Muscle Physiology

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Hierarchical Organization

Skeletal muscle exhibits a highly organized structural hierarchy essential for coordinated force generation:

Macroscopic to Microscopic Levels:

• Whole muscle: Ensheathed by epimysium (dense irregular connective tissue)
• Fascicles: Bundles of 10-100 fibers wrapped by perimysium containing neurovascular structures
• Muscle fibers (cells): Individual multinucleated cells (10-100 micrometers diameter, up to 30 cm length) surrounded by endomysium
• Myofibrils: Cylindrical organelles (1-2 micrometers diameter) comprising 80% of fiber volume
• Sarcomeres: Fundamental contractile unit (2.5 micrometers at rest) arranged in series within myofibrils

Sarcomere Architecture

The sarcomere extends from one Z-line to the next and contains precisely arranged contractile proteins:

Thick Filaments (15 nm diameter):

• Primarily myosin II (molecular weight 500 kDa)
• Each myosin molecule has 2 heavy chains forming tail and 2 head regions
• Myosin heads contain actin-binding site and ATP-binding site
• Arranged in bipolar fashion with heads projecting outward
• Located in A-band (1.6 micrometers length remains constant during contraction)

Thin Filaments (7 nm diameter):

• F-actin: double-stranded helix of G-actin monomers (375 monomers per filament)
• Tropomyosin: coiled-coil protein lying in actin grooves, blocking myosin-binding sites at rest
• Troponin complex: heterotrimeric regulatory protein every 7 actin monomers
  • TnT: binds tropomyosin
  • TnI: inhibitory subunit blocking actin-myosin interaction
  • TnC: calcium-binding subunit (4 binding sites, 2 high-affinity structural, 2 low-affinity regulatory)
• Anchored at Z-line via alpha-actinin

Sarcomere Bands and Zones:

• A-band: Contains entire thick filament length (appears dark on EM, anisotropic to polarized light)
• I-band: Thin filaments only, no overlap with thick filaments (appears light, isotropic)
• H-zone: Central region of A-band containing only thick filament tails (no actin overlap)
• M-line: Central anchoring proteins (myomesin, M-protein) connecting thick filaments
• Z-line (Z-disc): Anchoring point for thin filaments via alpha-actinin

Membrane Systems and Calcium Handling

Transverse Tubules (T-tubules):

• Invaginations of sarcolemma penetrating deep into fiber
• Located at A-I junction in mammalian muscle (2 per sarcomere)
• Contain voltage-gated L-type calcium channels (dihydropyridine receptors, DHPRs)
• Rapidly conduct action potentials to fiber interior
• Critical for synchronous activation of all myofibrils

Sarcoplasmic Reticulum (SR):

• Specialized smooth endoplasmic reticulum surrounding each myofibril
• Terminal cisternae: expanded SR regions flanking T-tubules forming triads
• Contains ryanodine receptors (RyR1) - calcium release channels
• SR lumen calcium concentration: 1-2 mM (10,000x higher than cytoplasm)
• SERCA pumps (SR Ca2+-ATPase) actively sequester calcium during relaxation
• Calsequestrin: calcium-binding protein in SR lumen (stores 80% of SR calcium)

Neuromuscular Junction:

• Specialized synapse between motor neuron terminal and muscle fiber
• Motor end plate: highly folded sarcolemma increasing surface area
• Acetylcholine receptors concentrated at junctional folds (10,000 per micrometer squared)
• Acetylcholinesterase in synaptic cleft rapidly hydrolyzes ACh (ensures brief signal)

Cross-Bridge Cycle

The molecular basis of muscle contraction involves cyclical interactions between myosin heads and actin binding sites, powered by ATP hydrolysis. Each cycle produces approximately 10 nm of filament sliding and generates 3-4 pN of force.

Four-Step Cross-Bridge Cycle:

1. ATP Binding and Myosin Detachment

   • ATP binds to myosin head causing conformational change
   • Myosin-actin affinity decreases dramatically (1000-fold)
   • Cross-bridge detaches from actin
   • In rigor mortis (no ATP), myosin remains bound creating muscle stiffness

2. ATP Hydrolysis and Myosin Cocking

   • Myosin ATPase hydrolyzes ATP to ADP plus Pi (both remain bound)
   • Energy from hydrolysis cocks myosin head into high-energy conformation
   • Myosin head rotates 90 degrees to "ready" position
   • This step stores elastic energy in myosin neck region

