Muscle Classification and Major Skeletal Muscles: Structure, Function, and Origins/Insertions

Muscle Classification and Nomenclature

Overview of Muscle Naming

Muscles are named according to several characteristics, which help in identifying their location, function, and structure. Understanding these naming conventions is essential for recognizing and describing muscles in human anatomy.

• Orientation of muscle fibers: Direction of fibers relative to the body or limb axis (e.g., rectus, oblique, transverse).

• Location or specific body region: Indicates where the muscle is found (e.g., pectoralis, gluteus, brachii).

• Muscle size: Describes relative size (e.g., maximus, minimus, longus, brevis).

• Muscle shape: Refers to the geometric shape (e.g., deltoid, trapezius, rhomboid).

• Muscle attachment: Named for points of origin and/or insertion (e.g., sternocleidomastoid, brachioradialis).

• Muscle action: Indicates the primary action (e.g., flexor, extensor, adductor, abductor).

Characteristic	Examples	Examples in Humans
Orientation of muscle fibers	Rectus (straight), Oblique (at an angle), Transverse (horizontal)	Rectus abdominis, Transversus abdominis
Location or specific body region	Pectoralis (chest), Gluteus (buttock), Brachii (arm)	Pectoralis major, Gluteus maximus, Biceps brachii
Muscle size	Maximus (largest), Minimus (smallest), Longus (long), Brevis (short)	Gluteus maximus, Peroneus longus
Muscle shape	Deltoid (triangle), Trapezius (trapezoid), Rhomboid (rhomboid)	Deltoid, Trapezius, Rhomboid major
Muscle attachment	Sternocleidomastoid (sternum, clavicle, mastoid), Brachioradialis (brachium, radius)	Sternocleidomastoid, Brachioradialis
Muscle action	Flexor, Extensor, Adductor, Abductor, Depressor	Flexor carpi radialis, Extensor digitorum, Adductor longus

Additional info: Muscle names often combine several characteristics, such as location and action (e.g., flexor carpi ulnaris).

Major Skeletal Muscles: Actions, Origins, and Insertions

Muscles of Facial Expression

These muscles control facial movements and expressions. They are primarily attached to the skin and underlying bones of the face.

Muscle	Action	Origin-Insertion
Frontalis	Elevates eyebrows, wrinkles skin of forehead horizontally	O: Epicranial aponeurosis I: Skin of eyebrows
Orbicularis oculi	Closes eye	O: Frontal and maxilla I: Skin of eyelid
Zygomaticus major/minor	Elevates corners of mouth and upper lip (smiling)	O: Zygomatic bone I: Skin at angle of mouth
Orbicularis oris	Closes and protrudes lips	O: Maxilla and mandible I: Skin around mouth
Buccinator	Pulls cheeks in	O: Maxilla and mandible I: Orbicularis oris
Platysma	Tenses skin of neck	O: Fascia of chest I: Mandible

Muscles of Mastication

These muscles are responsible for movements of the jaw during chewing.

Muscle	Action	Origin-Insertion
Masseter	Elevates mandible	O: Zygomatic arch I: Mandible
Temporalis	Elevates and retracts mandible	O: Temporal bone I: Mandible
Medial pterygoid	Elevates and protracts mandible	O: Sphenoid bone I: Mandible
Lateral pterygoid	Depresses and protracts mandible	O: Sphenoid bone I: Mandible

Muscles that Move the Neck and Trunk

These muscles are involved in flexion, extension, and rotation of the neck and trunk.

Muscle	Action	Origin-Insertion
Sternocleidomastoid	Flexes neck; rotates head	O: Sternum and clavicle I: Mastoid process
Scalenes	Flexes neck; elevates ribs	O: Cervical vertebrae I: Ribs 1 and 2
Trapezius (upper)	Extends neck	O: Occipital bone and cervical spine I: Clavicle and scapula
Erector spinae group	Extends trunk	O: Vertebrae, ribs, iliac crest I: Vertebrae, ribs, skull
Quadratus lumborum	Laterally flexes trunk	O: Iliac crest I: 12th rib and lumbar spine

Muscles of Respiration

These muscles facilitate breathing by changing the volume of the thoracic cavity.

Muscle	Action	Origin-Insertion
Diaphragm	Flattens, increases thoracic volume (inspiration)	O: Inferior border of rib cage I: Central tendon of diaphragm
External intercostals	Elevate ribs (inspiration)	O: Inferior border of rib above I: Superior border of rib below
Internal intercostals	Depress ribs (expiration)	O: Superior border of rib below I: Inferior border of rib above

Muscles of the Abdominal Wall

These muscles compress the abdominal cavity and assist in trunk movements.

Muscle	Action	Origin-Insertion
Rectus abdominis	Flexes trunk	O: Pubis I: Ribs 5-7
External oblique	Flexes and rotates trunk	O: Ribs 5-12 I: Iliac crest, linea alba
Internal oblique	Flexes and rotates trunk	O: Iliac crest I: Ribs 10-12
Transversus abdominis	Compresses abdominal cavity	O: Iliac crest, lumbar fascia I: Linea alba

Muscles that Move the Scapula

These muscles stabilize and move the scapula, allowing for shoulder movement.

Muscle	Action	Origin-Insertion
Levator scapulae	Elevates scapula	O: Cervical vertebrae I: Superior angle of scapula
Rhomboid major/minor	Retract scapula	O: Thoracic vertebrae I: Medial border of scapula
Serratus anterior	Protracts scapula	O: Ribs 1-8 I: Medial border of scapula

Muscles that Move the Humerus (Shoulder)

These muscles are responsible for movements of the upper arm at the shoulder joint.

