Muscle Overview and Actions

Overview

This lecture covers the major muscles of the head, neck, upper body, and lower limbs, focusing on origins, insertions, and actions relevant to joint movement.

Muscles of the Head and Neck

• Masseter: Origin—zygomatic arch/maxilla; insertion—angle/ramus of mandible; action—elevates mandible.
• Temporalis: Origin—temporal fossa; insertion—coronoid process/ramus of mandible; action—elevates and retracts mandible.
• Zygomaticus major: Origin—zygomatic bone; insertion—soft tissue/orbicularis oris; action—smiles by elevating mouth corners.
• Orbicularis oris: Origin—fascia of maxilla/mandible; insertion—mucosa of lips; action—closes lips, whistling/kissing motions.
• Orbicularis oculi: Origin—bones of orbit; insertion—eyelids; action—closes eyes.
• Sternocleidomastoid: Origin—sternum/clavicle; insertion—mastoid process/superior nuchal line; action—flexes neck (both), rotates head (single).
• Platysma: Origin—superior thorax; insertion—mandible/cheek tissue; action—depresses mandible, tenses neck skin.

Muscles of Upper Back, Chest, and Shoulder

• Levator scapulae: Origin—C1–C4; insertion—superior medial scapula; action—elevates scapula.
• Rhomboid minor & major: Minor—origin C7/T1, major—T2–T5; insertion—medial scapula; action—adducts scapula.
• Supraspinatus: Origin—supraspinatus fossa; insertion—greater tubercle of humerus; action—shoulder abduction.
• Infraspinatus, Teres minor: Origin—scapular fossae; insertion—greater tubercle; action—lateral rotation of humerus.
• Teres major: Origin—inferior scapula; insertion—intertubercular groove of humerus; action—medial rotation.
• Deltoid: Origin—clavicle/acromion/scapular spine; insertion—deltoid tuberosity; action—abduction, flexion, extension of shoulder.
• Pectoralis major: Origin—clavicle/sternum/ribs 2–6; insertion—intertubercular groove; action—adduction, flexion, medial rotation.
• Pectoralis minor: Origin—ribs 3–5; insertion—coracoid process; action—depresses scapula.
• Trapezius: Origin—occipital bone/C7–T12; insertion—clavicle/acromion/spine of scapula; action—elevate, adduct, depress scapula, head extension.
• Serratus anterior: Origin—ribs 1–8/9; insertion—anterior medial scapula; action—protracts shoulder.
• Latissimus dorsi: Origin—T5–L5, thoracolumbar fascia, ribs 9–12; insertion—intertubercular sulcus; action—extends, adducts, medially rotates humerus.

Muscles of the Upper Arm and Forearm

• Biceps brachii: Origin—scapula/coracoid; insertion—radial tuberosity; action—flexes, supinates forearm, flexes shoulder.
• Coracobrachialis: Origin—coracoid process; insertion—medial humerus; action—flexes, adducts humerus.
• Brachialis: Origin—anterior distal humerus; insertion—coronoid of ulna; action—elbow flexion.
• Brachioradialis: Origin—lateral humerus; insertion—styloid of radius; action—elbow flexion.
• Triceps brachii: Origin—scapula/humerus; insertion—olecranon of ulna; action—extends forearm and humerus.
• Flexor carpi radialis: Origin—medial epicondyle; insertion—2nd/3rd metacarpal; action—flexes, abducts wrist.
• Palmaris longus: Origin—medial epicondyle; insertion—palmar aponeurosis/retinaculum; action—flexes wrist.
• Flexor carpi ulnaris: Origin—medial epicondyle; insertion—pisiform, hamate, 5th metacarpal; action—flexes, adducts wrist.
• Pronator teres: Origin—medial epicondyle/coronoid of ulna; insertion—mid-radius; action—pronates forearm.

