion site is the middle region of the radius bone
• Flexor carpi radialis muscle: is finally inserted at the base of the second metacarpal of the hand.
• Flexor carpi ulnaris muscle: is finally inserted at the base of the fifth metacarpal of the hand.
• Palmar longus muscle- Attaches to the palmar fascia and is between the radial and ulnar carpal flexor muscles.

Second layer

• Superficial flexor muscle of the fingers: that facilitates the flexion of all fingers, except the thumb.

Third layer

• Deep common flexor muscle of the fingers: that facilitates the flexion of all fingers, except the thumb; born in the posterior three-quarters of the ulna.
• Flexor pollicis longus muscle: originates in the anterior portion of the radius bone.

Fourth layer

• Pronator square muscle: originates in the antero-medial part of the ulna and inserts in the antero-lateral region of the radius.

Posterior region or face (extension)

8 more muscles make up the posterior region of the forearm, which are arranged in two layers: a superficial and a deep one. In this region of the forearm are the muscles that allow the extension of the fingers of the hand.

Surface layer

Extensor finger muscle: it is one of the most important in this region, it extends from the elbow to the back of the wrist; participates in the extension of the proximal phalanx of the fingers.

• Extensor digiti minimi muscle.
• Extensor carpi ulnaris muscle.
• Anconeus muscle.

Deep layer

• Abductor pollicis longus muscle.
• Extensor pollicis brevis muscle.
• Extensor pollicis longus muscle.
• Extensor muscle of the second finger (the index).

Lateral region or face

This region is made up of four major muscles, which are responsible for the movement of the thumb and other movements related to the forearm and wrist..

• Brachioradialis muscle.
• Extensor carpi radialis longus muscle.
• Extensor carpi radialis brevis muscle.
• Supinator muscle.

References

1. Alshammari SM, Bordoni B. Anatomy, Shoulder and Upper Limb, Arm Muscles. [Updated 2020 Jul 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: ncbi.nlm.nih.gov/books/NBK554420/
2. Mitchell B, Whited L. Anatomy, Shoulder and Upper Limb, Forearm Muscles. [Updated 2020 Aug 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: ncbi.nlm.nih.gov/books/NBK536975/
3. Netter, F. H. (2014). Atlas of human anatomy, Professional Edition EBook: including NetterReference. com Access with full downloadable image Bank. Elsevier Health Sciences.
4. Pigeon, P., & Feldman, A. G. (1996). Moment arms and lengths of human upper limb muscles as functions of joint angles. Journal of biomechanics, 29(10), 1365-1370.
5. Saladin, K. S., & McFarland, R. K. (2008). Human anatomy (Vol. 2). New York: McGraw-Hill.