Muscle Groups and Their Actions

Overview

This study guide summarizes the major muscle groups of the human body, their origins and insertions, and their primary actions. Understanding these muscles is essential for comprehending movement, posture, and the functional anatomy of the musculoskeletal system.

Muscles of Facial Expression

Key Muscles and Actions

• Buccinator: Compresses cheek (e.g., blowing, sucking).

• Orbicularis oris: Closes and purses lips.

• Orbicularis oculi: Closes eyelids.

• Zygomaticus minor: Elevates upper lip.

• Zygomaticus major: Elevates corner of mouth (smiling).

• Platysma: Depresses mandible, draws corner of mouth laterally.

• Occipitofrontalis: Elevates eyebrows, wrinkles forehead.

• Depressor anguli oris: Depresses and draws corner of mouth laterally.

Example: Smiling involves the zygomaticus major and minor, while frowning uses the depressor anguli oris.

Muscles of Mastication

Key Muscles and Actions

• Masseter: Elevates mandible (chewing).

• Temporalis: Elevates mandible.

• Lateral pterygoid: Anterior and side-to-side movement of mandible.

• Medial pterygoid: Anterior and side-to-side movement of mandible.

Example: The masseter is one of the strongest muscles relative to its size, crucial for biting.

Muscles of the Neck and Cervical Vertebrae Movement

Key Muscles and Actions

• Sternocleidomastoid: Flexion, lateral flexion, and rotation of neck.

• Trapezius (upper, middle, lower): Extension at the neck, elevation, retraction, and depression/rotation of scapula.

Example: Turning the head to the side uses the sternocleidomastoid.

Deep Muscles of the Thorax (Breathing)

Key Muscles and Actions

• External Intercostals: Elevate ribs (inhalation).

• Internal Intercostals: Depress ribs (exhalation).

• Diaphragm: Changes volume of thoracic cavity; primary mover for inhalation.

Example: The diaphragm contracts and flattens during inhalation, increasing thoracic volume.

Muscles of the Abdominal Wall & Lumbar Vertebrae Movement

Key Muscles and Actions

• Rectus abdominis: Compresses abdomen, flexes vertebral column.

• Transverse abdominis: Compresses abdominal wall.

• External oblique: Compresses abdominal wall, trunk rotation, lateral flexion.

• Internal oblique: Compresses abdominal wall, trunk rotation, lateral flexion.

• Latissimus dorsi: Extends, adducts, and medially rotates humerus; trunk rotation.

• Erector spinae group: Extension and lateral flexion of vertebral column.

• Quadratus lumborum: Lateral flexion of spine, depresses ribs.

• Iliopsoas (Psoas major, Iliacus): Flexion of vertebral column and hip.

Example: Sit-ups primarily use the rectus abdominis.

Muscles of the Pectoral Girdle

Key Muscles and Actions

• Levator scapulae: Elevates scapula.

• Rhomboideus (major and minor): Adducts and medially rotates scapula.

• Trapezius: Elevates scapula, extends neck.

• Pectoralis major: Flexes and adducts humerus; pulls scapula forward and glenoid cavity down.

• Pectoralis minor: Pulls scapula forward and glenoid cavity upward.

• Coracobrachialis: Flexes and adducts humerus.

Example: Shrugging shoulders uses the trapezius and levator scapulae.

Muscles that Insert onto the Humerus

Key Muscles and Actions

• Deltoid (anterior, middle, posterior): Lateral and medial rotation, abduction, and flexion of humerus.

• Teres major: Adducts humerus, rotates humerus medially.

• Rotator cuff muscles: Supraspinatus (abducts and rotates humerus laterally), Infraspinatus (rotates humerus laterally), Teres minor (rotates humerus laterally), Subscapularis (rotates humerus medially).

Example: Throwing a ball involves the rotator cuff muscles and deltoid.

Muscles that Insert onto the Radius and Ulna

Key Muscles and Actions

• Biceps brachii (long and short head): Flexes elbow and shoulder.

• Brachialis: Flexes elbow.

• Brachioradialis: Flexes elbow.

• Triceps brachii: Extends elbow and adducts shoulder.

• Pronator teres: Pronates forearm and hand, flexion at elbow.

• Supinator: Supinates forearm and hand.

Example: Lifting objects uses the biceps brachii and brachialis.

Muscles that Insert onto the Carpals and Hand

Key Muscles and Actions

• Flexor digitorum superficialis: Flexes digits.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Flexor carpi radialis: Flexes and abducts wrist.

• Extensor digitorum: Extends fingers.

• Extensor carpi radialis group: Extends and abducts wrist.

• Extensor carpi ulnaris: Extends and adducts wrist.

Example: Typing uses flexors and extensors of the fingers and wrist.

Muscles that Move the Thigh (Hip)

Key Muscles and Actions

• Iliopsoas group (Iliacus, Psoas major): Flexion of vertebral column and hip.

• Gluteus maximus: Extends and rotates hip laterally.

• Gluteus medius: Abducts and rotates hip medially.

• Tensor fasciae latae: Abducts thigh at hip.

• Gracilis, Pectineus, Adductor longus, Adductor magnus: Adduct thigh at hip.

Example: Walking uses the iliopsoas, gluteal, and adductor muscles.

Muscles that Act on the Knee

Key Muscles and Actions

• Quadriceps group (Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius): Extension at knee.

• Hamstring group (Biceps femoris, Semitendinosus, Semimembranosus): Extension at hip and flexion at knee.

• Sartorius: Abducts and rotates thigh laterally, flexes hip and knee.

Example: Standing up from a chair uses the quadriceps group.

Muscles that Act on the Ankle, Foot, or Toes

Key Muscles and Actions

• Tibialis anterior: Dorsiflexes foot at ankle.

• Peroneus (fibularis) longus: Plantar flexes foot at ankle.

• Gastrocnemius: Plantar flexes foot at ankle.

Example: Walking on tiptoes uses the gastrocnemius and peroneus longus.

Origins and Insertions: Selected Examples

Muscle Attachments

Muscle origin is the fixed attachment, while insertion moves with contraction. Understanding these helps predict muscle action.

Additional info: Muscle actions depend on their attachments and the joints they cross. For example, the biceps brachii crosses both the shoulder and elbow, allowing it to flex both joints.

Summary Table: Major Muscle Groups and Actions

Key Terms and Definitions

• Origin: The fixed attachment of a muscle.

• Insertion: The movable attachment of a muscle.

• Flexion: Decreasing the angle between two bones.

• Extension: Increasing the angle between two bones.

• Abduction: Moving a limb away from the midline.

• Adduction: Moving a limb toward the midline.

• Rotation: Moving a bone around its longitudinal axis.

• Plantar flexion: Pointing the toes downward.

• Dorsiflexion: Raising the toes upward.

Additional info:

• Muscle names often reflect their location, action, or attachments (e.g., biceps brachii means "two-headed muscle of the arm").

• Muscle groups work together (agonists, antagonists, synergists) to produce smooth movements.

• Understanding muscle actions is essential for clinical assessment, injury diagnosis, and rehabilitation.