3. Cross-Bridge Formation (Power Stroke)

   • When calcium levels are high, tropomyosin moves, exposing actin binding sites
   • Cocked myosin head binds strongly to actin
   • Pi release triggers power stroke
   • Myosin head rotates, pulling actin filament toward M-line
   • Generates approximately 10 nm of sliding motion

4. ADP Release and Force Maintenance

   • ADP released from myosin completing power stroke
   • Myosin remains tightly bound to actin until next ATP binds
   • Multiple asynchronous cross-bridges maintain steady force
   • Cycle repeats if calcium and ATP remain available

Energetics:

• One ATP consumed per cross-bridge cycle
• Approximately 50% efficiency converting chemical to mechanical energy
• Resting muscle ATP concentration: 5 mM (sufficient for 2-3 seconds maximal contraction)
• Phosphocreatine provides rapid ATP regeneration (30 mM concentration)
• Glycolysis and oxidative phosphorylation sustain prolonged activity

Excitation-Contraction Coupling

This process links electrical excitation of the sarcolemma to mechanical contraction of myofibrils:

Step-by-Step Sequence:

1. Neuromuscular Transmission (1 ms)

   • Motor neuron action potential reaches axon terminal
   • Voltage-gated calcium channels open, calcium influx
   • Synaptic vesicles fuse, releasing ACh into synaptic cleft (200-300 vesicles)
   • Each vesicle contains 5,000-10,000 ACh molecules (quantum)

2. Muscle Fiber Depolarization (2-3 ms)

   • ACh binds nicotinic receptors on motor end plate
   • Sodium influx creates end-plate potential (50-70 mV)
   • Exceeds threshold, triggering action potential in adjacent sarcolemma
   • Action potential propagates bidirectionally along fiber at 3-5 m/s

3. T-tubule Conduction (1-2 ms)

   • Action potential rapidly conducted into T-tubules
   • Reaches A-I junctions throughout fiber cross-section
   • Ensures synchronous activation of all myofibrils
   • Voltage-gated DHPR (L-type calcium channels) detect depolarization

4. Calcium-Induced Calcium Release (5-10 ms)

   • DHPR conformational change mechanically coupled to RyR1
   • RyR1 calcium release channels open in SR membrane
   • SR calcium floods into cytoplasm (100-fold increase to 10 micrometers)
   • Unlike cardiac muscle, this is mechanical coupling NOT calcium-induced

5. Thin Filament Activation (10-20 ms)

   • Calcium binds to TnC (2 calcium ions per troponin)
   • Troponin complex undergoes conformational change
   • Tropomyosin shifts 25 Angstrom deeper into actin groove
   • Myosin-binding sites on actin exposed
   • Cross-bridge cycling initiated

6. Relaxation Phase (50-100 ms)

   • Sarcolemma repolarizes, DHPR returns to resting conformation
   • RyR1 channels close
   • SERCA pumps actively transport calcium back into SR
   • Requires ATP (1 ATP per 2 calcium ions)
   • Cytoplasmic calcium decreases below 0.1 micrometers
   • Calcium dissociates from TnC
   • Tropomyosin returns to blocking position
   • Myosin-actin interactions cease

Clinical Relevance:

• Malignant hyperthermia: RyR1 mutation causing uncontrolled calcium release
• Central core disease: RyR1 mutations causing structural abnormalities and weakness
• Periodic paralysis: mutations in DHPR or sodium channels causing episodic weakness
• Myasthenia gravis: antibodies against ACh receptors reducing end-plate potentials

Fiber Type Classification

Skeletal muscle fibers are classified based on contractile speed and metabolic profile:

Fiber Type Distribution Patterns:

• Most human muscles contain mixed fiber types (40-60% Type I, 30-50% Type IIa, 5-10% Type IIx)
• Postural muscles: predominantly Type I (soleus 80%, paravertebrals 70%)
• Phasic muscles: higher Type II (gastrocnemius 50/50, vastus lateralis 45/55)
• Within muscle: superficial regions often have more Type II than deep regions
• Individual variation: genetic factors determine baseline distribution

Fiber Type Plasticity:

• Type IIx to Type IIa transitions occur with training or detraining
• Endurance training: shifts IIx to IIa, increases oxidative capacity of Type II
• Strength training: hypertrophy of Type II fibers, minimal type conversion
• Denervation: shift toward faster phenotype (Type I to Type II characteristics)
• Cross-innervation experiments: motor neuron determines fiber type (neural control)
• Type I to Type II transformation extremely rare in adults (thyroid hormone can induce)

Motor Unit Organization

A motor unit consists of one alpha motor neuron and all muscle fibers it innervates.

