54 Muscles of the Upper Limb

Muscles of the Humerus that Act on the Forearm

Humerus that act on the forearm are primarily involved in flexion and extension.

The humerus is a long bone in the arm that runs from the shoulder to the elbow. Anatomically, it interacts with the scapula to form the shoulder joint and the radius and ulna of the lower arm to form the elbow joint. Forearm rotation is controlled by two joints: the proximal radioulnar joint which exists immediately below the elbow, and the distal radioulnar joint located immediately before the wrist.

Upper Arm

There are four muscles in the upper arm split into an anterior and posterior compartment.

Anterior Compartment

Three muscles are located in the anterior compartment of the upper arm.

• Biceps Brachii: The biceps brachii is a two-headed muscle. Although the majority of the muscle mass is located anteriorly to the humerus, it has no attachment to the bone itself.
  • Attachments: Both heads originate from the scapula and attach via the bicipital aponeurosis to the fascia of the forearm.
  • Action: Supination of the forearm. It also flexes the arm at the elbow and at the shoulder.
• Coracobrachialis: The coracobrachialis lies within the two heads of the biceps brachii.
  • Attachments: Originates from the scapula and attaches to the humerus.
  • Action: Flexing of the arm at the shoulder, and weak adduction.
• Brachialis: The brachialis muscle lies within the distal region of the biceps brachii.
  • Attachments: Originates from the humerus and attaches to the ulna.
  • Action: Flexing of the arm at the elbow.

Posterior Compartment

The posterior compartment of the upper arm contains only one muscle.

• Triceps Brachii: The triceps brachii is a three-headed muscle.
  • Attachments: The long head originates from the scapula, the lateral head from the proximal region of the humerus, and the medial head from the distal region of the humerus. All three converge into one tendon which attaches to the ulna.
  • Action: Extension of the arm at the elbow.

Forearm

As with the upper arm, the forearm is split into anterior and posterior compartments. Only those responsible for movement of the forearm are discussed below; the muscles responsible for movement of the hand and wrist are discussed in a later section.

Anterior

The anterior compartment of the forearm is split into superficial, intermediate, and deep regions.

• Pronator Teres: A rectangular muscle located in the superficial region of the anterior compartment.
  • Attachments: The pronator teres has two origins, one on the proximal end of the humerus and one of the distal end of the ulna. It attaches to the mid region of the radius.
  • Action: Pronates the forearm.
• Pronator Quadratus: A square shaped muscle located adjacent to the wrist in the deep region of the anterior compartment.
  • Attachments: Originates from the ulna and attaches to the radius.
  • Action: Pronates the forearm.

Posterior

The posterior compartment of the forearm is split into superficial and deep regions.

• Anconeus: The anconeus is located in the superficial region of the forearm posterior compartment and is blended with the triceps brachii.
  • Attachments: Originates from the humerus and attaches to the ulna.
  • Action: Moves the ulna during pronation and extends the forearm at the elbow.
• Brachioradialis: The brachioradialis is located in the superficial region of the forearm posterior compartment,
  • Attachments: Originates from the humerus and attaches to the distal end of the radius.
  • Action: Flexes the forearm at the elbow.
• Supinator: The supinator is located in the deep region of the forearm posterior compartment.
  • Attachments: The supinator has two heads: one originating from the humerus, the other from the ulna. Together they attach to the radius.
  • Action: Supinates the forearm.

Muscles of the Wrist and Hand

Muscles in the forearm move the wrists, and hand movement is caused by both forearm and hand muscles.

Muscles associated with the wrist include those of the forearm and hand that move the wrist and digits. The wrist and hand exhibit a remarkable range of movement, key for grasping and interaction with objects. These muscles can generate highly variable force, from the strong grip required when lifting a heavy object to the delicate movements required to write.

Muscles of the forearm that act on the wrist and hand are referred to as extrinsic muscles, or external to the hand. Those located within the hand are referred to as intrinsic.

Muscles of the Forearm

As with the upper arm, the forearm is split into anterior and posterior compartment. Each contains many more muscles than described below due to the requirement for more complex movements in the wrist and hand.

Anterior

The anterior compartment of the forearm is split into superficial, intermediate, and deep layers. These muscles are generally responsible for flexing of the wrist and fingers and pronation of the forearm.

