Skeletal muscles

Major Muscles of the Body

• Serratus Anterior

• Biceps

• Brachii

• Brachioradialis

• Pectoralis Major

• Deltoid

• Trapezius

• Latissimus Dorsi

• Rectus Abdominis

• Triceps Brachii

• Obliques

• Quadriceps Femoris

• Gluteus Maximus

• Hamstrings

• Gastrocnemius

Major Functions of Skeletal Muscle

1. Movement and Locomotion

2. Posture and Stability

3. Heat Production

4. Metabolic Functions

Function 1 - Movement and Locomotion

• Movement Generation

  • Primary function enabling human mobility

  • Creates forces for actions such as walking, running, jumping

  • Allows manipulation of the environment

  • Enables fine motor control

• Types of Movement:

  • Gross Motor: Large muscle groups (e.g., walking, lifting)

  • Fine Motor: Small precise movements (e.g., writing, typing)

  • Ballistic: Fast, powerful movements (e.g., throwing, kicking)

  • Controlled: Slow, precise movements (e.g., threading a needle)

• Bioengineering Applications:

  • Prosthetic limb control

  • Robotic actuator design

  • Human-machine interfaces

  • Rehabilitation devices

Function 2 - Posture and Stability

• Postural Control

  • Maintains body position against gravity

  • Provides dynamic stability during movement

  • Prevents falls and injuries

  • Enables efficient movement patterns

• Key Postural Systems:

  • Axial Muscles: Provide spine and trunk stability

  • Antigravity Muscles: Support body weight

  • Core Muscles: Act as a central stability platform

  • Stabilizer Muscles: Make fine postural adjustments

• Clinical Importance:

  • Poor posture can lead to musculoskeletal disorders

  • Essential for ergonomic design

  • Critical for balance training systems

  • Important for spinal orthotic design

Function 3 - Heat Production

• Thermogenesis

  • Muscles generate approximately 75% of body heat during cold exposure

  • Shivering can increase heat production by 3-5 times

  • Helps maintain core body temperature

  • Supports metabolic processes

• Heat Generation Methods:

  • Shivering: Involuntary muscle contractions

  • Voluntary Activity: Exercise and movement

  • Muscle Tone: Baseline muscle tension

  • Brown Fat Interaction: Supports metabolic heat

• Engineering Considerations:

  • Thermal management in prosthetics

  • Protective clothing design

  • Environmental control systems

  • Heat dissipation in wearable devices

Function 4 - Metabolic and Circulatory Support

• Metabolic Functions

  • Muscles are the largest glucose consumers in the body

  • They serve as the primary site of protein storage

  • Act as an energy substrate reservoir

  • Help regulate blood glucose

• Circulatory Support

  • Muscle pump mechanism aids venous return

  • Compresses blood vessels during contraction to prevent pooling in extremities

  • Supports overall cardiovascular function

• Clinical Applications:

  • Management of diabetes

  • Promotion of cardiovascular health

  • Prevention of deep vein thrombosis

  • Development of exercise prescription guidelines

Gross Anatomy of Skeletal Muscle 1

• A skeletal muscle is classified as an organ composed of multiple types of tissues working together, including skeletal muscle fibers, connective tissue, blood vessels, and nerves.

• Muscle fibers are grouped together within bundles called fascicles.

• A whole muscle contains multiple fascicles, and each fascicle consists of many muscle cells (fibers).

Connective Tissue Components

• Three concentric layers of wrapping:

  • Epimysium:

  • Dense irregular connective tissue encompassing the entire muscle

  • Perimysium:

  • Dense irregular connective tissue surrounding a fascicle

  • Houses many blood vessels and nerves

  • Endomysium:

  • Areolar connective tissue wrapping individual fibers

  • Provides electrical insulation, capillary support, and binds neighboring cells

• These hierarchical structures inspire biomimetic material design for soft robotics and tissue scaffolds.

Attachments of Muscle to Bone

• Tendons:

  • Cordlike structures made of dense regular connective tissue

• Aponeuroses:

  • Thin, flattened sheets of dense irregular connective tissue

• Deep Fascia:

  • Dense irregular connective tissue superficial to epimysium

  • Functions to separate individual muscles and bind those with similar functions

• Superficial Fascia:

  • Areolar and adipose connective tissue superficial to deep fascia

  • Separates muscles from the skin

Blood Vessels and Nerves

• Skeletal muscles are highly vascularized, containing extensive blood vessels that deliver oxygen and nutrients while removing waste products.

• They are innervated by somatic motor neurons; axons of these neurons branch and terminate at neuromuscular junctions, making skeletal muscles voluntary since contraction is consciously controlled.

