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What are the limiting factors of (human) stamina? Let's define stamina as the capacity to continue doing a physical activity. We clearly have our limits, so what aspects of physiology impose a limit on human physical exertion? How do these aspects work together and interact?

Great detail isn't necessary, but a concise overview is what I am looking for.

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  • $\begingroup$ I don't have a complete answer but in some ways your question is, as yet, unanswered. It is a bit like asking, "why do we need to sleep?" Also, there are things the body needs for energy that can run out, such as carbohydrates. However, to a limited extent we can increase our "stamina" by taking stimulants such as caffeine. $\endgroup$ – takintoolong Nov 26 '18 at 2:26
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    $\begingroup$ You might start be defining stamina, the word covers a wide variety of factors. $\endgroup$ – John Nov 26 '18 at 23:01
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    $\begingroup$ It seems this review has got you covered in pretty good detail: ncbi.nlm.nih.gov/pmc/articles/PMC2375555 $\endgroup$ – Armatus Nov 27 '18 at 0:39
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    $\begingroup$ Since your question is being considered too broad by some members of the community, I suggest you narrow it down to "What aspects of physiology impose a limit on human physical exertion?", and/or specifically ask for physiological quantities that correlate with the onset of perceived exhaustion. $\endgroup$ – Armatus Nov 27 '18 at 0:41
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    $\begingroup$ Stamina, even if you narrow it down to a capacity to continue a physical activity, is still influenced by diverse factors, including genetic, training, diet, hormonal, psychological, religious, social...It seems that one would need to write almost a book to make "a concise overview" about how various aspects work together. If you are looking for more physical factors, do you maybe want to use the term endurance instead of stamina? $\endgroup$ – Jan Nov 27 '18 at 12:56
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A few explanations at the cellular and molecular level:

First, for contraction to happen, muscles need signals from nerves. Replenishing the pool of neurotransmitters (used for each synaptic burst) takes a bit of time, and short-term exhaustion may happen at this level.

In the muscle cells, contraction is caused by a calcium release within the cell, from a cellular compartment called the sarcoplasmic reticulum. This reticulum can only contain so many ions, so if the muscle contracts a lot, it will eventually need to rest for a while to re-accumulate calcium in the sarcoplasmic reticulum to prepare for more contractions.

Muscles need molecular "fuel" (like ATP molecules) to contract/relax. After an intense or sustained effort, these molecules are partially consumed and the muscle simply runs out of energy; it needs a bit of time to resore the pool of metabolites needed to work properly. Conversely, muscle contraction also causes the accumulation of metabolites and ions like potassium, which partially inhibit muscle contraction. Fatigue occurs when these accumulate at high level, and they must be cleared before making new efforts.

For references see:

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