Muscular system: Part 2

Neck and back regions

• Sternocleidomastoid

• Trapezius

• Splenius 

• Latissimus dorsi 

• Erector spinae 

Erector spinae

spinalis, longissimus iliocostalis

Sternocleidomastoid

• Origin: Sternum & clavicle

• Insertion: Mastoid process

• Actions:

  • Bilateral: Neck flexion

  • Unilateral: Contralateral rotation

Trapezius

• Origin: Occipital bone, cervical & thoracic vertebrae

• Insertion: Acromion, spine of scapula, clavicle

• Actions:

  • Neck extension (bilateral)

  • Lateral flexion (unilateral)

  • Scapula: Elevate, retract, depress

Splenius

• Origin: Cervical & thoracic vertebrae

• Insertion: Mastoid process, occipital bone

• Actions: Neck extension (bilateral), ipsilateral rotation (unilateral)

Erector spinae group

• Iliocostalis, Longissimus, Spinalis

• Actions: Extend & laterally flex vertebral column

Latissimus dorsi 

• Origin: Thoracic/lumbar vertebrae, iliac crest, ribs 9–12

• Insertion: Intertubercular sulcus of humerus

• Actions: Extend, adduct, medially rotate shoulder

Rotator cuff muscles

• Supraspinatus: Abducts humerus (0–15°)

• Infraspinatus: Lateral rotation

• Teres Minor: Lateral rotation & extension

• Subscapularis: Medial rotation

Teres major

Medial rotation & extension (not part of rotator cuff)

Biceps brachii

• Origin: Short head – coracoid process; Long head – supraglenoid tubercle

• Insertion: Radial tuberosity

• Actions: Elbow flexion, forearm supination

Brachialis

• Origin: Distal humerus

• Insertion: Ulnar tuberosity

• Action: Elbow flexion

Triceps brachii 

• Origin: Long head – infraglenoid tubercle; Lateral/medial heads – posterior humerus

• Insertion: Olecranon

• Action: Elbow extension

Anterior (flexor) compartment 

• Superficial Layer (all from medial epicondyle):

  • Pronator teres – pronation

  • Flexor carpi radialis – flex & abduct wrist

  • Palmaris longus – accessory wrist flexor

  • Flexor carpi ulnaris – flex & adduct wrist

• Intermediate Layer:

  • Flexor digitorum superficialis – flex digits

• Deep Layer:

  • Flexor digitorum profundus

  • Flexor pollicis longus

  • Pronator quadratus

Posterior (extensor) compartments

• Superficial Layer (common origin: lateral epicondyle):

  • Brachioradialis – accessory elbow flexor

  • Extensor carpi radialis longus/brevis – extend & abduct wrist

  • Extensor digitorum – extend digits 2–5

  • Extensor digiti minimi – extend little finger

  • Extensor carpi ulnaris – extend & adduct wrist

• Deep Layer:

  • Supinator

  • Abductor pollicis longus

  • Extensor pollicis longus/brevis

  • Extensor indicis

3 Muscle groups of the hand 

• Thenar (3) 

• Hypothenar (3) 

• Midpalmar 

Interossei 

• DAB = Dorsal - ABduction

• PAD = Palmar - ADduction

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Pronator teres

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Flexor carpi radialis

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Palmaris longus 

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Flexor carpi ulnaris

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Brachioradialis 

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Extensor carpi radialis longus & brevis 

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Extensor digitorum

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Extensor digitorum minimi

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Extensor carpi radialis 

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Blue: Thenar 

Green: Hypothenar 

Red: Midpalmar