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 examples.

Muscles of the Head and Neck

Facial Muscles

• Orbicularis oculi: Closes the eyes, enables blinking and squinting.

• Zygomaticus: Raises the lateral corners of the mouth, producing a smile.

• Orbicularis oris: Closes the mouth, purses lips (as in kissing).

• Platysma: Depresses the mandible, pulls the lower lip back and down.

• Frontal belly of the epicranius: Raises eyebrows, wrinkles the skin of the forehead.

• Masseter: Closes the jaw, elevates the mandible; important for chewing.

Example: The orbicularis oculi is used when blinking to protect the eye from debris.

Neck Muscles

• Sternocleidomastoid: Flexes the neck when both sides contract; rotates the head toward the opposite shoulder when one side contracts.

• Digastric: Elevates the hyoid bone, opens the mouth, and depresses the mandible.

Example: The sternocleidomastoid is activated when turning the head to look over the shoulder.

Muscles of the Shoulder and Chest

Shoulder Muscles

• Deltoid: Prime mover of arm abduction.

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

• Serratus anterior: Moves the scapula forward (protraction).

Example: The deltoid is used when lifting the arm to the side.

Muscles of the Abdomen and Back

Abdominal Muscles

• Transversus abdominis: Compresses abdominal contents.

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

• External obliques: Similar actions to internal obliques.

• Rectus abdominis: Flexes vertebral column, depresses ribs, stabilizes pelvis during walking.

Example: The rectus abdominis is engaged during sit-ups.

Back Muscles

• Trapezius: Extends the head, retracts, elevates, and depresses the scapula (shrugging shoulders).

• Teres major: Extends, medially rotates, and adducts the humerus.

• Latissimus dorsi: Prime mover of arm extension; adducts and medially rotates the arm.

• Supraspinatus: Assists in abduction of the humerus; stabilizes the shoulder.

• Infraspinatus: Lateral rotation of the humerus.

• Erector spinae: Extends and abducts the vertebral column.

Example: The latissimus dorsi is used during swimming strokes.

Muscles of the Arm

Upper Arm Muscles

• Triceps brachii: Extension of the forearm.

• Biceps brachii: Flexion and supination of the forearm.

Example: The biceps brachii is used when lifting objects.

Forearm Muscles

• Flexor carpi radialis: Wrist flexion, abducts hand.

• Flexor carpi ulnaris: Wrist flexion, adducts hand.

• Flexor digitorum superficialis: Flexes wrist and fingers 2-5.

• Flexor pollicis longus: Flexes thumb.

• Extensor carpi ulnaris: Extends and adducts wrist.

• Extensor carpi radialis longus: Extends and abducts wrist.

• Extensor digitorum: Finger extension.

• Extensor pollicis longus: Extends thumb.

Example: The flexor digitorum superficialis is used when gripping objects.

Muscles of the Hip and Thigh

Hip Muscles

• Iliopsoas: Flexes trunk on thigh, hip flexor.

• Tensor fasciae latae: Flexes, abducts, and medially rotates thigh.

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

Example: The sartorius is used when sitting cross-legged.

Thigh Muscles

• Rectus femoris: Extends knee and flexes thigh at hip.

• Vastus lateralis: Extends knee.

• Vastus medialis: Extends knee.

• Adductor longus: Adducts, medially rotates, and flexes thigh.

• Gracilis: Adducts thigh and medially rotates leg.

Example: The quadriceps group (including rectus femoris, vastus lateralis, vastus medialis) is used during running.

Muscles of the Gluteal Region and Posterior Thigh

• Gluteus maximus: Hip extensor, laterally rotates thigh.

• Biceps femoris: Extends thigh, flexes knee, laterally rotates leg.

• Semitendinosus: Extends thigh, flexes knee, medially rotates leg.

• Semimembranosus: Extends thigh, flexes knee, medially rotates leg.

Example: The gluteus maximus is used when rising from a sitting position.

Muscles of the Leg and Foot

Posterior Leg Muscles

• Gastrocnemius: Flexion of knee when foot is dorsiflexed.

• Soleus: Plantarflexion of foot.

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

• Popliteus: Flexes and medially rotates leg.

• Fibularis longus: Plantarflexes and everts foot.

• Flexor hallucis longus: Flexes great toe; pushes off when walking.

• Tibialis posterior: Inversion and plantarflexion of foot.

Example: The gastrocnemius is used when jumping.

Anterior Leg Muscles

• Tibialis anterior: Dorsiflexes and inverts foot.

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

• Extensor hallucis longus: Extends great toe.

Example: The tibialis anterior is used when walking on heels.

Summary Table: Major Muscles and Their Actions

Muscle	Action	Region
Deltoid	Arm abduction	Shoulder
Biceps brachii	Forearm flexion, supination	Arm
Rectus femoris	Extends knee, flexes thigh	Thigh
Gastrocnemius	Knee flexion, plantarflexion	Leg
Orbicularis oculi	Closes eyes	Face
Gluteus maximus	Hip extension, lateral rotation	Gluteal
Trapezius	Extends head, moves scapula	Back
Tibialis anterior	Dorsiflexes, inverts foot	Leg
Flexor digitorum longus	Flexes toes, plantarflexes foot	Leg

Additional info: This table summarizes a selection of major muscles and their primary actions for quick reference. For comprehensive study, refer to the full list above.

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.

• Plantarflexion: Pointing the toes downward.

• Dorsiflexion: Raising the foot upward toward the shin.

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

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

Conclusion

Mastery of muscle identification and understanding their actions is fundamental for students of anatomy and physiology. Use this guide to review the names, locations, and functions of key muscles for lab practicals and exams.