Muscle Physiology

Hierarchical Organisation

Skeletal muscle is organised as a hierarchy, and that organisation is essential for coordinated force generation. From the outside in:

• Whole muscle, ensheathed by epimysium (dense irregular connective tissue)
• Fascicles, bundles of 10-100 fibres wrapped in perimysium, which carries the neurovascular structures
• Muscle fibres, the cells themselves: multinucleated, 10-100 micrometres in diameter and up to 30 cm long, each surrounded by endomysium
• Myofibrils, cylindrical organelles 1-2 micrometres in diameter that make up 80% of fibre volume
• Sarcomeres, the fundamental contractile unit, 2.5 micrometres long at rest and arranged in series within each myofibril

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Sarcomere Architecture

The sarcomere runs from one Z-line to the next.

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Thick filaments. Thick filaments are 15 nm in diameter and made mainly of myosin II (molecular weight 500 kDa). Each myosin molecule has two heavy chains forming a tail and two heads, and each head carries an actin-binding site and an ATP-binding site. The molecules are arranged in bipolar fashion with the heads projecting outward, and they occupy the A-band, whose length of 1.6 micrometres remains constant during contraction.

Thin filaments. Thin filaments are 7 nm in diameter. Their backbone is F-actin, a double-stranded helix of G-actin monomers (375 per filament), and tropomyosin, a coiled-coil protein, lies in the actin grooves and blocks the myosin-binding sites at rest. A troponin complex, a heterotrimeric regulatory protein, sits every seven actin monomers:

• TnT binds tropomyosin
• TnI is the inhibitory subunit, blocking actin-myosin interaction
• TnC binds calcium, with four binding sites: two high-affinity structural and two low-affinity regulatory

Bands and zones. Each band is defined by the filaments it contains, and the lines by the proteins that anchor them:

• A-band: the entire length of the thick filaments; dark on electron microscopy and anisotropic to polarised light
• I-band: thin filaments only, with no thick-filament overlap; light and isotropic
• H-zone: the central region of the A-band, containing only thick filament tails with no actin overlap
• M-line: the central anchoring proteins (myomesin, M-protein) that connect the thick filaments
• Z-line (Z-disc): the anchoring point for the thin filaments, via alpha-actinin

Membrane Systems and Calcium Handling

T-tubules. Transverse tubules are invaginations of the sarcolemma that penetrate deep into the fibre; in mammalian muscle they lie at the A-I junction, two per sarcomere. They conduct action potentials rapidly to the fibre interior, which is critical for synchronous activation of all the myofibrils. Their membrane carries voltage-gated L-type calcium channels, the dihydropyridine receptors (DHPRs).

Sarcoplasmic reticulum. The SR is specialised smooth endoplasmic reticulum surrounding each myofibril. It contains the calcium-release channels, the ryanodine receptors (RyR1), and its terminal cisternae are expanded regions that flank the T-tubules to form triads. The SR lumen holds calcium at 1-2 mM, 10,000 times the cytoplasmic concentration, and calsequestrin, a calcium-binding protein in the lumen, stores 80% of it. SERCA pumps (SR calcium-ATPase) actively sequester calcium back into the SR during relaxation.

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Neuromuscular junction. This is the specialised synapse between the motor neuron terminal and the muscle fibre. The motor end plate is highly folded sarcolemma, which increases its surface area, and acetylcholine receptors are concentrated at the junctional folds at 10,000 per square micrometre. Acetylcholinesterase in the synaptic cleft hydrolyses ACh rapidly, which keeps the signal brief.

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Cross-Bridge Cycle

Contraction is a cyclical interaction 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, in four steps:

1. ATP binding and detachment. ATP binds the myosin head and causes a conformational change that lowers myosin-actin affinity dramatically (1000-fold), so the cross-bridge detaches from actin.
2. Hydrolysis and cocking. Myosin ATPase hydrolyses ATP to ADP and Pi, both of which remain bound. The energy cocks the head into a high-energy conformation, rotating it 90 degrees to the "ready" position and storing elastic energy in the myosin neck region.
3. Cross-bridge formation and power stroke. When calcium levels are high, tropomyosin moves and exposes the actin binding sites, and the cocked head binds actin strongly. Pi release triggers the power stroke: the head rotates and pulls the actin filament toward the M-line.
4. ADP release. ADP is released, completing the power stroke, and myosin remains tightly bound to actin until the next ATP binds. Multiple asynchronous cross-bridges maintain steady force, and the cycle repeats while calcium and ATP remain available.

