Muscle Fatigue
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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.