Muscles of the Human Body: Structure, Function, and Classification

Muscular System Overview

The muscular system is responsible for movement, posture, and various involuntary processes in the human body. Muscles are classified based on their location, structure, and function. This guide summarizes the major skeletal muscles, their anatomical groupings, and primary actions.

Facial Muscles

Muscles of Facial Expression

These muscles are primarily responsible for facial movements such as smiling, frowning, and other expressions. They are innervated mainly by the facial nerve (cranial nerve VII).

• Buccinator: Compresses the cheek, as in blowing or sucking.

• Corrugator supercilii: Draws eyebrows downward and medially, producing frowning.

• Depressor anguli oris: Depresses the corner of the mouth (frowning).

• Levator labii superioris: Elevates the upper lip.

• Mentalis: Protrudes lower lip and wrinkles chin.

• Nasalis: Compresses the nasal cartilage, flaring nostrils.

• Epicranius (Frontalis and Occipitalis): Raises eyebrows (frontal belly); pulls scalp posteriorly (occipital belly).

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes lips (kissing muscle).

• Risorius: Draws corner of mouth laterally.

• Zygomaticus major and minor: Elevate the corners of the mouth (smiling).

Muscles Moving the Eye

These six extrinsic eye muscles control the movement of the eyeball within the orbit.

• Superior rectus: Elevates and turns eye medially.

• Inferior rectus: Depresses and turns eye medially.

• Lateral rectus: Moves eye laterally.

• Medial rectus: Moves eye medially.

• Superior oblique: Depresses and turns eye laterally.

• Inferior oblique: Elevates and turns eye laterally.

Muscles of Mastication and Swallowing

Mastication (Chewing)

• Temporalis: Elevates and retracts mandible.

• Masseter: Elevates mandible (primary chewing muscle).

• Lateral and Medial pterygoids: Side-to-side movement and protraction of mandible.

Swallowing

• Inferior pharyngeal constrictors: Constrict pharynx during swallowing.

• Genioglossus, Hyoglossus, Styloglossus: Move the tongue for speech and swallowing.

Muscles of the Neck

Hyoid/Anterior Neck Muscles

• Digastric, Mylohyoid, Stylohyoid: Elevate hyoid bone and floor of mouth during swallowing.

• Sternohyoid, Thyrohyoid: Depress hyoid bone and larynx during swallowing and speech.

Vertebral/Posterior Neck Muscles

• Splenius capitis: Extends, rotates, and laterally flexes the head.

• Scalenes: Flex and slightly rotate neck; elevate first two ribs.

• Sternocleidomastoid: Flexes neck; rotates head to opposite side.

• Erector spinae: Extends vertebral column.

• Levator scapulae: Elevates scapula.

• Trapezius: Elevates, retracts, and rotates scapula.

Muscles of the Thorax

• Diaphragm: Primary muscle of inspiration; flattens to increase thoracic cavity volume.

• Internal intercostals: Depress ribs during forced expiration.

• External intercostals: Elevate ribs during inspiration.

Abdominal Muscles

These muscles support the abdominal wall, aid in trunk movement, and assist in respiration.

• Rectus abdominis: Flexes vertebral column; compresses abdomen.

• Internal and External obliques: Rotate and laterally flex trunk; compress abdomen.

• Transverse abdominis: Compresses abdominal contents.

Muscles of the Pelvic Floor

These muscles support pelvic organs and control openings of the urethra, anus, and vagina.

• Levator ani: Supports pelvic viscera; forms pelvic floor.

• External urethral sphincter: Controls urination.

• Deep and Superficial transverse perineal: Support pelvic organs.

• Bulbospongiosus, Ischiocavernosus: Assist in erection and compress vaginal or penile base.

Muscles of the Upper Body

Shoulder and Back

• Levator scapulae: Elevates scapula.

• Pectoralis minor: Draws scapula anteriorly and inferiorly.

• Trapezius: Elevates, retracts, and rotates scapula.

• Rhomboids (Major and Minor): Retract scapula.

• Supraspinatus, Infraspinatus, Subscapularis, Teres minor: Rotator cuff muscles; stabilize shoulder joint.

• Serratus anterior: Protracts scapula; assists in upward rotation.

• Subclavius: Stabilizes and depresses clavicle.

Arm and Forearm

• Coracobrachialis: Flexes and adducts arm.

• Deltoid: Abducts, flexes, and extends arm.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

• Pectoralis major: Flexes, adducts, and medially rotates arm.

• Teres major: Extends, adducts, and medially rotates arm.

• Biceps brachii: Flexes elbow and supinates forearm.

• Triceps brachii: Extends elbow.

• Brachioradialis, Brachialis: Flex forearm at elbow.

Forearm and Hand

• Flexor carpi radialis, Palmaris longus, Flexor carpi ulnaris: Flex wrist.

• Finger flexors group: Flex fingers.

• Extensor digitorum, Extensor indicis: Extend fingers.

• Lumbricals: Flex metacarpophalangeal joints, extend interphalangeal joints.

• Adductor pollicis: Adducts thumb.

Muscles of the Lower Body

Hip and Thigh

• Tensor fascia latae: Abducts and medially rotates thigh.

• Iliopsoas: Flexes thigh at hip joint.

• Sartorius: Flexes, abducts, and laterally rotates thigh; flexes knee.

• Adductor magnus, longus, brevis: Adduct and medially rotate thigh.

• Biceps femoris, Semitendinosus, Semimembranosus: Hamstring muscles; extend thigh and flex knee.

• Gluteus maximus, medius, minimus: Extend, abduct, and medially rotate thigh.

Quadriceps Group

• Rectus femoris, Vastus lateralis, Vastus medialis: Extend knee; rectus femoris also flexes hip.

Leg and Foot

• Tibialis anterior: Dorsiflexes and inverts foot.

• Gastrocnemius, Soleus: Plantarflex foot at ankle.

• Gracilis: Adducts thigh, flexes and medially rotates leg.

• Flexor digitorum brevis, Flexor hallucis brevis: Flex toes.

• Abductor hallucis, Adductor hallucis: Abduct and adduct great toe.

Summary Table: Major Muscle Groups and Actions

Region	Muscle Group	Main Actions
Face	Facial expression muscles	Facial movements (smiling, frowning, etc.)
Eye	Extraocular muscles	Eye movement
Mastication	Masseter, Temporalis, Pterygoids	Chewing
Neck	Sternocleidomastoid, Scalenes, etc.	Head movement, swallowing
Thorax	Diaphragm, Intercostals	Breathing
Abdomen	Rectus abdominis, Obliques	Trunk movement, compression
Pelvic Floor	Levator ani, Sphincters	Support pelvic organs
Upper Limb	Deltoid, Biceps, Triceps, etc.	Arm and hand movement
Lower Limb	Quadriceps, Hamstrings, Gluteals, etc.	Leg and foot movement

Additional info:

• Muscle names often reflect their location, shape, size, direction of fibers, number of origins, or action (e.g., biceps = two heads, rectus = straight).

• Muscles work in groups: agonists (prime movers), antagonists (oppose movement), synergists (assist movement), and fixators (stabilize origin).

• Understanding muscle anatomy is essential for clinical applications such as diagnosing injuries, planning surgeries, and physical therapy.