Skeletal Muscles: Origins, Insertions, and Actions

Skeletal Muscles: Overview

Introduction to Skeletal Muscles

Skeletal muscles are voluntary muscles attached to bones and are responsible for movement, posture, and stability. Each muscle has a specific origin (attachment point), insertion (end point), and action (movement produced). Understanding these features is essential for identifying muscles and their functions.

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

• Insertion: The movable attachment point, usually distal.

• Action: The movement produced when the muscle contracts.

Axial Skeletal Muscles

Anterior Axial Muscles

The anterior muscles of the axial skeleton are primarily involved in facial expression, head movement, and respiration.

• Orbicularis Oculi: Origin: Frontal and Maxilla bones Insertion: Skin of the eyelids Action: Closes the eyelids

• Orbicularis Oris: Origin: Maxilla and Mandible Insertion: Skin of the lips Action: Closes/Purses the lips ("kissing muscle")

• Masseter: Origin: Zygomatic arch Insertion: Mandibular ramus Action: Elevates jaw

• Sternocleidomastoid: Origin: Manubrium, clavicle Insertion: Mastoid process Action: Flex and laterally rotate head

• Pectoralis Major: Origin: Clavicle, sternum Insertion: Greater tubercle of humerus Action: Flex, adduct, and medially rotate arm

• Serratus Anterior: Origin: Ribs 1-8 Insertion: Medial border of scapula Action: Rotates scapula

Intercostal Muscles

Intercostal muscles are located between the ribs and play a crucial role in breathing.

• Internal Intercostals: Origin: Superior border of inferior rib Insertion: Inferior border of superior rib Action: Depresses ribcage during exhalation

• External Intercostals: Origin: Inferior border of superior rib Insertion: Superior border of inferior rib Action: Elevates ribcage during inhalation

Abdominal Muscles

The abdominal muscles support trunk movement and stability.

• Rectus Abdominis: Origin: Pubic symphysis Insertion: Xiphoid process, costal cartilage Action: Flex and rotate lumbar spine

• External Oblique: Origin: Lower 8 ribs Insertion: Linea alba Action: Trunk rotation

Posterior Axial Muscles

The posterior muscles of the axial skeleton are involved in movements of the back, shoulders, and hips.

• Trapezius: Origin: Occipital bone, spinous processes of C7-T12 Insertion: Acromion process and scapular spine Action: Raise, retract, and rotate scapular spine

• Latissimus Dorsi: Origin: Thoracic and lumbar vertebrae, ribs, iliac crest Insertion: Proximal humerus Action: Extend, adduct, and medially rotate arm

• Gluteus Maximus: Origin: Sacrum, coccyx, ilium Insertion: Iliotibial tract Action: Extends thigh

Summary Table: Muscle Origins, Insertions, and Actions

Muscle	Origin	Insertion	Action
Orbicularis Oculi	Frontal & Maxilla bones	Skin of eyelids	Closes eyelids
Orbicularis Oris	Maxilla & Mandible	Skin of lips	Closes/Purses lips
Masseter	Zygomatic arch	Mandibular ramus	Elevates jaw
Sternocleidomastoid	Manubrium, clavicle	Mastoid process	Flex & rotate head
Pectoralis Major	Clavicle, sternum	Greater tubercle of humerus	Flex, adduct, medially rotate arm
Serratus Anterior	Ribs 1-8	Medial border of scapula	Rotates scapula
Internal Intercostals	Superior border of inferior rib	Inferior border of superior rib	Depresses ribcage
External Intercostals	Inferior border of superior rib	Superior border of inferior rib	Elevates ribcage
Rectus Abdominis	Pubic symphysis	Xiphoid process, costal cartilage	Flex & rotate lumbar spine
External Oblique	Lower 8 ribs	Linea alba	Trunk rotation
Trapezius	Occipital bone, C7-T12	Acromion process, scapular spine	Raise, retract, rotate scapula
Latissimus Dorsi	Thoracic/lumbar vertebrae, ribs, iliac crest	Proximal humerus	Extend, adduct, medially rotate arm
Gluteus Maximus	Sacrum, coccyx, ilium	Iliotibial tract	Extends thigh
Deltoid	Clavicle, acromion process	Diaphysis of humerus	Arm abduction
Biceps Brachii	Coracoid process	Radial tuberosity	Elbow flexion
Brachialis	Distal humerus	Coronoid process	Lifts ulna with elbow flexion
Brachioradialis	Distal humerus	Styloid process of radius	Lifts radius with elbow flexion
Tensor Fascia Latae	Iliac crest	Iliotibial tract	Steadies knee
Rectus Femoris	Anterior iliac spine	Patella, tibial tuberosity	Extend knee, flex thigh
Vastus Lateralis	Greater trochanter	Patella, tibial tuberosity	Extend knee, flex thigh
Vastus Medialis	Diaphysis of femur	Patella, tibial tuberosity	Extend knee, flex thigh
Sartorius	Anterior iliac spine	Proximal tibia	Flex, abduct, laterally rotate thigh
Gracilis	Pubis, ischium	Medial tibia	Adduct thigh
Tibialis Anterior	Tibia	Medial cuneiform, 1st metatarsal	Dorsiflexion
Triceps Brachii	Posterior humerus	Olecranon process	Extends elbow
Biceps Femoris	Ischial tuberosity	Fibula	Flex knee, extend thigh
Semitendinosus	Ischial tuberosity	Tibia	Flex knee, extend thigh
Semimembranosus	Ischial tuberosity	Tibia	Flex knee, extend thigh
Gastrocnemius	Distal femur	Calcaneus	Plantar flexion
Soleus	Fibula, tibia	Calcaneus	Plantar flexion

Key Concepts and Applications

• Muscle identification: Recognizing muscles by their origin, insertion, and action is essential for anatomy and physiology studies.

• Clinical relevance: Understanding muscle actions helps diagnose injuries and plan rehabilitation.

• Movement analysis: Muscles work in groups to produce complex movements; knowing their functions aids in understanding biomechanics.

Example: The sternocleidomastoid muscle is often palpated during physical exams to assess neck movement and flexibility. Additional info: Muscles are often named based on their location, shape, or function (e.g., deltoid for its triangular shape, pectoralis major for its location in the chest).