ATP is needed to let go, not to attach. ATP powers the stroke and also allows detachment, so it is essential for relaxation as well as contraction. Without ATP, myosin stays bound to actin and the muscle is permanently stiff: that is rigor mortis. The same principle matters for understanding muscle cramps and contractures.

Energetics. One ATP is consumed per cross-bridge cycle, and chemical energy is converted to mechanical work at approximately 50% efficiency. Resting muscle holds ATP at 5 mM, enough for 2-3 seconds of maximal contraction. Phosphocreatine (30 mM) regenerates ATP rapidly, and glycolysis and oxidative phosphorylation sustain prolonged activity.

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Excitation-Contraction Coupling

Excitation-contraction coupling links electrical excitation of the sarcolemma to mechanical contraction of the myofibrils. The sequence, with the time each step takes:

1. Neuromuscular transmission (1 ms). The motor neuron action potential reaches the axon terminal, voltage-gated calcium channels open, and calcium influx makes 200-300 synaptic vesicles fuse and release ACh into the cleft. Each vesicle (a quantum) contains 5,000-10,000 ACh molecules.
2. Muscle fibre depolarisation (2-3 ms). ACh binds nicotinic receptors on the motor end plate, and sodium influx creates an end-plate potential of 50-70 mV. This exceeds threshold and triggers an action potential in the adjacent sarcolemma, which propagates in both directions along the fibre at 3-5 m/s.
3. T-tubule conduction (1-2 ms). The action potential is conducted rapidly into the T-tubules and reaches the A-I junctions throughout the fibre cross-section, where the voltage-gated DHPRs detect the depolarisation.
4. SR calcium release (5-10 ms). The conformational change in the DHPR is mechanically coupled to RyR1, whose calcium-release channels open in the SR membrane. SR calcium floods the cytoplasm, a 100-fold increase to 10 micromolar.
5. Thin filament activation (10-20 ms). Calcium binds TnC (two calcium ions per troponin) and the troponin complex changes conformation. Tropomyosin shifts 25 Angstrom deeper into the actin groove, exposing the myosin-binding sites on actin, and cross-bridge cycling begins.
6. Relaxation (50-100 ms). The sarcolemma repolarises, the DHPR returns to its resting conformation and the RyR1 channels close. SERCA pumps transport calcium back into the SR, using 1 ATP per 2 calcium ions, until cytoplasmic calcium falls below 0.1 micromolar. Calcium dissociates from TnC, tropomyosin returns to its blocking position and myosin-actin interaction ceases.

The calcium source. Skeletal muscle differs from cardiac muscle on both counts: the calcium for contraction comes from the sarcoplasmic reticulum, not the extracellular space, and it is released by mechanical coupling, not calcium-induced calcium release. The DHPR acts as the voltage sensor but does not conduct significant calcium; it couples mechanically to RyR1.

Clinical relevance. Disorders along this chain:

• Malignant hyperthermia: RyR1 mutation causing uncontrolled calcium release with volatile anaesthetics; dantrolene treats it by blocking RyR1
• Central core disease: RyR1 mutations causing structural abnormalities and weakness
• Periodic paralysis: mutations in the DHPR or sodium channels causing episodic weakness
• Myasthenia gravis: antibodies against ACh receptors, reducing end-plate potentials

Neuromuscular blockers also work at the ACh receptor.

Fibre Type Classification

Skeletal muscle fibres are classified by contractile speed and metabolic profile.

Distribution. Most human muscles contain mixed fibre types, typically 40-60% Type I, 30-50% Type IIa and 5-10% Type IIx, and no muscle should be called 100% one type. Postural muscles are predominantly Type I (soleus 80%, paravertebrals 70%). Phasic muscles carry more Type II (gastrocnemius 50/50, vastus lateralis 45/55), and the extraocular muscles are mostly Type II, for rapid movement.

Variation. Within a muscle, superficial regions often have more Type II fibres than deep regions, and genetic factors determine each individual's baseline distribution. Elite marathon runners have 80% Type I fibres in their leg muscles; sprinters have 60% Type IIx.