Motor Unit Characteristics:

• Innervation ratio: varies from 10:1 (extraocular muscles) to 2000:1 (gastrocnemius)
• Fine motor control: small motor units (finger muscles: 100:1)
• Gross movement: large motor units (back muscles: 1000-2000:1)
• All fibers in a motor unit are the same type (Type I or Type II)
• Fiber territory: distributed over 5-10 mm diameter region (interspersed with other motor units)

Size Principle of Motor Unit Recruitment: Henneman's size principle describes orderly recruitment based on motor neuron size:

1. Low-Force Tasks: Small Type I motor units recruited first

   • Small motor neurons have lower activation threshold
   • Higher input resistance amplifies synaptic input
   • Provides fine gradation of force at low levels

2. Moderate-Force Tasks: Type IIa motor units added

   • Larger motor neurons require greater synaptic drive
   • Increases force production while maintaining some fatigue resistance

3. High-Force Tasks: Type IIx motor units recruited last

   • Largest motor neurons, highest threshold
   • Maximal force production but rapid fatigue
   • Only recruited for ballistic movements or maximal efforts

Rate Coding:

• Once recruited, motor units increase firing frequency to generate more force
• Initial firing rate: 8-12 Hz (unfused tetanus, visible twitches)
• Maximal firing rate: 50-60 Hz (fused tetanus, smooth contraction)
• Rate coding contributes 50% of force modulation in intermediate force ranges

Clinical Implications:

• Denervation: loss of motor units, remaining units reinnervate orphaned fibers (enlarged motor units)
• EMG shows increased motor unit amplitude and duration with reinnervation
• Primary muscle disease: motor units normal but reduced force per fiber
• Upper motor neuron lesions: impaired recruitment, firing rate abnormalities (spasticity)

Length-Tension Relationship

Force generation depends critically on sarcomere length due to actin-myosin filament overlap:

Zones of the Length-Tension Curve:

1. Over-Stretched Position (sarcomere length greater than 3.6 micrometers)

   • Minimal actin-myosin overlap
   • Few cross-bridges can form
   • Force production reduced to 0-20% maximum
   • Clinical example: over-lengthened muscle after nerve palsy

2. Descending Limb (3.6-2.5 micrometers)

   • Progressive increase in filament overlap
   • Linear relationship between length and force
   • Each 0.1 micrometer decrease adds approximately 10% more cross-bridges
   • Passive elastic elements contribute minimal force

3. Plateau Region (2.5-2.0 micrometers)

   • Optimal actin-myosin overlap throughout entire thick filament
   • Maximum active force generation (100%)
   • Corresponds to resting muscle length in situ
   • Most stable operating range for physiological function

4. Ascending Limb (2.0-1.5 micrometers)

   • Actin filaments begin overlapping from opposite ends
   • Interference reduces available binding sites
   • Thick filament compression against Z-lines
   • Force decreases to 60-70% at 1.5 micrometers

5. Extreme Shortening (less than 1.5 micrometers)

   • Severe actin overlap and structural distortion
   • Force production drops below 50%
   • Rarely achieved in vivo except in specialized muscles

Passive Tension Component:

• At lengths beyond resting, elastic elements (titin, connective tissue) generate passive force
• Titin: giant protein (3-4 MDa) spanning from Z-line to M-line
• Acts as molecular spring resisting over-stretch
• Total force = active (cross-bridges) plus passive (elastic) components

Clinical Applications:

• Joint immobilization in shortened position: sarcomeres adapt by reducing number in series
• Results in shortened optimal length and contracture formation
• Immobilization in lengthened position: sarcomeres added in series
• Tendon transfers: muscle set to appropriate tension intraoperatively for optimal length-tension
• Achilles tendon lengthening: must preserve sufficient overlap for push-off strength

Force-Velocity Relationship

Shortening velocity inversely relates to force production:

Concentric Contractions (Muscle Shortening):

• Maximum velocity (Vmax) occurs at zero load
• Type I fibers: Vmax = 4-5 fiber lengths/second
• Type IIx fibers: Vmax = 15-20 fiber lengths/second
• As load increases, velocity decreases hyperbolically
• At maximum isometric force, velocity = 0
• Power (force times velocity) peaks at approximately 30% Vmax and 30% maximum force

Isometric Contractions:

• Velocity = 0, force determined by activation level and length
• Reference point for force-velocity curve
• Maximum tetanic force defined as F0 (100%)

Eccentric Contractions (Muscle Lengthening):

