Muscular System: Structure, Function, and Disorders

Muscular System

Overview of Skeletal Muscles

The human body contains approximately 700 identified skeletal muscles. These muscles function primarily by pulling (never pushing) due to the arrangement of their contractile units, the sarcomeres. Muscles typically operate in groups to produce coordinated movements, with the insertion (moveable attachment) moving toward the origin (immovable attachment) during contraction.

• Muscles can only pull: Contraction shortens the muscle, pulling on bones or tissues.

• Group action: Muscles rarely act alone; they work in functional groups.

• Movement: Insertion moves toward origin as muscle shortens.

Factors Determining Muscle Contraction Effects

• Anatomical arrangement of muscle fibers: The orientation and structure of muscle fibers affect the force and direction of contraction.

• Attachment to bone: The way a muscle connects to bone influences its action and efficiency.

Classification of Muscles

By Shape (Fascicle Arrangement)

Muscles are classified based on the arrangement of their fascicles (bundles of muscle fibers), which determines their shape and function.

• Parallel Muscles: Fascicles run parallel to the muscle's long axis; spindle-shaped or strap-like. Example: Biceps brachii, Sartorius

• Pennate Muscles: Fascicles are short and attach obliquely to a central tendon, resembling a feather.

  • Unipennate: Fascicles insert into one side of the tendon. Example: Extensor digitorum

  • Bipennate: Fascicles insert into both sides of the tendon. Example: Rectus femoris

  • Multipennate: Multiple fascicles insert into a branching tendon. Example: Deltoid

• Convergent Muscles: Fan-shaped, with fascicles converging toward a single point (raphe). Can pull in different directions. Example: Pectoralis major

• Circular (Sphincter) Muscles: Fascicles arranged in concentric rings; contraction closes an opening. Example: Orbicularis oris, Orbicularis oculi

By Action

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

• Insertion: The movable attachment point.

• Movements can be named for the bone (e.g., flexion of forearm) or the joint (e.g., flexion of elbow).

By Functional Group

• Agonist (Prime Mover): Main muscle responsible for a movement. Example: Biceps brachii for elbow flexion.

• Antagonist: Opposes or reverses a movement; located on the opposite side of the joint from the agonist. Example: Triceps brachii vs. Biceps brachii

• Synergist: Assists the agonist by adding force or reducing unwanted movement.

  • Example (force): External intercostals aid inspiration.

  • Example (stabilization): Brachioradialis in forearm flexion.

  • Fixator: A synergist that immobilizes a bone or muscle origin. Example: Muscles stabilizing the scapula.

Note: A muscle can act as agonist, antagonist, or synergist depending on the movement. Example: Deltoid is a prime mover for arm abduction, antagonist for adduction, and synergist for flexion.

Levers in the Muscular System

Lever Mechanics

Most skeletal muscles use levers to move body parts. A lever is a rigid bar (bone) that moves on a fixed point (fulcrum, or joint) when a force (muscle contraction) is applied to move a resistance (load).

• Lever: Bone

• Fulcrum: Joint

• Applied Force (Effort): Muscle contraction

• Resistance (Load): Bone, tissue, or object being moved

Mechanical Advantage and Disadvantage

• Mechanical Advantage (Power Lever): Effort is farther from the fulcrum than the load; less force needed, but movement is slower and more stable. Example: Jacking a car.

• Mechanical Disadvantage (Speed Lever): Effort is closer to the fulcrum than the load; more force needed, but movement is faster and covers greater distance. Example: Shoveling.

Classes of Levers

Class	Arrangement	Example (Body)	Mechanical Advantage?
First Class	Resistance - Fulcrum - Applied Force	Seesaw, scissors; lifting head off chest (advantage), triceps extending forearm (disadvantage)	Can be either
Second Class	Fulcrum - Resistance - Applied Force	Wheelbarrow; standing on toes	Advantage (rare in body)
Third Class	Resistance - Applied Force - Fulcrum	Tweezers, forceps; biceps flexing forearm	Disadvantage (most common in body)

Naming Skeletal Muscles

Muscle names often reflect their characteristics. The term "muscle" is usually included, except for a few exceptions (e.g., platysma, diaphragm).

• Location: Fibularis, Tibialis, Sternocleidomastoid

• Shape: Deltoid (triangle), Trapezius (trapezoid)

• Position: Externus (superficial), Internus (deep), Extrinsic (outside organ), Intrinsic (within organ)

• Relative Size: Maximus (largest), Minimus (smallest), Longus (long), Brevis (short)

• Number of Origins: Biceps (2), Triceps (3), Quadriceps (4)

• Location of Attachments: Origin (O) listed first, then Insertion (I). Example: Sternocleidomastoid

• Direction of Fibers: Rectus (straight), Oblique (angled), Transverse (horizontal)

• Action: Flexor, Extensor, Adductor, Abductor

Effects of Aging on the Muscular System

Aging leads to a decline in muscle mass, strength, and function. These changes are due to both cellular and structural alterations in muscle tissue.

• Decrease in muscle size and power: Fewer and smaller myofibrils result in reduced strength and increased fatigue.

• Loss of elasticity: Increased fibrous connective tissue (fibrosis) makes muscles less flexible.

• Lower exercise tolerance: Reduced ability to handle fatigue and regulate body temperature.

• Impaired muscle repair: Fewer myoblasts and satellite cells slow recovery from injury.

• Performance decline: 30-50% decrease by age 65.

• Prevention: Maintaining good physical condition early in life helps preserve muscle function later.

Disorders of the Muscular System

Common Muscle Injuries and Conditions

• Charley Horse: Muscle tear with bleeding into tissue, causing severe, prolonged pain.

• Calcaneal (Achilles) Tendon Rupture: Loss of plantar flexion; exaggerated dorsiflexion.

• Shin Splints: Irritation of the anterior tibialis muscle or surrounding tissues; may also refer to stress fractures or inflammation.

Hernias

A hernia is the protrusion of an organ through a muscle wall. Risk factors include obesity, age, lack of abdominal muscle use, and heavy lifting.

Type	Description	Common Population
Inguinal Hernia	Defect in the inguinal canal; most common (80%)	Adults, especially males
Femoral Hernia	Enlargement of femoral ring; affects passage of blood vessels	More common in females
Umbilical Hernia	Occurs around the umbilicus	Infants
Hiatus Hernia	Protrusion of stomach into thoracic cavity via diaphragm	Adults, especially older individuals
Incisional Hernia	Protrusion at surgical incision site	Post-surgical patients

Treatment: Surgical repair is often required, especially for incisional hernias.

Additional info: Understanding muscle structure and function is essential for diagnosing and treating musculoskeletal disorders, as well as for optimizing physical performance and rehabilitation.