Comprehensive Study Guide: Muscles of the Human Body

Muscular System Overview

Introduction to the Muscular System

The muscular system is responsible for movement, posture, and heat production in the human body. Muscles are classified based on their location, function, and structure. This guide covers the identification and function of major skeletal muscles, organized by anatomical region.

• Skeletal muscles are voluntary muscles attached to bones and responsible for body movements.

• Muscle actions include flexion, extension, abduction, adduction, rotation, and more.

• Muscles often work in groups: agonists (prime movers), antagonists, and synergists.

Superficial Muscles: Posterior View

Major Muscles of the Back and Posterior Limbs

The posterior view highlights muscles responsible for extension, rotation, and stabilization of the trunk and limbs.

• Trapezius: Elevates, retracts, and rotates the scapula.

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

• Deltoid: Abducts the arm.

• Gluteus maximus: Extends and laterally rotates the thigh.

• Hamstrings (biceps femoris, semitendinosus, semimembranosus): Flex the knee and extend the hip.

• Gastrocnemius: Plantar flexes the foot and flexes the knee.

Muscles of Facial Expression and Mastication

Muscles of Facial Expression

These muscles allow for a wide range of facial movements, including smiling, frowning, and blinking.

• Frontalis: Raises eyebrows and wrinkles forehead.

• Orbicularis oculi: Closes eyelids.

• Orbicularis oris: Closes and protrudes lips.

• Zygomaticus major: Elevates corners of the mouth (smiling).

• Buccinator: Compresses cheek (as in blowing).

Muscles of Mastication

• Masseter: Elevates mandible (closes jaw).

• Temporalis: Elevates and retracts mandible.

• Medial and lateral pterygoids: Move jaw side-to-side and assist in opening.

Muscles of the Head and Neck

Muscles of Swallowing

These muscles facilitate the movement of food from the mouth to the esophagus.

• Digastric: Depresses mandible and elevates hyoid during swallowing.

• Mylohyoid: Elevates floor of mouth.

• Stylohyoid: Elevates and retracts hyoid bone.

Muscles That Move the Head and Neck

• Sternocleidomastoid: Flexes and rotates the head.

• Splenius capitis: Extends and rotates the head.

• Semispinalis capitis: Extends head and neck.

Muscles of the Vertebral Column

Deep and Superficial Back Muscles

These muscles stabilize and move the vertebral column.

• Erector spinae group: Extends and laterally flexes the spine.

• Multifidus: Stabilizes vertebrae during movement.

• Quadratus lumborum: Flexes the vertebral column laterally.

Muscles of the Trunk

Abdominal Muscles

The abdominal wall muscles support trunk movement, protect internal organs, and assist in respiration.

• Rectus abdominis: Flexes vertebral column and compresses abdomen.

• External oblique: Rotates and flexes trunk.

• Internal oblique: Rotates and flexes trunk.

• Transversus abdominis: Compresses abdominal contents.

Intercostal Muscles

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

Muscles of the Pelvic Floor and Perineum

Pelvic Floor Muscles

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

• Levator ani: Supports pelvic viscera and aids in continence.

• Coccygeus: Supports pelvic organs.

• External anal sphincter: Controls defecation.

• Urogenital diaphragm: Supports the pelvic floor and assists in urinary continence.

Muscles of the Shoulder and Arm

Muscles That Move the Scapula

• Trapezius: Elevates, retracts, and rotates scapula.

• Serratus anterior: Protracts scapula.

• Rhomboids: Retract scapula.

Muscles That Move the Arm

• Deltoid: Abducts arm.

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

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

• Teres major: Extends and medially rotates arm.

Muscles That Move the Forearm

• Biceps brachii: Flexes elbow and supinates forearm.

• Triceps brachii: Extends elbow.

• Brachialis: Flexes elbow.

• Brachioradialis: Flexes forearm.

Muscles of the Forearm and Hand

Muscles That Move the Hand and Fingers

• Flexor carpi radialis: Flexes and abducts wrist.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Extensor carpi radialis: Extends and abducts wrist.

• Extensor digitorum: Extends fingers.

• Thenar and hypothenar muscles: Move the thumb and little finger.

• Lumbricals and interossei: Flex and extend fingers.

Muscles That Move the Thigh and Knee

Anterior and Medial Muscles

• Quadriceps femoris group (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius): Extend the knee.

• Sartorius: Flexes, abducts, and laterally rotates thigh.

• Adductor group (adductor longus, adductor magnus, gracilis): Adduct thigh.

Posterior Muscles

• Hamstrings (biceps femoris, semitendinosus, semimembranosus): Flex knee and extend hip.

• Gluteus maximus: Extends and laterally rotates thigh.

Muscles That Move the Ankle, Foot, and Toes

Anterior, Lateral, and Posterior Muscles

• Tibialis anterior: Dorsiflexes and inverts foot.

• Fibularis (peroneus) longus: Everts and plantar flexes foot.

• Gastrocnemius: Plantar flexes foot and flexes knee.

• Soleus: Plantar flexes foot.

• Extensor digitorum longus: Extends toes.

• Flexor digitorum longus: Flexes toes.

Muscles That Move the Toes

• Flexor hallucis longus: Flexes big toe.

• Extensor hallucis longus: Extends big toe.

• Lumbricals and interossei: Flex and extend toes.

Summary Table: Major Muscle Groups and Their Actions

Region	Major Muscles	Primary Actions
Face	Frontalis, Orbicularis oculi, Zygomaticus	Facial expression
Mastication	Masseter, Temporalis	Chewing
Neck	Sternocleidomastoid, Splenius capitis	Head movement
Back	Trapezius, Latissimus dorsi, Erector spinae	Trunk movement
Abdomen	Rectus abdominis, Obliques	Trunk flexion, rotation
Pelvic Floor	Levator ani, Coccygeus	Support pelvic organs
Shoulder/Arm	Deltoid, Pectoralis major, Biceps brachii	Arm movement
Forearm/Hand	Flexor/extensor groups	Hand/finger movement
Thigh/Knee	Quadriceps, Hamstrings, Adductors	Thigh/knee movement
Ankle/Foot/Toes	Tibialis anterior, Gastrocnemius, Flexor/extensor digitorum	Foot/toe movement

Additional info:

• Muscle identification is essential for understanding movement, diagnosing injuries, and planning rehabilitation.

• Muscles are named based on location, shape, size, direction of fibers, number of origins, and action.

• Muscle contraction is governed by the sliding filament theory, involving actin and myosin filaments.

• Muscle actions can be described using anatomical terms: flexion, extension, abduction, adduction, rotation, circumduction.