• Force production exceeds isometric maximum (up to 150% F0)
• Mechanisms:
  • Cross-bridges forcibly detached while bound (energy absorbed)
  • Titin and elastic elements stretched, contributing passive force
  • Some cross-bridges remain attached longer during forced lengthening
• Lower metabolic cost per unit force (ATP consumed only during attachment)
• Muscle damage occurs more readily (delayed-onset muscle soreness)
• Training effect: rapid strength gains, protective adaptation to eccentric stress

Muscle Architecture and Force Production

Muscle architecture determines functional capacity:

Architectural Parameters:

1. Physiological Cross-Sectional Area (PCSA)

   • Total cross-sectional area of all fibers perpendicular to fiber direction
   • PCSA = (muscle mass times cosine pennation angle) divided by (fiber length times muscle density)
   • Directly proportional to maximum force (specific tension: 20-30 N/cm squared)
   • Example: gastrocnemius PCSA 50 cm squared, maximum force 1000-1500 N

2. Fiber Length

   • Determines maximum shortening distance and velocity
   • Longer fibers = greater excursion and higher Vmax
   • Sarcomeres in series multiply shortening distance
   • Example: sartorius (long fibers) vs. soleus (short fibers)

3. Pennation Angle

   • Angle between fiber direction and muscle's line of action
   • Ranges from 0 degrees (fusiform) to 30 degrees (unipennate) to 45 degrees (multipennate)
   • Packing more fibers (higher PCSA) but reducing effective force transmission
   • Force along tendon = fiber force times cosine (pennation angle)
   • Example: deltoid pennation 20 degrees, force reduction 6% but allows 3x more fibers

Muscle Architectural Types:

• Fusiform (parallel fibers): sartorius, biceps brachii

  • Long excursion, high velocity, lower force

• Unipennate: extensor digitorum longus, vastus lateralis

  • Fibers on one side of central tendon
  • Intermediate force and excursion

• Bipennate: rectus femoris, flexor hallucis longus

  • Fibers on both sides of central tendon
  • Higher force, moderate excursion

• Multipennate: deltoid, subscapularis

  • Multiple pennation planes
  • Highest force production, shortest excursion

Force Transmission:

• Longitudinal transmission: via tendons to skeleton
• Lateral transmission: through endomysium/perimysium to adjacent structures (30% of force)
• Costameres: link sarcolemma to extracellular matrix
• Dystrophin-glycoprotein complex: critical for membrane stability
• Muscular dystrophies: defects in force transmission proteins causing progressive weakness

Training Adaptations

Endurance Training:

• Mitochondrial biogenesis (PGC-1alpha upregulation): 50-100% increase in mitochondrial density
• Capillary angiogenesis: 20-30% increase in capillary:fiber ratio
• Oxidative enzyme upregulation (citrate synthase, succinate dehydrogenase)
• Fiber type shift: IIx to IIa conversion (more fatigue-resistant)
• Myoglobin content increases 80%
• Glycogen storage capacity increases
• Improved lactate clearance
• Minimal hypertrophy (fiber size increases less than 10%)

Resistance Training:

• Muscle hypertrophy: 20-50% fiber cross-sectional area increase in 12 weeks
• Type II fibers hypertrophy more than Type I (especially IIx)
• Satellite cell activation and myonuclear addition
• Protein synthesis exceeds breakdown (mTOR pathway activation)
• Neural adaptations (first 4-6 weeks):
  • Increased motor unit recruitment
  • Improved firing rate synchronization
  • Reduced antagonist co-contraction
• Tendon stiffness increases (force transmission efficiency)
• Minimal metabolic adaptations

Eccentric Training:

• Rapid strength gains (20% in 4 weeks)
• Addition of sarcomeres in series (shifts optimal length)
• Protective "repeated bout effect" - reduced DOMS with subsequent sessions
• Z-line remodeling and cytoskeletal reinforcement
• Desmin and dystrophin upregulation
• Useful for tendinopathy rehabilitation (Alfredson protocol for Achilles)

Immobilization and Denervation

Immobilization Effects (Time Course):

Week 1:

• Protein synthesis decreases 50%
• Type I fibers preferentially affected initially
• Mitochondrial enzyme activity decreases
• 3-5% strength loss

Week 2:

• 20-30% strength loss
• Muscle atrophy 10-15% (fiber cross-sectional area)
• Shift toward Type II fiber characteristics
• Collagen deposition begins in endomysium

Week 4-6:

• 30-40% strength loss
• Sarcomere number adaptation:
  • Shortened position: sarcomeres in series decrease (contracture develops)
  • Lengthened position: sarcomeres added in series
• Joint stiffness from capsular contracture and muscle shortening
• Type I fiber atrophy up to 30%