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Plasticity. Cross-innervation experiments show that the motor neuron determines fibre type: the control is neural. The changes seen in adult fibres:

• IIx-IIa transitions occur in both directions: training shifts IIx to IIa, and detraining shifts back, taking IIx above its baseline (the IIX overshoot)
• Endurance training shifts IIx to IIa and increases the oxidative capacity of Type II fibres
• Strength training hypertrophies Type II fibres and also shifts IIx to IIa
• Training does not convert Type II to Type I
• Denervation shifts fibres toward a faster phenotype, Type I towards Type II characteristics
• Type I to Type II transformation is extremely rare in adults, though thyroid hormone can induce it

Motor Unit Organisation

A motor unit is one alpha motor neuron and all the muscle fibres it innervates. All the fibres in a unit are the same type, Type I or Type II, and they are distributed over a territory 5-10 mm in diameter, interspersed with the fibres of other units.

Innervation ratio. Small motor units serve fine motor control and large ones gross movement. The ratio ranges from 10:1 in the extraocular muscles and 100:1 in the finger muscles to 1000-2000:1 in the back muscles and 2000:1 in gastrocnemius.

Henneman's size principle. Motor units are recruited in order of motor neuron size. Small motor neurons have a lower activation threshold, and their higher input resistance amplifies synaptic input, so recruitment runs from small to large:

1. Low-force tasks recruit small Type I units first, giving fine gradation of force at low levels.
2. Moderate-force tasks add Type IIa units. Their larger motor neurons need greater synaptic drive, and they increase force while maintaining some fatigue resistance.
3. High-force tasks recruit Type IIx units last. These have the largest, highest-threshold motor neurons and give maximal force with rapid fatigue, so they are recruited only for ballistic movements or maximal efforts.

Rate coding. Once recruited, a motor unit increases its firing frequency to generate more force. It starts at 8-12 Hz, an unfused tetanus with visible twitches, and reaches a maximum of 50-60 Hz, a fused tetanus and smooth contraction. Rate coding contributes 50% of force modulation in the intermediate force range.

When the unit fails. In primary muscle disease the motor units are normal but force per fibre is reduced. Upper motor neuron lesions impair recruitment and disturb firing rates, producing spasticity. Denervation changes the units themselves, as described under denervation below.

Length-Tension Relationship

Force generation depends critically on sarcomere length, because length sets the overlap of actin and myosin. Force is optimal at resting muscle length, where overlap is maximal; at short lengths the actin filaments overlap and interfere, and at long lengths fewer cross-bridges can form. The curve has five zones:

• Over-stretched, above 3.6 micrometres: minimal overlap, few cross-bridges, and force reduced to 0-20% of maximum. The clinical example is over-lengthened muscle after nerve palsy.
• Descending limb, 3.6-2.5 micrometres: overlap increases progressively, with a linear relationship between length and force. Each 0.1 micrometre decrease adds approximately 10% more cross-bridges, and passive elastic elements contribute minimal force.
• Plateau, 2.5-2.0 micrometres: optimal overlap throughout the entire thick filament and maximum active force (100%). This corresponds to resting muscle length in situ and is the most stable operating range for physiological function.
• Ascending limb, 2.0-1.5 micrometres: actin filaments begin overlapping from opposite ends, reducing the available binding sites, and the thick filaments compress against the Z-lines. Force falls to 60-70% at 1.5 micrometres.
• Extreme shortening, below 1.5 micrometres: severe actin overlap and structural distortion drop force below 50%. This is rarely achieved in vivo except in specialised muscles.

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Passive tension. Beyond resting length, elastic elements (titin and connective tissue) generate passive force. Titin is a giant protein (3-4 MDa) spanning from Z-line to M-line that acts as a molecular spring resisting over-stretch. Total force is the active (cross-bridge) plus the passive (elastic) component.

Clinical applications. A joint immobilised in the shortened position loses sarcomeres in series, which shortens the optimal length and produces a contracture with reduced functional range; immobilisation in the lengthened position adds sarcomeres in series. In a tendon transfer the muscle is set to the appropriate tension intraoperatively for optimal length-tension, and an Achilles tendon lengthening must preserve sufficient overlap for push-off strength.

Force-Velocity Relationship

Concentric contraction. Shortening velocity is inversely related to force. Maximum velocity (Vmax) occurs at zero load: 4-5 fibre lengths per second in Type I fibres and 15-20 in Type IIx. As load increases, velocity decreases hyperbolically, reaching zero at maximum isometric force, and power (force times velocity) peaks at approximately 30% of Vmax and 30% of maximum force.

Isometric contraction. Velocity is zero, and force is determined by activation level and length. This is the reference point for the force-velocity curve: maximum tetanic force is defined as F0 (100%).