Muscles of the Thorax and Abdomen

• External intercostals: Origin—inferior border of superior rib; insertion—superior border of inferior rib; action—elevates ribs.
• Internal intercostals: Origin—superior border of inferior rib; insertion—inferior border of superior rib; action—depresses ribs.
• Diaphragm: Origin—xiphoid, ribs 4–10, lumbar vertebrae; insertion—central tendon; action—increases thoracic volume (breathing).
• Rectus abdominis: Origin—pubic crest; insertion—xiphoid cartilage/ribs 5–7; action—spinal flexion, abdominal compression.
• External oblique: Origin—ribs 5–12; insertion—linea alba, iliac crest; action—spinal flexion/rotation, abdominal compression.
• Internal oblique: Origin—iliac crest/lumbar fascia; insertion—linea alba, costal cartilage 8–10; action—spinal flexion/rotation, abdominal compression.
• Transverse abdominis: Origin—iliac crest, lumbar fascia, ribs 6–12; insertion—linea alba, pubic crest; action—abdominal compression.

Muscles of the Hip and Thigh

• Psoas major: Origin—T12, L1–L5; insertion—lesser trochanter of femur; action—flexes femur.
• Iliacus: Origin—iliac fossa; insertion—lesser trochanter; action—flexes femur.
• Tensor fascia latae: Origin—iliac crest; insertion—IT band; action—flexes, abducts femur, tenses IT band.
• Sartorius: Origin—anterior superior iliac spine; insertion—proximal/medial tibia; action—flexes/rotates hip, flexes knee.
• Gracilis: Origin—pubic bone; insertion—proximal/medial tibia; action—flexes knee, adducts femur.
• Quadriceps group: Rectus femoris—origin ASIS, insertion tibial tuberosity, action—hip flexion, knee extension; vastus lateralis/medialis/intermedius—origins on femur, insert tibial tuberosity, action—knee extension.
• Pectineus: Origin—pubic bone; insertion—medial femur; action—adducts femur.
• Adductor longus/magnus: Origin—pubic bone; insertion—linea aspera; action—adducts femur.
• Semimembranosus: Origin—ischial tuberosity; insertion—medial tibia; action—knee flexion, hip extension.

Muscles of the Lower Leg and Gluteal Region

• Gastrocnemius: Origin—femoral condyles; insertion—calcaneus via Achilles; action—foot extension (plantarflexion), knee flexion.
• Soleus: Origin—head fibula/posterior tibia; insertion—calcaneus via Achilles; action—foot extension.
• Tibialis anterior: Origin—lateral condyle/proximal tibia; insertion—medial cuneiform, 1st metatarsal; action—ankle flexion (dorsiflexion).
• Tibialis posterior: Origin—interosseous membrane; insertion—metatarsals 2, 3, 5; action—foot extension, inversion.
• Fibularis (peroneus) longus: Origin—head/proximal fibula; insertion—medial cuneiform, 1st metatarsal; action—foot extension, eversion.
• Gluteus maximus: Origin—iliac crest, sacrum, coccyx; insertion—gluteal tuberosity, IT band; action—femur extension.
• Gluteus medius: Origin—superior lateral ilium; insertion—greater trochanter; action—abducts, medially rotates femur.
• Gluteus minimus: Origin—inferior lateral ilium; insertion—greater trochanter; action—abducts, medially rotates femur.
• Biceps femoris: Origin—ischial tuberosity, linea aspera; insertion—head of fibula, lateral tibia; action—knee flexion, hip extension.
• Semitendinosus: Origin—ischial tuberosity; insertion—medial proximal tibia; action—knee flexion, hip extension.

Key Terms & Definitions

• Origin — the fixed attachment site of a muscle, usually proximal.
• Insertion — the movable attachment site of a muscle, usually distal.
• Action — the main movement that occurs when the muscle contracts.
• Abduction — movement away from the body's midline.
• Adduction — movement toward the body's midline.
• Flexion — decreasing the angle at a joint.
• Extension — increasing the angle at a joint.
• Medial rotation — rotation toward the body's midline.
• Lateral rotation — rotation away from the body's midline.

Action Items / Next Steps

• Review muscle origins, insertions, and actions for all regions covered.
• Practice labeling diagrams of muscles.
• Prepare for questions on muscle actions, especially prime movers at each major joint.
• Study for upcoming practical exams using cadaveric images.