# Muscle Fatigue > Muscle fatigue in MBBS Physiology: peripheral metabolites, central fatigue, glycogen depletion and how myasthenic fatigue differs from normal fatigue. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/fatigue-muscle-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Muscle Fatigue", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/fatigue-muscle-physiology ## Direct answer Whether fatigue begins in the motor cortex, the neuromuscular junction or the contractile machinery itself, its definition is the same: force falls despite unchanged drive. Peripheral, intramuscular fatigue is driven chiefly by accumulation of inorganic phosphate and hydrogen ions (pH may fall to 6.2-6.5), which impair calcium release from the sarcoplasmic reticulum and reduce myofibrillar calcium sensitivity; lactate, blamed for a century, is better seen as fuel and buffer than culprit. Central fatigue shows as declining voluntary activation — group III and IV muscle afferents feed back to inhibit the motor cortex, a protective governor on effort. Glycogen depletion sets the marathon runner's wall at around 30-32 kilometres. Pathological fatigue is different in kind: weakness worsening through the day and with repeated use, with a demonstrable decrement on repetitive nerve stimulation, points to myasthenia gravis. ## What you must remember - **Three sites:** central (cortex and spinal drive), junctional (neuromuscular transmission), peripheral (muscle fibre); twitch interpolation quantifies the central component by superimposing electrical twitches on maximal voluntary contraction. - **Peripheral culprits:** inorganic phosphate first among equals, then hydrogen ion and ADP accumulation; all reduce sarcoplasmic reticulum calcium release and crossbridge force. - **Lactate rehabilitated:** lactate is a shuttle substrate for heart, slow fibres and brain, and buffers protons; it correlates with fatigue but does not cause it — a modern viva answer. - **Threshold numbers:** lactate appears in blood above about 55-65% of maximal oxygen uptake (lactate threshold), 4 mmol/L defining onset of blood lactate accumulation (OBLA); training shifts both rightward. - **Central mechanisms:** group III and IV afferents signal metabolic disturbance and reflexly inhibit motor output; serotonin and rating of perceived exertion rise — the sense of effort is itself a measured variable. - **Glycogen economy:** muscle glycogen roughly 300-400 g in a 70 kg man; marathon hitting-the-wall coincides with depletion in recruited fibres despite abundant fat, because fat oxidation cannot match the required ATP flux. - **Myasthenic decrement:** repetitive stimulation at 3 Hz shows more than 10% amplitude decrement from acetylcholine receptor antibody disease — fatigue made electrical and quantifiable. ## Two patients who tire, read physiologically A healthy runner slows at 32 kilometres: force per activation has fallen because pH and phosphate have risen in type II fibres, central drive is throttled by afferent feedback, and glycogen in the recruited pool is spent — carbohydrate feeding and pacing, not willpower, are the levers. A myasthenic patient develops drooping eyelids by evening and slurred speech after a long conversation: each activation releases quanta that must find fewer functioning acetylcholine receptors, so successive volleys yield smaller end-plate potentials until safety factor is lost — the decrement. The bedside analogues separate them: the myasthenic improves within a minute of rest (ice-pack test on ptotic lid is famous) and worsens with sustained upgaze, while the healthy fatigued runner recovers over hours with glycogen resynthesis taking 24-48 hours. Same word, two physiologies — which is precisely the examiner's point. ## Where students slip Blaming lactate remains the classic error; the candidate who says "inorganic phosphate and acidosis reduce calcium release and calcium sensitivity, while lactate is a buffer and fuel" has answered the question the modern way. The second slip is treating central fatigue as psychological: it is a measurable reflex inhibition with an afferent limb, demonstrated when twitch-interpolation shows force reserve the subject cannot access voluntarily. Third, the junctional tier is forgotten — physiological neuromuscular fatigue exists at extreme stimulation rates, and its pathological exaggeration is myasthenia, so the three-site answer (central, junctional, peripheral) should be volunteered as a framework before details are sought. A final number worth having: post-exercise phosphagen recovery takes seconds to minutes; full glycogen restoration needs 24-48 hours. ## Frequently asked questions ### What is the definition of muscle fatigue? A reversible reduction in force-generating capacity despite continued neural drive, classified by site into central, neuromuscular junctional and peripheral (intramuscular) fatigue. ### Which metabolites mainly cause peripheral fatigue? Accumulated inorganic phosphate and hydrogen ions (with ADP), which impair sarcoplasmic reticulum calcium release and myofibrillar calcium sensitivity. ### Why is lactate no longer considered the cause of fatigue? Lactate serves as an oxidative fuel shuttled to heart, brain and slow fibres and buffers protons; it tracks fatigue intensity but does not cause the force loss. ### How is central fatigue demonstrated objectively? Twitch interpolation superimposes an electrical stimulus on maximal voluntary contraction; extra force reveals incomplete voluntary activation, quantifying central fatigue. ### What distinguishes myasthenic fatigue from physiological fatigue? Myasthenic fatigue is junctional — antibody-mediated acetylcholine receptor loss lowers safety factor, giving a greater-than-10% decrement on 3 Hz repetitive stimulation and rapid brief improvement with rest.