Human Muscles: Identification and Actions – Study Guide

Muscles of the Human Body

Overview

The human muscular system consists of numerous muscles responsible for movement, posture, and various bodily functions. Understanding the location, identification, and actions of these muscles is essential for anatomy and physiology students. This guide summarizes key muscles, their actions, and relevant anatomical context.

Muscles of the Head and Neck

Facial Muscles

• Orbicularis oculi: Closes the eyes, enables blinking and squinting. Example: Used when winking or protecting the eyes from bright light.

• Zygomaticus: Raises the lateral corners of the mouth, producing a smile. Example: Smiling or laughing.

• Orbicularis oris: Closes the mouth, purses lips (as in kissing). Example: Whistling or blowing a kiss.

• Platysma: Depresses the mandible, pulls the lower lip back and down. Example: Expressions of surprise or fright.

• Frontal belly of the epicranius: Raises eyebrows, wrinkles skin of forehead. Example: Expressing curiosity or shock.

• Masseter: Closes jaw, elevates mandible. Example: Chewing food.

Neck Muscles

• Sternocleidomastoid: Flexes neck (both sides together); rotates head toward opposite shoulder (individually). Example: Turning the head to look over the shoulder.

• Digastric: Elevates hyoid, opens mouth, depresses mandible. Example: Swallowing or opening the mouth widely.

Muscles of the Shoulder and Chest

• Deltoid: Prime mover of arm abduction. Example: Raising the arm to the side.

• Pectoralis major: Arm flexion, adducts and medially rotates arm. Example: Hugging or pushing movements.

• Serratus anterior: Moves scapula forward. Example: Punching or reaching forward.

Muscles of the Abdomen

• Transversus abdominis: Compresses abdominal contents. Example: Forced expiration or stabilizing the core.

• Internal obliques: Flexes vertebral column, fixes and depresses ribs, stabilizes pelvis during walking, aids in trunk rotation and lateral flexion. Example: Twisting the torso.

• External obliques: Similar actions as internal obliques. Example: Bending sideways or rotating the trunk.

• Rectus abdominis: Flexes vertebral column, fixes and depresses ribs, stabilizes pelvis during walking. Example: Sit-ups or crunches.

Muscles of the Back

• Trapezius: Extends head, retracts scapula, elevates and depresses scapula (shrug shoulders). Example: Shrugging or pulling shoulders back.

• Teres major: Extends, medially rotates, and adducts humerus. Example: Pulling arms down and back.

• Latissimus dorsi: Prime mover of arm extension; adducts and medially rotates arm. Example: Swimming or climbing.

• Supraspinatus: Assists in abduction of humerus; stabilizes shoulder. Example: Initiating arm lift.

• Infraspinatus: Lateral rotation of humerus. Example: Rotating arm outward.

• Erector spinae: Extends and abducts vertebral column. Example: Standing up straight or bending backward.

Muscles of the Arm

• Triceps brachii: Extension of forearm. Example: Straightening the elbow.

• Biceps brachii: Flexion of forearm, supination of forearm. Example: Bending the elbow or turning the palm upward.

Muscles of the Forearm

Flexor Group

• Flexor carpi radialis: Wrist flexion, abducts hand. Example: Waving or gripping.

• Flexor carpi ulnaris: Wrist flexion, adducts hand. Example: Moving hand toward the body.

• Flexor digitorum superficialis: Flexes wrist and phalanges of fingers 2-5. Example: Making a fist.

• Flexor pollicis longus: Flexes thumb. Example: Grasping objects.

Extensor Group

• Extensor carpi ulnaris: Extends and adducts wrist. Example: Moving hand away from the body.

• Extensor carpi radialis longus: Extends and abducts wrist. Example: Lifting hand upward and outward.

• Extensor digitorum: Finger extension. Example: Opening the hand.

• Extensor pollicis longus: Extends thumb. Example: Thumbs-up gesture.

Muscles of the Hip and Thigh

• Iliopsoas: Flex trunk on thigh, hip flexor. Example: Walking or running.

• Tensor fasciae latae: Flexes, abducts, and medially rotates thigh. Example: Stabilizing pelvis during movement.

• Sartorius: Flexes and laterally rotates thigh (cross-legged sitting). Example: Sitting cross-legged.

• Rectus femoris: Extends knee and flexes thigh at hip. Example: Kicking a ball.

• Vastus lateralis: Extends knee. Example: Standing up from a chair.

• Vastus medialis: Extends knee. Example: Walking or running.

• Adductor longus: Adducts and medially rotates and flexes thigh. Example: Bringing legs together.

• Gracilis: Adducts thigh and medially rotates leg. Example: Crossing legs.

Muscles of the Gluteal Region and Posterior Thigh

• Gluteus maximus: Hip extensor, laterally rotates thigh. Example: Rising from a sitting position.

• Biceps femoris: Extends thigh, flexes knee, laterally rotates leg. Example: Running or jumping.

• Semitendinosus: Extends thigh, flexes knee, medially rotates leg. Example: Walking or squatting.

• Semimembranosus: Extends thigh, flexes knee, medially rotates leg. Example: Similar to semitendinosus.

Muscles of the Leg

Posterior Compartment

• Gastrocnemius: Flexion of knee when foot is dorsiflexed. Example: Jumping or running.

• Soleus: Plantarflexion of foot. Example: Standing on tiptoe.

• Flexor digitorum longus: Flexes toes; plantarflexes and inverts foot. Example: Curling toes.

• Popliteus: Flexes and medially rotates leg. Example: Unlocking the knee during walking.

• Fibularis longus: Plantarflexes and everts foot. Example: Balancing on one foot.

• Flexor hallucis longus: Flexes great toe; pushes off when walking. Example: Propelling forward during gait.

• Tibialis posterior: Inversion and plantarflexion of foot. Example: Stabilizing the arch of the foot.

Anterior Compartment

• Tibialis anterior: Dorsiflexes and inverts foot. Example: Lifting toes upward.

• Extensor digitorum longus: Toe extension; dorsiflexes and everts foot. Example: Raising toes.

• Extensor hallucis longus: Extends great toe. Example: Lifting big toe.

Summary Table: Major Muscles and Their Actions

Muscle	Primary Action	Region
Deltoid	Arm abduction	Shoulder
Biceps brachii	Forearm flexion, supination	Arm
Triceps brachii	Forearm extension	Arm
Rectus femoris	Knee extension, hip flexion	Thigh
Gastrocnemius	Knee flexion, plantarflexion	Leg
Gluteus maximus	Hip extension, lateral rotation	Gluteal
Orbicularis oculi	Closes eyes	Face
Masseter	Jaw closure	Face
Tibialis anterior	Dorsiflexion, inversion	Leg
Latissimus dorsi	Arm extension, adduction	Back

Key Terms and Definitions

• Abduction: Movement away from the midline of the body.

• Adduction: Movement toward the midline of the body.

• Flexion: Decreasing the angle between two body parts.

• Extension: Increasing the angle between two body parts.

• Rotation: Moving a bone around its longitudinal axis.

• Plantarflexion: Pointing the toes downward.

• Dorsiflexion: Raising the toes upward.

• Supination: Rotating the forearm so the palm faces up.

• Pronation: Rotating the forearm so the palm faces down.

Additional info:

• Muscle actions are often described relative to anatomical position and joint movement.

• Muscles may act as agonists (prime movers), antagonists, synergists, or fixators depending on the movement.

• Understanding muscle actions is essential for clinical assessment, rehabilitation, and movement analysis.