Eccentric contraction. A lengthening muscle produces force above its isometric maximum, up to 150% of F0, at a lower metabolic cost per unit force, because ATP is consumed only during attachment. That makes eccentric work efficient for deceleration and shock absorption. The mechanisms:

• Cross-bridges are forcibly detached while bound, absorbing energy
• Titin and the elastic elements are stretched and contribute passive force
• Some cross-bridges remain attached longer during forced lengthening

Muscle Architecture and Force Production

Muscle architecture determines functional capacity, through three parameters.

Physiological cross-sectional area (PCSA). PCSA is the total cross-sectional area of all fibres, taken perpendicular to the fibre direction: muscle mass times the cosine of the pennation angle, divided by fibre length times muscle density. It is directly proportional to maximum force, through a specific tension of 20-30 N per square centimetre; gastrocnemius, with a PCSA of 50 square centimetres, produces a maximum force of 1000-1500 N.

Fibre length. Fibre length determines maximum shortening distance and velocity. Sarcomeres in series multiply shortening distance, so longer fibres give greater excursion and a higher Vmax, as in sartorius with its long fibres against soleus with its short ones.

Pennation angle. This is the angle between fibre direction and the muscle's line of action, ranging from 0 degrees (fusiform) to 30 degrees (unipennate) and 45 degrees (multipennate). Pennation packs in more fibres, raising PCSA, but reduces effective force transmission, because the force along the tendon is fibre force times the cosine of the pennation angle. In the deltoid, a pennation of 20 degrees reduces force by 6% but allows three times as many fibres.

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Architectural types. Fibre arrangement sorts muscles into four types:

• Fusiform (parallel fibres), such as sartorius and biceps brachii: long excursion, high velocity, lower force
• Unipennate, such as extensor digitorum longus and vastus lateralis: fibres on one side of a central tendon, with intermediate force and excursion
• Bipennate, such as rectus femoris and flexor hallucis longus: fibres on both sides of a central tendon, with higher force and moderate excursion
• Multipennate, such as deltoid and subscapularis: multiple pennation planes, with the highest force production and the shortest excursion

Force transmission. Force travels longitudinally through the tendons to the skeleton, and laterally through the endomysium and perimysium to adjacent structures, which carries 30% of it. Costameres link the sarcolemma to the extracellular matrix, and the dystrophin-glycoprotein complex is critical for membrane stability. In the muscular dystrophies, defects in these force-transmission proteins cause progressive weakness.

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Training Adaptations

Endurance training. Endurance work builds oxidative capacity, with minimal hypertrophy (fibre size increases less than 10%). PGC-1alpha upregulation drives mitochondrial biogenesis, raising mitochondrial density 50-100%, and angiogenesis raises the capillary-to-fibre ratio 20-30%. Oxidative enzymes (citrate synthase, succinate dehydrogenase) are upregulated, myoglobin content increases 80%, glycogen storage capacity increases and lactate clearance improves.

Resistance training. Fibre cross-sectional area increases 20-50% in 12 weeks, Type II fibres more than Type I, and IIx especially. Satellite cells are activated and myonuclei added, and protein synthesis exceeds breakdown through mTOR pathway activation. Tendon stiffness increases, improving force transmission, while metabolic adaptations are minimal. Neural adaptations come in the first 4-6 weeks:

• Increased motor unit recruitment
• Improved firing rate synchronisation
• Reduced antagonist co-contraction

Eccentric training. Strength gains are rapid, 20% in 4 weeks. Sarcomeres are added in series, which shifts optimal length; the Z-lines are remodelled and the cytoskeleton reinforced, with desmin and dystrophin upregulated. The protective "repeated bout effect" reduces DOMS with subsequent sessions, and eccentric work is useful in tendinopathy rehabilitation (the Alfredson protocol for the Achilles).

Immobilisation and Denervation

Immobilisation. The effects accumulate over time:

• Week 1: protein synthesis decreases 50%, Type I fibres are preferentially affected initially, mitochondrial enzyme activity decreases, and strength falls 3-5%
• Week 2: strength loss of 20-30% and atrophy of 10-15% of fibre cross-sectional area; fibres shift toward Type II characteristics and collagen deposition begins in the endomysium
• Weeks 4-6: strength loss of 30-40% and Type I fibre atrophy of up to 30%; sarcomere number adapts to the immobilised length, as described under the length-tension relationship, and the joint stiffens from capsular contracture and muscle shortening
• Month 3 onwards: fatty infiltration begins (adipogenic differentiation of muscle progenitors), fibrosis replaces functional tissue and the contracture resists stretching; strength loss plateaus at 50-60% if immobilisation continues

Recovery. Strength returns faster than muscle mass, through neural adaptations, and complete recovery takes 2-3 times the immobilisation duration. Contractures may be permanent if immobilisation exceeds 12 weeks, so early mobilisation is critical for preventing irreversible changes.