Superficial Layer

Three muscles are located in the superficial layer of the anterior compartment of the forearm.

• Flexor Carpi Ulnaris: A long muscle originating near the elbow and passing through into the wrist.
  • Attachments: Originates from the humerus and ulna and attaches to one of the carpal bones in the wrist.
  • Actions: Flexion and adduction at the wrist.
• Palmaris Longus: A long muscle originating near the elbow and passing through into the wrist.
  • Attachments: Originates from the humerus and attaches to the base of the hand.
  • Actions: Flexion at the wrist.
• Flexor Carpi Radialis: A long muscle originating near the elbow and passing through into the wrist.
  • Attachments: Originates from the humerus and attaches to the base of the digits.
  • Actions: Flexion and abduction at the wrist.
• Pronator Teres: A rectangular muscle.
  • Attachments: The pronator teres has two origins, one on the proximal end of the humerus and one of the distal end of the ulna. It attaches to the mid region of the radius.
  • Actions: Pronates the forearm.

Intermediate Layer

There is just one muscle in the intermediate layer of the anterior compartment of the forearm.

• Flexor Digitorum Superficialis: Lying below the superficial region, the flexor digitorum superficialis is a key muscle controlling wrist and finger flex.
  • Attachments: Originates from the humerus and the radius, splitting into four tendons at the wrist which travel through the carpal tunnel and attach to the fingers.
  • Actions: Flexes fingers and wrist.

Deep Layer

There are three muscles in the deep layer of the anterior compartment of the forearm.

• Flexor Digitorum Profundus: A long muscle originating near the elbow and passing through into the wrist, lying adjacent to the flexor pollicis longus.
  • Attachments: Originates from the ulna, splitting into four tendons at the wrist which travel through the carpal tunnel and attach distally to the fingers.
  • Actions: Flexes the wrist and the most distal regions of the fingers.
• Flexor Pollicis Longus: A long muscle originating near the elbow and passing through into the wrist, lying adjacent to the flexor digitorum profundus.
  • Attachments: Originates from the radius and attaches to the base of the thumb.
  • Actions: Flexes the thumb.
• Pronator quadratus: A square-shaped muscle located adjacent to the wrist.
  • Attachments: Originates from the ulna and attaches to the radius.
  • Actions: Pronates the forearm.

Posterior

The posterior compartment of the forearm is split into superficial and deep regions. The muscles are generally responsible for extension of the wrist and fingers.

Superficial Layer

The superficial layer of the posterior forearm contains seven muscles.

• Aconeus: The aconeus is located in the superficial region of the forearm posterior compartment and is blended with the triceps brachii.
  • Attachments: Originates from the humerus and attaches to the ulna.
  • Actions: Moves the ulna during pronation and extends the forearm at the elbow.
• Brachioradialis: The brachioradialis is located in the superficial region of the forearm posterior compartment.
  • Attachments: Originates from the humerus and attaches to the distal end of the radius.
  • Actions: Flexes the forearm at the elbow.
• Extensor Carpi Radialis Longus and Brevis: A pair of muscles located on the side of the forearm, allowing them to control extension and abduction of the wrist.
  • Attachments: Both originate from the humerus and attach to the base of the hand.
  • Actions: Extend and abduct the wrist.
• Extensor Digitorum: The extensor digitorum is the main extensor of the fingers.
  • Attachments: Originates from the humerus, splitting into four tendons at the wrist which travel through the carpal tunnel and attach to the digits.
  • Actions: Extends fingers.
• Extensor Digiti Minimi: Originates from the extensor digitorum. In some people, these muscles cannot be individually defined.
  • Attachments: Originates from the humerus and attaches to the little finger.
  • Actions: Extends the little finger, and contributes to extension at the wrist.
• Extensor Carpi Ulnaris: Located on the other side of the forearm to the extensor carpi radialis longus and brevis, it performs a similar role.
  • Attachments: Originates from the humerus and attaches to the base of the hand.
  • Actions: Extension and adduction of wrist.

Deep Layer

There are four muscles in the deep layer of the posterior compartment of the forearm.