Regional Muscle Anatomy: Head and Neck Muscles

• Major Functional Groups:

  • Muscles of Facial Expression

  • Muscles of Mastication

  • Neck Muscles

  • Muscles of the Eye (affiliated with special senses)

1. Muscles of Facial Expression

• Key Muscles:

  • Forehead and Eye Region:

  • Frontalis: Raises eyebrows, wrinkles forehead

  • Orbicularis Oculi: Closes eyelids, allows for squinting

  • Mouth and Cheek Region:

  • Orbicularis Oris: Closes lips, facilitates puckering

  • Zygomaticus Major: Elevates corners of the mouth (smiling)

  • Buccinator: Compresses cheeks, aids in blowing

2. Muscles of Mastication

• Key Muscles:

  • Masseter:

  • Location: Cheek region

  • Function: Powerful jaw closing

  • Force: Up to 200 lbs of bite force

  • Temporalis:

  • Location: Temple region

  • Function: Jaw closing and retracting

  • Shape: Fan-shaped muscle

  • Medial Pterygoid:

  • Location: Deep to mandible

  • Function: Jaw closing, lateral movement

  • Lateral Pterygoid:

  • Location: Deep muscle

  • Function: Jaw protrusion, lateral movement

3. Major Muscles of the Neck

• Anterior Neck:

  • Sternocleidomastoid (SCM):

  • Origin: Sternum and clavicle

  • Insertion: Mastoid process

  • Actions: Head rotation, neck flexion

  • Scalene Muscles:

  • Anterior, middle, posterior scalenes

  • Function: Neck flexion and first rib elevation

• Posterior Neck:

  • Trapezius (Upper Portion):

  • Function: Neck extension, scapular elevation

Trunk Muscles Overview

• Major Functional Divisions:

  • Abdominal Wall Muscles

  • Back Muscles

  • Respiratory Muscles

  • Pelvic Floor Muscles

  • Core Concept: The "core" refers to all muscles that stabilize the spine and pelvis, serving as a foundation for limb movement

1. Abdominal Wall Muscles

• Rectus Abdominis:

  • Commonly known as the "six-pack" muscle

  • Function: Trunk flexion, posterior pelvic tilt

• External Oblique:

  • Fiber direction: Down and inward

  • Function: Trunk flexion, rotation, lateral flexion

• Internal Oblique:

  • Fiber direction: Up and inward

  • Function: Trunk rotation, compression

• Transversus Abdominis:

  • Deepest layer with horizontal fibers

  • Function: Abdominal compression, spinal stability

2. Back Muscles

• Key Muscles:

  • Superficial Muscles of the Back:

  • Trapezius:

    • Three functional portions:

    • Upper: Elevates scapula

    • Middle: Retracts scapula

    • Lower: Depresses scapula

  • Latissimus Dorsi: Known as the "swimmer's muscle"

    • Function: Shoulder adduction, extension, internal rotation

  • Rhomboids (Major and Minor):

    • Function: Scapular retraction

  • Levator Scapulae:

    • Function: Elevates scapula

  • Deep Muscles of the Back:

  • Erector Spinae:

3. Respiratory Muscles

• Key Muscles:

  • Diaphragm:

  • Most important respiratory muscle

  • Dome-shaped and separates thorax from abdomen

  • Function: Responsible for 75% of quiet inspiration

  • Innervation: Phrenic nerve (C3-C5)

  • External Intercostals:

  • Located between ribs

  • Function: Elevate ribs during inspiration

  • Comprises 11 pairs of muscles

Upper Extremity Muscles

• Major Shoulder Muscle Groups:

  • Rotator Cuff Muscles

  • Prime Movers

  • Scapular Stabilizers

  • Muscles of the Arm:

  • Flexors

  • Extensors

  • Muscles of the Forearm:

  • Flexors

  • Extensors

  • Muscles of the Hand

Rotator Cuff Muscles (SITS)

• 4 Muscles to Know:

  • Supraspinatus:

  • Function: Initiates shoulder abduction (first 15°)

  • Clinical: Most commonly torn rotator cuff muscle

  • Infraspinatus:

  • Function: External rotation, posterior stability

  • Teres Minor:

  • Function: External rotation, assists infraspinatus

  • Subscapularis:

  • Function: Internal rotation, anterior stability

• Major Shoulder Movers:

  • Deltoid:

  • Three Portions:

    • Anterior Deltoid: Shoulder flexion, internal rotation

    • Middle Deltoid: Primary abductor (15-90°)

    • Posterior Deltoid: Extension, external rotation

  • Pectoralis Major:

  • Function: Adduction, internal rotation, horizontal flexion

  • Latissimus Dorsi (covered previously):

  • Function: Adduction, extension, internal rotation

Scapular Stabilizers

• Trapezius (covered previously):

  • Contains three portions with distinct functions

• Serratus Anterior:

  • Function: Scapular protraction, upward rotation

• Rhomboids (covered previously):

  • Function: Scapular retraction, downward rotation

• Levator Scapulae (covered previously):

  • Function: Elevates the scapula

Arm Muscles

• Anterior Compartment (Flexors):

  • Biceps Brachii:

  • Two heads: Long head, Short head

  • Functions: Elbow flexion, forearm supination

  • Brachialis:

  • Function: Primary elbow flexor

  • Coracobrachialis:

  • Function: Shoulder flexion, adduction

• Posterior Compartment (Extensors):

  • Triceps Brachii:

  • Three heads: Long, lateral, medial

  • Function: Primary elbow extensor

Forearm Muscles and Intrinsic Muscles of the Hand

• Forearm Muscles:

  • Anterior Compartment: Primarily flexors

  • Posterior Compartment: Primarily extensors

• Intrinsic Muscles of the Hand:

  • Thenar Muscles (Thumb):

  • Functions: Thumb abduction, adduction, flexion, and opposition

  • Hypothenar Muscles (Little Finger):

  • Functions: Movements of the 5th finger

  • Central Muscles:

  • Functions: Finger abduction and adduction

Lower Extremity Muscles

• Muscles of the Thigh:

  • Hip Flexors

  • Hip Extensors

  • Hip Abductors

  • Hip Adductors

• Muscles of the Leg:

  • Flexors

  • Extensors

• Muscles of the Foot

Hip Flexor Muscles

• Iliopsoas:

  • Function: Primary hip flexor

• Quadriceps Muscles:

  • Rectus Femoris + 3 vastus (medialis, lateralis, intermedius)

  • Function: Hip flexion, knee extension

  • Essential for walking, standing, and stair climbing

  • Capable of generating tremendous force (>3000N)

Hip Extensor Muscles

• Gluteus Maximus:

  • Function: Primary hip extensor, external rotator

  • Important for rising from a seated position

• Hamstring Group (Three Muscles):

  • Biceps Femoris:

  • Function: Hip extension, knee flexion

  • Semitendinosus:

  • Function: Hip extension, knee flexion

  • Semimembranosus:

  • Function: Hip extension, knee flexion

Hip Abductor and Adductor Muscles

• Hip Abductors:

  • Gluteus Medius:

  • Function: Primary hip abductor

• Hip Adductors:

  • Adductor Magnus: Largest adductor

  • Adductor Longus: Superficial adductor

  • Adductor Brevis: Deep adductor

  • Gracilis: Crosses knee joint

Leg Muscles

• Anterior Compartment:

  • Tibialis Anterior:

  • Function: Ankle dorsiflexion, inversion

  • Extensor Hallucis Longus:

  • Function: Great toe extension, ankle dorsiflexion

  • Extensor Digitorum Longus:

  • Function: Toe extension, ankle dorsiflexion

• Posterior Compartment (Calf):

  • Gastrocnemius:

  • Two heads: Medial and lateral

  • Function: Plantar flexion, knee flexion

Muscle Groups by Function - Breathing

• Primary Respiratory Muscles:

  • Diaphragm: Main inspiratory muscle, innervated by the phrenic nerve

  • External Intercostals: Elevate ribs

  • Internal Intercostals: Depress ribs

  • Abdominal Muscles: Act during forced expiration

Muscle Groups by Function - Lifting

• Extensors of the Head:

  • Trapezius

• Spinal Extensors:

  • Spinal Erectors

• Hip Extensors:

  • Gluteus Maximus: Primary hip power

  • Hamstrings: Assist with hip extensions

• Knee Extensors:

  • Quadriceps: Support body weight

• Knee Stabilizers:

  • Adductor Magnus

Muscle Groups by Function - Balance

• Postural Control System:

  • Ankle Strategy:

  • Gastrocnemius: Controls backward sway

  • Tibialis Anterior: Controls forward sway

  • Hip Strategy:

  • Gluteus Maximus: Responsible for hip extension

  • Hip Flexors: Responsible for hip flexion

  • Abdominals: Control trunk stability

  • Core Stability:

  • Deep Abdominals: Provide central stability

  • Deep Back Muscles: Support spinal control

  • Diaphragm: Regulates pressure

Muscle Groups by Function - Balance Control Exercises

• Deltoids

  • Shoulder exercises (e.g., military presses, lateral raises)

• Latissimus Dorsi

  • Exercises like lat pulldowns, seated rows

• Pectorals

  • Chest press, chest flys

• Trapezius

  • Shoulder shrugs, upright rows

• Abdominals

  • Ab crunches, planks

• Erector Spinae

  • Back extensions

• Biceps

  • Barbell curls, preacher curls

• Triceps

  • Cable pressdowns, barbell tricep extensions

• Hip Abductors

  • Lying leg lifts, clams

• Hip Adductors

  • Band hip adduction, adductor machine

• Iliopsoas and Gluteals:

  • Exercises include leg lifts and bridges

• Gastrocnemius:

  • Standing calf raises, donkey calf raises

• Tibialis Anterior:

  • Toe raises

• Quadriceps and Hamstrings:

  • Leg extensions, lunges, leg curls, straight leg deadlifts

Practice Quiz

• Review all key muscle functions, anatomical landmarks, and their roles in rehabilitation and human movement.