Muscle	Action	Origin-Insertion
Pectoralis major	Flexes, adducts, and medially rotates shoulder	O: Sternum, costal cartilage I: Proximal humerus
Latissimus dorsi	Extends, adducts, and medially rotates shoulder	O: Iliac crest, lower vertebrae I: Proximal humerus
Deltoid	Abducts shoulder	O: Clavicle, scapula I: Deltoid tuberosity of humerus
Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis)	Stabilize and rotate shoulder	O: Scapula I: Humerus (greater/lesser tubercle)

Muscles that Move the Elbow

These muscles flex or extend the forearm at the elbow joint.

Muscle	Action	Origin-Insertion
Biceps brachii	Flexes elbow	O: Scapula I: Radial tuberosity
Brachialis	Flexes elbow	O: Humerus I: Ulna
Triceps brachii	Extends elbow	O: Scapula and humerus I: Olecranon process of ulna

Muscles that Move the Wrist and Fingers

These muscles control movements of the wrist and fingers, allowing for fine motor skills.

Muscle	Action	Origin-Insertion
Flexor carpi radialis/ulnaris	Flexes wrist	O: Medial epicondyle of humerus I: Metacarpals
Extensor carpi radialis/ulnaris	Extends wrist	O: Lateral epicondyle of humerus I: Metacarpals
Flexor digitorum superficialis/profundus	Flexes digits 2-5	O: Humerus, ulna I: Middle/distal phalanges
Extensor digitorum	Extends digits 2-5	O: Lateral epicondyle of humerus I: Distal phalanges

Muscles that Move the Thumb

These muscles allow for thumb opposition, abduction, and extension, which are essential for grasping objects.

Muscle	Action	Origin-Insertion
Abductor pollicis longus	Abducts thumb	O: Radius and ulna I: 1st metacarpal
Extensor pollicis longus	Extends thumb	O: Ulna I: Distal phalanx of thumb

Muscles that Move the Hip

These muscles are responsible for movements of the thigh at the hip joint, including flexion, extension, abduction, and rotation.

Muscle	Action	Origin-Insertion
Iliopsoas (psoas major & iliacus)	Flexes hip	O: Iliac fossa, lumbar vertebrae I: Lesser trochanter of femur
Gluteus maximus	Extends and laterally rotates hip	O: Ilium, sacrum, coccyx I: Gluteal tuberosity of femur
Gluteus medius/minimus	Abducts and medially rotates hip	O: Ilium I: Greater trochanter of femur
Adductor group (longus, brevis, magnus)	Adducts hip	O: Pubis I: Femur

Muscles that Move the Knee

These muscles flex or extend the leg at the knee joint.

Muscle	Action	Origin-Insertion
Quadriceps femoris group (rectus femoris, vastus lateralis, medialis, intermedius)	Extends knee	O: Ilium, femur I: Tibial tuberosity
Hamstring group (biceps femoris, semitendinosus, semimembranosus)	Flexes knee	O: Ischial tuberosity I: Tibia, fibula

Muscles that Move the Ankle and Toes

These muscles control movements of the foot and toes, essential for walking and balance.

Muscle	Action	Origin-Insertion
Tibialis anterior	Dorsiflexes ankle	O: Anterior tibia I: Metatarsal #1
Gastrocnemius	Plantarflexes ankle	O: Femur I: Calcaneus via Achilles tendon
Soleus	Plantarflexes ankle	O: Tibia, fibula I: Calcaneus
Extensor digitorum longus	Extends toes 2-5	O: Tibia, fibula I: Distal phalanges of toes 2-5
Flexor digitorum longus	Flexes toes 2-5	O: Tibia I: Distal phalanges of toes 2-5

Summary Table: Muscle Actions and Attachments

This table summarizes the main actions and attachments of major muscle groups discussed above.

Region	Primary Actions	Key Muscles
Face	Facial expression	Frontalis, orbicularis oculi, zygomaticus
Jaw	Mastication	Masseter, temporalis
Neck/Trunk	Flexion, extension, rotation	Sternocleidomastoid, trapezius, erector spinae
Shoulder	Abduction, adduction, rotation	Deltoid, pectoralis major, rotator cuff
Elbow	Flexion, extension	Biceps brachii, triceps brachii
Wrist/Fingers	Flexion, extension	Flexor carpi radialis, extensor digitorum
Hip	Flexion, extension, abduction, adduction	Iliopsoas, gluteus maximus, adductors
Knee	Flexion, extension	Quadriceps, hamstrings
Ankle/Toes	Dorsiflexion, plantarflexion, toe movement	Tibialis anterior, gastrocnemius, flexor digitorum longus

Key Terms and Concepts

• Origin: The fixed attachment point of a muscle, usually proximal.

• Insertion: The movable attachment point, usually distal.

• Action: The movement produced when the muscle contracts.

• Prime mover (agonist): Main muscle responsible for a movement.

• Antagonist: Muscle that opposes the action of the prime mover.

• Synergist: Muscle that assists the prime mover.

• Fixator: Muscle that stabilizes the origin of the prime mover.

Example: Biceps Brachii

• Origin: Scapula (coracoid process and supraglenoid tubercle)

• Insertion: Radial tuberosity

• Action: Flexes elbow and supinates forearm

Additional info: Understanding muscle actions and attachments is crucial for clinical applications, such as diagnosing injuries or planning rehabilitation exercises.