• Supinator: The supinator is located in the deep region of the forearm posterior compartment.
  • Attachments: The supinator has two heads: one originates from the humerus, the other from the ulna. Together they attach to the radius.
  • Actions: Supinates the forearm.
• Abductor Pollicis Longus: The abductor pollicis longus is situated immediately distal to the supinator muscle.
  • Attachments: Originates from the radius and ulna attaching to the base of the thumb.
  • Actions: Abducts the thumb.
• Extensor Pollicis Brevis: The extensor pollicis brevis is located below the abductor pollicis longus.
  • Attachments: Originates from radius and attaches to the base of the thumb.
  • Actions: Extends the thumb.
• Extensor Indicis Proprius: This muscle allows the index finger to be independent of the other fingers during extension.
  • Attachments: Originates from the ulna and attaches to the index finger.
  • Actions: Extends the index finger.

Muscles of the Hand

The extrinsic muscles of the hand are responsible for the larger scale, stronger movements of the wrist and hand. The intrinsic muscles produce finer, more controlled movements and play important roles in maintaining grip.

Thenar Muscles

The thenar muscles are three short muscles located at the base of the thumb and responsible for its fine movement.

• Opponens Pollicis: The opponens pollicis is the largest and deepest-lying of the thenar muscles.
  • Attachments: Originates from the wrist and attaches to the thumb.
  • Actions: Rotates the thumb towards the palm, producing opposition and improving grip.
• Abductor Pollicis Brevis : Located anteriorly to the opponens pollicis and proximal to the flexor pollicis brevis.
  • Attachments: Originates from the wrist and attaches to the thumb.
  • Actions: Abducts the thumb.
• Flexor Pollicis Brevis: The smallest and most distal of the thenar muscles.
  • Attachments: Originates from the wrist and attaches to the thumb.
  • Actions: Flexes the thumb.

Hypothenar Muscles

The hypothenar muscles are located at the base of the little finger. Their naming, function, and organization are similar to those of the thenar muscles.

• Opponens Digiti Minimi: The opponens digit minimi is the deepest-lying of the hypothenar muscles.
  • Attachments: Originates from the wrist and attaches to the little finger.
  • Actions: Rotates little finger towards the palm, producing opposition and improving grip.
• Abductor Digiti Minimi: The most superficial of the hypothenar muscles.
  • Attachments: Originates from the wrist and attaches to the little finger.
  • Actions: Abducts the little finger.
• Flexor Digiti Minimi Brevis: Located laterally to the digiti minimi.
  • Attachments: Originates from the wrist and attaches to the little finger.
  • Actions: Flexes little finger.

Lumbricals

These are four lumbricals in the hand, each associated with an individual finger.

• Attachments: Originates from a tendon of attached to the flexor digitorum profundus of the forearm, each attaching to an individual finger
• Actions: Flexes and extends the fingers.

Interossei

The interossei muscles are located between the fingers; they can be split into two groups.

• Dorsal  Interossei: Located superficially on the dorsal side of the hand, there are four dorsal interossei muscles.
  • Attachments: Originates from the base of the finger, each attaching after the first finger joint.
  • Actions: Abducts the fingers.
• Palmar Interossei: Located on the anterior side of the hand, there are three palmar interossei, with the index finger controlled by the extensor indicis proprius.
  • Attachments: Originates from the base of the finger, each attaching after the first finger joint.
  • Actions: Adducts the fingers.

Other Muscles

One other muscle in the hand is not easily grouped with the above categories.

• Palmaris Brevis: The palmaris brevis is a small superficial muscle found in the palm.
  • Attachments: Originates from the fascia of the palm and attaches to the dermis.
  • Actions: Wrinkles the skin and deepens the curvature of the palm improving grip.

Muscles of the Shoulder

Muscles of the shoulder include those that attach to the bones of the shoulder to move and stabilize the joint.

The shoulder or glenohumeral joint is a ball and socket joint formed between the humerus and scapula. Due to the shallowness of the socket and relatively loose connections, the shoulder joint allows for a wide range of motion; however, this wide range makes the joint unstable and thus more prone to dislocation and injury than other joints.

Two other joints make up the shoulder; the acromioclavicular joint of the clavicle and scapula, which allows the arm to be raised above the head, and the sternoclavicular joint of the clavicle and sternum, which plays an important role in facilitating movement of the upper arm and connecting it to the rest of the skeleton.