Month 3 plus:

• Fatty infiltration begins (adipogenic differentiation of muscle progenitors)
• Fibrosis replaces functional tissue
• Contracture formation resistant to stretching
• Strength loss plateaus at 50-60% if immobilization continues

Recovery from Immobilization:

• Strength returns faster than muscle mass (neural adaptations)
• Complete recovery takes 2-3x immobilization duration
• Contractures may be permanent if immobilization exceeds 12 weeks
• Early mobilization critical for preventing irreversible changes

Denervation Effects:

Acute Phase (0-4 weeks):

• Muscle fiber membrane instability (fibrillation potentials on EMG)
• Acetylcholine receptors spread beyond motor end plate
• Fiber atrophy begins (10% loss in first month)
• Preserved fiber type characteristics initially

Subacute Phase (1-6 months):

• Progressive atrophy (50% fiber size reduction by 6 months)
• Reinnervation potential if nerve regenerates (1 mm/day)
• Surviving motor neurons sprout collaterals to reinnervate orphaned fibers
• Motor unit territory expansion (giant motor units on EMG)
• Fiber type grouping (all fibers in a region become same type)

Chronic Phase (beyond 6 months):

• Irreversible fatty infiltration and fibrosis
• Motor end plates degenerate
• Muscle loses capacity for reinnervation (point of no return 12-18 months)
• Goutallier grading on MRI:
  • Grade 0: Normal muscle
  • Grade 1: Some fatty streaks
  • Grade 2: Less than 50% fat
  • Grade 3: Equal muscle and fat
  • Grade 4: More fat than muscle
• Grade 3-4 indicates poor surgical prognosis for rotator cuff repair

Muscle Injury and Regeneration

Strain Injury Mechanisms:

• Most common during eccentric contractions at long muscle length
• Myotendinous junction: weakest structural point (75% of strains)
• Grade I: micro-tears, less than 5% fibers, 1-2 weeks recovery
• Grade II: partial tear, 5-50% fibers, 3-6 weeks recovery
• Grade III: complete tear, surgical consideration, 3 plus months recovery

Regeneration Process:

1. Destruction Phase (1-3 days):

   • Membrane disruption, calcium influx, protein degradation
   • Neutrophil infiltration, inflammatory cytokines
   • Satellite cell activation (normally quiescent between basal lamina and sarcolemma)

2. Repair Phase (3-14 days):

   • Satellite cells proliferate and differentiate into myoblasts
   • Myoblasts fuse forming myotubes
   • Macrophages clear debris, release growth factors (IGF-1)
   • Angiogenesis and neural sprouting

3. Remodeling Phase (2-6 weeks):

   • Myotube maturation and sarcomere organization
   • Connective tissue scar formation
   • Functional strength returns to 80-90%
   • Residual scar remains potential weak point

Factors Impairing Regeneration:

• Age: satellite cell number and function decline with age
• Corticosteroids: inhibit satellite cell proliferation
• NSAIDs: controversial, may impair early inflammation but long-term effect minimal
• Excessive fibrosis: TGF-beta pathway overactivation
• Severe injury: complete disruption of basal lamina scaffold prevents organized regeneration

Muscle physiology encompasses the structural, biochemical, and biomechanical principles underlying skeletal muscle function. Understanding sarcomere architecture, excitation-contraction coupling mechanisms, fiber type characteristics, motor unit organization, and force generation relationships is essential for orthopaedic practice.

The sliding filament theory explains contraction at the molecular level through ATP-dependent cross-bridge cycling between actin and myosin filaments, regulated by calcium-mediated troponin-tropomyosin interactions. Fiber type diversity (Type I slow oxidative, Type IIa fast oxidative-glycolytic, Type IIx fast glycolytic) provides functional specialization for different muscular demands, with recruitment following Henneman's size principle.

Force production depends on length-tension relationships (optimal at resting length), force-velocity relationships (inverse during concentric contractions, enhanced during eccentric contractions), and architectural parameters (PCSA, fiber length, pennation angle). Muscle adapts to training stimuli through metabolic, structural, and neural mechanisms, while immobilization and denervation cause progressive atrophy, fatty infiltration, and potentially irreversible functional loss.

Clinical applications include understanding tendon transfer biomechanics, rotator cuff tear prognosis based on fatty infiltration grading, rehabilitation protocol design incorporating fiber type and motor unit recruitment principles, and recognition of time-sensitive intervention windows for denervation injuries. These fundamental concepts underpin evidence-based orthopaedic decision-making across trauma, reconstructive, and sports medicine specialties.