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Denervation. Denervation means a loss of motor units, and the muscle passes through three phases:

• Acute (0-4 weeks): the fibre membrane becomes unstable (fibrillation potentials on EMG), acetylcholine receptors spread beyond the motor end plate, and fibre atrophy begins (10% loss in the first month); fibre type characteristics are preserved initially
• Subacute (1-6 months): atrophy progresses to a 50% reduction in fibre size by 6 months. Reinnervation is possible if the nerve regenerates (1 mm/day): surviving motor neurons sprout collaterals to reinnervate orphaned fibres, motor unit territory expands (giant motor units on EMG, with increased amplitude and duration), and fibre type grouping appears as all fibres in a region become the same type
• Chronic (beyond 6 months): fatty infiltration and fibrosis are irreversible, motor end plates degenerate and the muscle loses its capacity for reinnervation

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The chronic rotator cuff tear. Chronic cuff tears show fibre type change, increased Type II fibres, along with fatty infiltration. Goutallier grading on MRI correlates with irreversible muscle changes:

• 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 a poor surgical prognosis for rotator cuff repair, which is why massive chronic tears have poor surgical outcomes: the muscle has undergone irreversible transformation. After repair, graduated loading is required to restore Type I fibre oxidative capacity.

Muscle Injury and Regeneration

Strain. Strains occur most commonly during eccentric contraction at long muscle length, and the myotendinous junction, the weakest structural point, accounts for 75% of them. They are graded by the proportion of fibres torn:

• Grade I: micro-tears of less than 5% of fibres; recovery 1-2 weeks
• Grade II: partial tear of 5-50% of fibres; recovery 3-6 weeks
• Grade III: complete tear; surgical consideration; recovery 3 months or more

Regeneration. Repair runs in three phases:

1. Destruction (1-3 days): membrane disruption, calcium influx and protein degradation; neutrophil infiltration and inflammatory cytokines; and activation of the satellite cells, normally quiescent between the basal lamina and the sarcolemma.
2. Repair (3-14 days): satellite cells proliferate and differentiate into myoblasts, which fuse to form myotubes. Macrophages clear debris and release growth factors (IGF-1), and angiogenesis and neural sprouting follow.
3. Remodelling (2-6 weeks): myotubes mature and sarcomeres organise, and connective tissue scar forms. Functional strength returns to 80-90%, and the residual scar remains a potential weak point.

What impairs regeneration. Regeneration is slowed or disorganised by:

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

Muscle weakness/wasting is a common viva stem. The key is to localise the lesion (upper motor neuron, lower motor neuron, neuromuscular junction, or muscle) using the pattern of weakness, reflexes, EMG, and CK.

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• Peripheral fatigue (within the muscle) - dominant in high-intensity activity.
  • Inorganic phosphate (Pi) accumulation (from phosphocreatine and ATP breakdown) is the major cause: Pi reduces the force per cross-bridge, lowers myofilament calcium sensitivity, and precipitates with calcium inside the SR to impair calcium release.
  • Impaired SR calcium handling - reduced RyR1 release and slowed SERCA reuptake shrink the calcium transient, so fewer cross-bridges cycle.
  • Metabolic acidosis (H+ from ATP hydrolysis / lactate) - historically blamed, but its direct depressant effect on force is now considered modest at physiological temperature.
  • Glycogen depletion limits prolonged endurance activity and independently impairs SR calcium release.
  • Extracellular/T-tubular potassium accumulation can impair action-potential propagation.
• Central fatigue (within the nervous system). A progressive decline in voluntary neural drive - reduced motor-cortical output and altered afferent feedback (including group III/IV muscle afferents signalling the metabolic milieu) - lowers motor-unit recruitment and firing rate. It is demonstrated by the twitch-interpolation (superimposed-twitch) technique: an extra force increment when a maximal stimulus is superimposed on a "maximal" voluntary contraction proves the drive was sub-maximal.
• Why fibre type maps onto it. Type I fibres resist peripheral fatigue because their high mitochondrial/oxidative capacity minimises Pi and H+ accumulation; Type IIx fibres rely on glycolysis, rapidly accumulate Pi/H+ and deplete glycogen - the mechanism behind the fatigue-resistance column graded throughout.