Muscles that act on the shoulder can be classified as extrinsic, intrinsic, pectoral, or upper arm. Upper arm muscles will be discussed in a later section since they primarily promote forearm movement.

Extrinsic Shoulder Muscles

Extrinsic muscles of the shoulder originate from the trunk and attach to the bones of the shoulder. They can be further subdivided into superficial and deep layers.

Superficial

As suggested by the name, superficial muscles lie on the surface. There are two superficial extrinsic muscles.

• Trapezius: The trapezius is the most superficial muscle of the back and forms a broad flat triangle.
  • Attachments: The trapezius originates from the skull and spine of the upper back and neck. It attaches to the clavicle and scapula.
  • Actions: The superior region supports the arm and elevates and rotates the scapula, the intermediate region retracts the scapula, and the inferior region rotates and depresses the scapula.
• Latissimus Dorsi: The latissimus dorsi originates from the lower back and covers a wide area.
  • Attachments: The latissimus dorsi originates from the lower spine and ribs and the upper pelvis and fascia of the deep trunk muscles. The muscle converges into a tendon attaching to the humerus.
  • Actions: Extends, adducts, and medially rotates the upper arm.

Deep

Three deep muscles lie below the superficial muscles of the shoulder.

• Levator Scapulae: A small, strap-like muscle that joins the neck to the scapula.
  • Attachments: Originates from the side of the spine in the neck and attaches to the scapula.
  • Actions: Elevates the scapula.
• Rhomboid Major: Sits inferiorly to the levator scapulae.
  • Attachments: Originates from the spine in the upper back and attaches to the scapula in an inferior position to the levator scapulae attachment.
  • Actions: Retracts and rotates the scapula.
• Rhomboid Minor: Sits between the levator scapulae and  rhomboid major, with which it is paired in action and function. It retracts and rotates the scapula.

Intrinsic

Intrinsic muscles originate from the scapula or clavicle and attach to the humerus. There are six intrinsic muscles, four of which form the rotator cuff.

• Deltoid: The deltoid muscle is a triangular muscle which covers the shoulder. The action of the muscle is complex, with the components acting in opposing and separate ways during the course of a contraction.
  • Attachments: The deltoid muscle originates from the scapula and clavicle and attaches to the lateral surface of the humerus.
  • Actions: The anterior region assists the pectoralis major during transverse flexion of the shoulder and acts weakly in strict transverse flexion. The lateral region assists in shoulder flexion when the shoulder is rotating, although it also assists the transverse abduction of the shoulder. The posterior region is the hyperextensor of the shoulder, contributing to transverse
    extension.
• Teres Major: The teres major is a thick flattened muscle connecting the lower scapula with the humerus.
  • Attachments: Originates from the posterior of the scapula and attaches to the humerus.
  • Actions: Adducts the shoulder and assists in rotation of the arm.

Rotator Cuff

The rotator cuff is a group of four muscles that pull the ball of the humerus into the shallow socket of the scapula, adding required stability. The rotator cuff complex is composed of the supraspinatus, infraspinatus, subscapularis, and teres minor all of which originate from the scapula and connect to the humerus. The supraspinatus is involved in abduction of the arm in association with the deltoid, while the other muscles facilitate rotation of the arm.

Pectoral

Pectoral muscles lie in the chest and exert force through the shoulder to move the upper arm. Three pectoral muscles interact with the shoulder.

• Pectoralis Major: The pectoralis major is a large, fan-shaped muscle covering the chest. It is comprised of clavicular and sternocostal regions.
  • Attachments: The clavicular region originates from the clavicle and the sternocostal region originates from the sternum and the fascia of the oblique muscles of the abdomen. Both attach to the humerus.
  • Actions: Adducts and rotates the upper arm.
• Pectoralis Minor: The pectoralis minor muscle is smaller and lies beneath the pectoralis major.
  • Attachments: The pectoralis minor originates from the third to fifth ribs and attaches to the scapula.
  • Actions: Supports and depresses the scapula.
• Serratus Anterior: The serratus anterior is located in the lateral wall of the chest.
  • Attachments: The muscle is formed of several strips originating from the second to eight ribs, each of which attaches to the scapula.
  • Actions: Supports the scapula allowing for elevation of the upper arm.