Terms for Muscle Action

Key Definitions and Concepts

• Origin: The fixed attachment point of a muscle, typically on an immovable bone.

• Insertion: The movable attachment point of a muscle, usually on a bone that moves during contraction.

• Prime Mover (Agonist): The main muscle responsible for a specific movement.

• Synergist: Muscles that assist and stabilize the prime mover.

• Antagonist: Muscles that oppose the action of the prime mover.

• Note: Muscle actions depend on their position relative to the joint:

  • Anterior side: usually flexion

  • Posterior side: usually extension

  • Lateral side: abduction

  • Medial side: adduction

Muscles of Facial Expression

Major Muscles and Their Functions

• Occipitofrontalis (Epicranius): Raises eyebrows, pulls scalp posteriorly.

• Orbicularis oculi: Closes eyelids, enables blinking and squinting.

• Orbicularis oris: Closes and protrudes lips, "kissing muscle".

• Buccinator: Compresses cheek, aids in chewing and whistling.

• Zygomaticus: Raises corners of mouth, "smiling muscle".

• Risorius: Draws corner of mouth laterally, "laughing muscle".

• Platysma: Tenses skin of neck, depresses mandible.

Muscles That Move the Head

Neck and Head Movement

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

• Capitis Group: Extends, hyperextends, and rotates head.

• Splenius capitis: Extends and rotates head.

• Semispinalis capitis: Extends and rotates head.

• Longissimus capitis: Extends and rotates head.

• Scalene group: Flexes and slightly rotates neck.

Muscles That Move the Vertebral Column

Postural and Movement Muscles

• Erector spinae group: Extends and laterally bends vertebral column.

• Quadratus lumborum: Laterally bends vertebral column.

Muscles of the Abdominal Wall

Structure and Function

• Internal oblique: Compresses abdomen, flexes vertebral column.

• Transversus abdominis: Deepest abdominal muscle, compresses abdomen.

• Rectus abdominis: Flexes vertebral column, compresses abdomen.

Breathing Muscles

Muscles Involved in Respiration

• Diaphragm: Separates thoracic and abdominal cavities, main muscle of inspiration.

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

Muscles That Move the Pectoral Girdle

Shoulder and Scapular Movement

• Trapezius: Elevates, retracts, and rotates scapula.

• Serratus anterior: Protracts and holds scapula against thoracic wall.

• Rhomboids: Retract and stabilize scapula.

• Pectoralis minor: Depresses and protracts scapula.

• Levator scapulae: Elevates scapula.

Muscles That Move the Arm

Major Muscles and Their Actions

• Coracobrachialis: Flexes and adducts arm.

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

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

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

• Supraspinatus: Abducts arm.

• Deltoid: Abducts, flexes, and extends arm.

Rotator Cuff Muscles

• Subscapularis: Medial rotation of arm.

• Infraspinatus: Lateral rotation of arm.

• Teres minor: Lateral rotation and adduction of arm.

• All three form the rotator cuff, stabilizing the shoulder joint.

Muscles That Move the Forearm

Flexors, Extensors, and Rotators

• Biceps brachii: Flexes forearm, supinates forearm.

• Brachialis: Flexes forearm.

• Brachioradialis: Flexes forearm.

• Triceps brachii: Extends forearm.

• Supinator: Supinates forearm.

• Pronator teres: Pronates forearm.

• Pronator quadratus: Pronates forearm.

Muscles That Move the Wrist, Hand, and Fingers

Flexors and Extensors

• Flexor carpi radialis: Flexes and abducts wrist.

• Palmaris longus: Flexes wrist.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Extensor carpi radialis longus & brevis: Extend and abduct wrist.

• Extensor digitorum: Extends fingers.

• Extensor carpi ulnaris: Extends and adducts wrist.

Muscles That Move the Thigh

Anterior, Lateral, Posterior, and Adductor Groups

• Iliopsoas: Flexes thigh.

• Tensor fasciae latae: Abducts and medially rotates thigh.

• Gluteus maximus: Extends and laterally rotates thigh.

• Gluteus medius: Abducts and medially rotates thigh.

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

• Gracilis: Adducts thigh, flexes leg.

Muscles That Move the Leg

Flexors and Extensors

• Biceps femoris, semitendinosus, semimembranosus: Flex knee, extend thigh (hamstrings).

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

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

Muscles That Move the Foot

Dorsiflexors, Plantar Flexors, Inverters, and Everters

• Tibialis anterior: Dorsiflexes and inverts foot.

• Extensor digitorum longus: Extends toes, dorsiflexes foot.

• Gastrocnemius: Plantar flexes foot, flexes knee.

• Soleus: Plantar flexes foot.

• Tibialis posterior: Inverts and plantar flexes foot.

• Peroneus (fibularis) longus: Everts and plantar flexes foot.

Summary Table: Major Muscle Groups and Actions

Additional info:

• Muscle actions are determined by their origin, insertion, and orientation relative to the joint.

• Rotator cuff injuries are common in athletes due to repetitive overhead movements.

• The quadriceps femoris group is essential for knee extension and is a common site for intramuscular injections.