Muscles Crossing the Shoulder, Elbow, and Forearm: Movements of the Arm, Forearm, Wrist, Hand, and Fingers

The Muscular System: Muscles Crossing the Shoulder, Elbow, and Forearm

Shoulder (Glenohumeral) Joint

The shoulder joint, also known as the glenohumeral joint, is the most freely moving joint in the human body, allowing a wide range of motion at the expense of stability.

• Ball-and-socket joint: The large, hemispherical head of the humerus fits into the small, shallow glenoid cavity of the scapula, similar to a golf ball on a tee.

• Articular capsule: The capsule enclosing the cavity is thin and loose, contributing to the joint's freedom of movement.

Glenoid Labrum and Ligaments

• Glenoid labrum: A fibrocartilaginous rim around the glenoid cavity that adds depth to the shallow cavity, though it only holds about one-third of the humeral head.

• Reinforcing ligaments:

  • Coracohumeral ligament: Supports the weight of the upper limb.

  • Glenohumeral ligaments (three): Strengthen the anterior capsule but provide weak support.

Reinforcing Muscle Tendons

• Tendon of long head of biceps brachii: Acts as a 'superstabilizer' by securing the humerus to the glenoid cavity.

• Rotator cuff tendons: Four tendons encircle the shoulder joint, originating on the scapula and inserting on the humerus:

  • Subscapularis

  • Supraspinatus

  • Infraspinatus

  • Teres minor

Clinical Note: Shoulder Dislocations

• Shoulder dislocations are common due to the joint's mobility and weak reinforcement anteriorly and inferiorly.

• The glenoid cavity provides poor support when the humerus is rotated laterally and abducted (e.g., football tackles).

• Blows to the top or back of the shoulder can also cause dislocations.

Muscles Crossing the Shoulder Joint: Movements of the Arm (Humerus)

Nine muscles cross the shoulder joint, inserting on and moving the humerus. These muscles originate from either the scapula or the axial skeleton.

• Prime movers of arm movement:

  • Pectoralis major: Agonist for flexion and adduction; assists subscapularis in medial rotation.

  • Latissimus dorsi: Agonist for extension and adduction.

  • Deltoid: Extension (posterior fibers), abduction (middle fibers).

• Rotator cuff muscles: Act as synergists and fixators, reinforcing the shoulder capsule and preventing dislocation.

Table: Muscles Crossing the Shoulder Joint

Muscle	Origin	Insertion	Action
Pectoralis major	Clavicle, sternum	Humerus	Flexion, adduction, medial rotation
Latissimus dorsi	Lower spine, iliac crest	Humerus	Extension, adduction, medial rotation
Deltoid	Clavicle, acromion, scapula	Humerus	Abduction, flexion, extension
Rotator cuff (Subscapularis, Supraspinatus, Infraspinatus, Teres minor)	Scapula	Humerus	Stabilization, rotation

Muscles Crossing the Elbow Joint: Flexion and Extension of the Forearm

The muscles acting on the elbow are divided into anterior (flexors) and posterior (extensors) compartments.

• Anterior muscles (flexors):

  • Biceps brachii: Prime mover of forearm flexion.

  • Brachialis: Agonist of forearm flexion.

  • Brachioradialis: Synergist in forearm flexion.

• Posterior muscles (extensors):

  • Triceps brachii: Prime mover of forearm extension.

  • Anconeus: Synergist of triceps brachii.

Table: Muscles Crossing the Elbow Joint

Muscle	Compartment	Action
Biceps brachii	Anterior	Flexion
Brachialis	Anterior	Flexion
Brachioradialis	Anterior	Flexion (synergist)
Triceps brachii	Posterior	Extension
Anconeus	Posterior	Extension (synergist)

Muscles of the Forearm: Movements of the Wrist, Hand, and Fingers

Forearm muscles are divided into anterior (flexors) and posterior (extensors) groups, further subdivided into superficial and deep muscles. Most tendons are held in place at the wrist by the flexor retinaculum (anterior) or extensor retinaculum (posterior).

Anterior Muscles (Flexors)

• Consist of five superficial and three deep muscles.

• Most arise from the common flexor tendon attached to the medial epicondyle of the humerus.

• Key muscles and actions:

  • Pronator teres: Pronates the forearm.

  • Pronator quadratus: Deep muscle, prime mover of forearm pronation.

  • Flexor carpi radialis: Flexes the wrist and abducts the hand.

  • Palmaris longus: Inserts in palmar aponeurosis, weak flexor.

  • Flexor carpi ulnaris: Prime wrist flexor, adducts the hand.

  • Flexor digitorum superficialis: Flexes the wrist and phalanges of fingers 2-5.

Posterior Muscles (Extensors)

• Extend the wrist and fingers; consist of four superficial and four deep muscles.

• Most arise from the common extensor tendon attached to the lateral epicondyle of the humerus.

• Superficial muscles:

  • Brachioradialis

  • Extensor carpi radialis longus: Extends and abducts the wrist.

  • Extensor carpi radialis brevis: Extends and abducts the wrist.

  • Extensor digitorum: Prime mover of finger extension.

  • Extensor carpi ulnaris: Extends and adducts the wrist.

• Deep muscles:

  • Supinator: Supinates the forearm; synergist of biceps brachii.

  • Abductor pollicis longus: Abducts and extends the thumb.

  • Extensor pollicis brevis: Extends the thumb.

  • Extensor indicis: Extends the index finger.

Table: Muscles of the Forearm

Muscle	Compartment	Action
Pronator teres	Anterior	Pronation
Flexor carpi radialis	Anterior	Flexion, abduction
Flexor carpi ulnaris	Anterior	Flexion, adduction
Flexor digitorum superficialis	Anterior	Flexion of fingers
Extensor carpi radialis longus	Posterior	Extension, abduction
Extensor digitorum	Posterior	Extension of fingers
Extensor carpi ulnaris	Posterior	Extension, adduction
Supinator	Posterior	Supination

Intrinsic Muscles of the Hand

Intrinsic hand muscles are responsible for fine motor movements such as opposition, abduction, and adduction of the fingers and thumb.

• Opposition: Touching the thumb to another finger.

• Abduction: Moving fingers away from the midline of the hand.

• Adduction: Moving fingers toward the midline of the hand.

Summary Table: Actions of Muscles Acting on the Arm, Forearm, and Hand

The following table summarizes the actions of major muscles at the shoulder:

Muscle	Flexion	Extension	Abduction	Adduction	Medial Rotation	Lateral Rotation
Pectoralis major	✓			✓	✓
Latissimus dorsi		✓		✓	✓
Deltoid	✓ (anterior fibers)	✓ (posterior fibers)	✓ (middle fibers)
Subscapularis					✓
Supraspinatus			✓
Infraspinatus						✓
Teres minor						✓

Example: Carpal Tunnel Syndrome

Carpal tunnel syndrome is a clinical condition resulting from compression of the median nerve

Additional info:

• Muscle actions are often coordinated, with agonists, antagonists, and synergists working together to produce smooth movement.

• Understanding the origin, insertion, and action of each muscle is essential for clinical assessment and diagnosis of musculoskeletal injuries.