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• The muscle spindle (length / stretch sensor). Encapsulated intrafusal fibres (nuclear-bag and nuclear-chain fibres) lie in parallel with the force-generating extrafusal fibres. They are wrapped by Ia (primary) afferents (sensing the rate and magnitude of stretch) and group II (secondary) afferents (sensing static length).
• The gamma (fusimotor) system and alpha-gamma co-activation. Gamma motor neurons innervate the contractile poles of the intrafusal fibres, keeping the spindle taut and sensitive as the whole muscle shortens. During voluntary movement the CNS co-activates alpha and gamma neurons so the spindle continues to report length throughout the contraction.
• The stretch (myotatic) reflex. Stretch increases Ia firing, which monosynaptically excites the homonymous alpha motor neuron, producing a contraction that opposes the stretch (the basis of the tendon jerk), with reciprocal inhibition of the antagonist. Loss of descending inhibition of this arc produces the hyper-reflexia, increased tone and clonus listed in the upper-motor-neuron column of the weakness table, and its exaggeration underlies spasticity.
• The Golgi tendon organ (force sensor). Sited at the myotendinous junction, in series with the fibres, GTOs are innervated by Ib afferents that sense muscle tension/force. Through an inhibitory interneuron they produce autogenic inhibition of the same muscle - a force-feedback and protective mechanism (the basis of the "clasp-knife" response and of proprioceptive-neuromuscular-facilitation stretching).

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Muscle physiology is a basic-science topic, so formal disease-specific society guidelines are limited; the most relevant guidance concerns conditions where muscle physiology drives clinical decisions (rotator cuff disease, malignant hyperthermia, denervation/nerve repair).

Global epidemiology and relevance

• Skeletal muscle is approximately 40% of body mass and is the largest protein reservoir; sarcopenia (age-related muscle loss) affects an estimated 10-16% of older adults worldwide and is a growing global burden.
• Malignant hyperthermia susceptibility may be present in up to roughly 1 in 400 individuals genetically, though clinical reactions are rare (about 1:10,000 to 1:250,000 anaesthetics).

Side-by-side guidance

Registry and high- vs limited-resource notes

• No registry tracks muscle physiology per se, but arthroplasty/shoulder registries (NJR-UK, AOANJRR-Australia, AJRR-US) capture outcomes of procedures (e.g. reverse shoulder arthroplasty) chosen when muscle is irreparably degenerated.
• In high-resource settings, MRI Goutallier grading, EMG/nerve conduction studies, and IVCT/genetic MH testing are routinely available. In limited-resource settings, decisions rely more on clinical pattern, ultrasound, and serum CK; dantrolene stocking and MH testing may be unavailable, shifting practice toward total intravenous anaesthesia to avoid triggers.

• Reversibility of fatty infiltration after cuff repair. Goutallier and most series show fatty degeneration does not regress and often progresses despite anatomic healing, yet some report stabilisation or modest improvement after early, successful repair. There is no consensus on a precise Goutallier threshold above which repair is futile; Grade 3-4 is widely treated as a relative contraindication, but biology, tear size, and patient demand all modify the decision.

• Type I to Type II conversion. Adult human muscle shows robust IIX-IIA interconversion with training and detraining (Andersen and Aagaard), but whether true slow-to-fast (Type I to Type II) conversion ever occurs physiologically remains debated; cross-innervation and chronic stimulation studies suggest the neural firing pattern is the dominant determinant, with hybrid fibers as transitional states.

• The denervation "point of no return". The commonly quoted 12-18 month window for irreversible motor end-plate loss is derived largely from animal and observational human data; outcomes are highly variable, and some nerve transfers succeed beyond this window. Timing remains individualised.

• NSAIDs and muscle healing. Short-term NSAIDs may blunt the early inflammatory/satellite-cell response in animal models, but clinical significance in humans is uncertain and the analgesic and anti-heterotopic-ossification benefits often dominate practice.

• Eccentric loading for tendinopathy. Eccentric protocols (e.g. Alfredson for Achilles) are well established, but head-to-head trials increasingly show heavy slow resistance and combined loading are at least equivalent, so "eccentric-only" dogma is being revised.

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.