|MadSci Network: General Biology|
There are many factors to explain physical fatigue. Muscle fatigue happens when work is done at a level that uses up substrate (fuel) faster than it can be replenished. Muscle can uptake oxygen from bloodstream only so fast, and when work load demand exceeds that rate, then anaerobic metabolism takes over.
Muscle fuel (glucose) is metabolized incompletely and the build up of byproduct lactic acids inhibit muscle output. Rest is then required to oxidatively metabolize the lactate and allow full strength to return.
On a longer term time scale, and lower level of exertion, supply of muscle glucose precursor (glycogen) can be depleted. At this point, longer rest and food intake is needed to replenish.
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