Muscles: Origin, Insertion, and Action

Muscular System Notes

Frontalis

  • Origin: Frontal bone
  • Insertion: Skin of eyebrows
  • Action: Raises eyebrows, wrinkles forehead

Sternocleidomastoid

  • Origin: Sternum and clavicle
  • Insertion: Mastoid process of temporal bone
  • Action:
    • Flexes neck
    • Rotates head to opposite side
  • Agonist: Neck flexion

Orbicularis Oculi

  • Origin: Frontal bone and maxilla
  • Insertion: Skin around the eyelids
  • Action: Closes eyelids (blinking/squinting)

Orbicularis Oris

  • Origin: Maxilla and mandible
  • Insertion: Skin and muscle around mouth
  • Action: Closes and protrudes lips ("kissing")

Masseter

  • Origin: Zygomatic arch
  • Insertion: Angle and ramus of mandible
  • Action:
    • Elevates mandible (closes jaw/chewing)
  • Agonist: Jaw elevation

Trapezius

  • Origin: Occipital bone, cervical and thoracic vertebrae
  • Insertion: Clavicle and scapular spine
  • Action:
    • Elevates, retracts, rotates scapula
  • Agonist: Scapular elevation and retraction
  • Antagonist: Serratus anterior (protraction)

Latissimus Dorsi

  • Origin: Lower spine, iliac crest
  • Insertion: Intertubercular groove of humerus
  • Action:
    • Extends, adducts, and medially rotates arm
  • Agonist: Shoulder extension/adduction
  • Antagonist: Deltoid (anterior)

Pectoralis Major

  • Origin: Clavicle, sternum, costal cartilage
  • Insertion: Greater tubercle of humerus
  • Action:
    • Flexes, adducts, medially rotates humerus
  • Agonist: Shoulder flexion/adduction
  • Antagonist: Latissimus dorsi (extension)

Serratus Anterior

  • Origin: Ribs 1-8
  • Insertion: Medial border of scapula
  • Action: Protracts and stabilizes scapula
  • Agonist: Scapular protraction

Biceps Brachii

  • Origin:
    • Short head: Coracoid process of scapula
    • Long head: Supraglenoid tubercle
  • Insertion: Radial tuberosity (Radius)
  • Action:
    • Flexes elbow, supinates forearm
  • Agonist: Elbow flexion

Triceps Brachii

  • Origin: Scapula and humerus
  • Insertion: Olecranon of ulna
  • Action: Extends elbow
  • Agonist: Elbow extension
  • Antagonist: Biceps brachii

Brachialis

  • Origin: Distal anterior humerus
  • Insertion: Coronoid process of ulna
  • Action: Primary elbow flexor (stronger than biceps)
  • Agonist: Elbow flexion
  • Antagonist: Triceps brachii

Rectus Femuris

  • Origin: Anterior inferior iliac spine (pelvic bone: ileum)
  • Insertion: Tibial tuberosity via patellar tendon
  • Action:
    • Extends knee, flexes hip
  • Agonist: Knee extension
  • Antagonist: Hamstrings

Vastus Lateralis

  • Origin: Greater trochanter of femur
  • Insertion: Tibial tuberosity
  • Action: Extends knee
  • Agonist: Knee extension
  • Antagonist: Hamstrings

Vastus Medialis

  • Origin: Linea aspera of femur
  • Insertion: Tibial tuberosity
  • Action: Extends knee
  • Agonist: Knee extension
  • Antagonist: Hamstrings

Vastus Intermedius

  • Origin: Anterior and lateral femur
  • Insertion: Tibial tuberosity
  • Action: Extends knee
  • Agonist: Knee extension
  • Antagonist: Hamstrings

Sartorius

  • Origin: Anterior superior iliac spine
  • Insertion: Medial tibia
  • Action:
    • Flexes hip and knee, laterally rotates thigh
  • Agonist: Hip/knee flexion
  • Antagonists: Hamstrings (knee), gluteus maximus (hip extension)

Biceps Femoris

  • Origin:
    • Long Head: Ischial tuberosity (pelvic bone)
    • Short Head: Linea aspera (femur)
  • Insertion: Head of fibula
  • Action:
    • Extends hip, flexes knee
  • Agonist: Hip extension and knee flexion
  • Antagonist: Quadriceps

Semitendinosus

  • Origin: Ischial tuberosity (pelvic bone)
  • Insertion: Medial tibia
  • Action:
    • Extends hip, flexes knee
  • Agonist: Hip extension and knee flexion
  • Antagonist: Quadriceps

Gastrocnemius

  • Origin: Medial and lateral condyles of femur
  • Insertion: Calcaneus via Achilles tendon
  • Action:
    • Plantarflexes foot, flexes knee
  • Agonist: Plantarflexion
  • Antagonist: Tibialis anterior

Soleus

  • Origin: Tibia and fibula
  • Insertion: Calcaneus via Achilles tendon
  • Action: Plantarflexes foot
  • Agonist: Plantarflexion (when knee is flexed)
  • Antagonist: Tibialis anterior

Tibialis Anterior

  • Origin: Lateral condyle of tibia
  • Insertion: Medial cuneiform and 1st metatarsal
  • Action: Dorsiflexes and inverts foot
  • Agonist: Dorsiflexion
  • Antagonist: Gastrocnemius/Soleus

Gluteus Maximus

  • Origin: Posterior ilium, sacrum, and coccyx
  • Insertion: Gluteal tuberosity of the femur and iliotibial tract (IT band)
  • Action:
    • Primary: Hip extension (e.g., standing from sitting, climbing stairs)
    • Secondary: Lateral rotation and abduction of thigh
  • Role at Hip Joint: Agonist for hip extension (Antagonist to hip flexors like iliopsoas)

Gluteus Medius

  • Origin: Outer surface of the ilium (below iliac crest)
  • Insertion: Lateral surface of the greater trochanter of femur
  • Action:
    • Primary: Abduction of thigh
    • Secondary: Medial rotation of thigh; stabilizes pelvis during walking
  • Role at Hip Joint: Agonist for hip abduction (Antagonist to hip adductors like adductor longus)