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  • You're lifting weights.

  • The first time feels easy,

  • but each lift takes more and more effort until you can't continue.

  • Inside your arms, the muscles responsible for the lifting have become unable to contract.

  • Why do our muscles get fatigued?

  • We often blame lactic acid or running out of energy,

  • but these factors alone don't account for muscle fatigue.

  • There's another major contributor:

  • the muscle's ability to respond to signals from the brain.

  • To understand the roots of muscle fatigue,

  • it helps to know how a muscle contracts in response to a signal from a nerve.

  • These signals travel from the brain to the muscles in a fraction of a second via long, thin cells called motor neurons.

  • The motor neuron and the muscle cell are separated by a tiny gap,

  • and the exchange of particles across this gap enables the contraction.

  • On one side of the gap, the motor neuron contains a neurotransmitter called acetylcholine.

  • On the other side, charged particles, or ions, line the muscle cell's membrane:

  • potassium on the inside, and sodium on the outside.

  • In response to a signal from the brain,

  • the motor neuron releases acetylcholine,

  • which triggers pores on the muscle cell membrane to open.

  • Sodium flows in, and potassium flows out.

  • The flux of these charged particles is a crucial step for muscle contraction:

  • the change in charge creates an electrical signal called an action potential

  • that spreads through the muscle cell,

  • stimulating the release of calcium that's stored inside it.

  • This flood of calcium causes the muscle to contract

  • by enabling proteins buried in the muscle fibers to lock together

  • and ratchet towards each other, pulling the muscle tight.

  • The energy used to power the contraction comes from a molecule called ATP.

  • ATP also helps pump the ions back across the membrane afterward,

  • resetting the balance of sodium and potassium on either side.

  • This whole process repeats every time a muscle contracts.

  • With each contraction,

  • energy in the form of ATP gets used up,

  • waste products like lactic acid are generated,

  • and some ions drift away from the muscle's cell membrane,

  • leaving a smaller and smaller group behind.

  • Though muscle cells use up ATP as they contract repeatedly,

  • they are always making more,

  • so most of the time even heavily fatigued muscles still have not depleted this energy source.

  • And though many waste products are acidic,

  • fatigued muscles still maintain pH within normal limits,

  • indicating that the tissue is effectively clearing these wastes.

  • But eventually, over the course of repeated contractions

  • there may not be sufficient concentrations of potassium, sodium or calcium ions

  • immediately available near the muscle cell membrane to reset the system properly.

  • So even if the brain sends a signal,

  • the muscle cell can't generate the action potential necessary to contract.

  • Even when ions like sodium, potassium or calcium are depleted in or around the muscle cell,

  • these ions are plentiful elsewhere in the body.

  • With a little time,

  • they will flow back to the areas where they're needed,

  • sometimes with the help of active sodium and potassium pumps.

  • So if you pause and rest,

  • muscle fatigue will subside as these ions replenish throughout the muscle.

  • The more regularly you exercise,

  • the longer it takes for muscle fatigue to set in each time.

  • That's because the stronger you are,

  • the fewer times this cycle of nerve signal from the brain to contraction in the muscle has to be repeated to lift a certain amount of weight.

  • Fewer cycles means slower ion depletion,

  • so as your physical fitness improves,

  • you can exercise for longer at the same intensity.

  • Many muscles grow with exercise,

  • and larger muscles also have bigger stores of ATP

  • and a higher capacity to clear waste,

  • pushing fatigue even farther into the future.

You're lifting weights.

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筋肉が疲れる意外な理由 - クリスチャン・モロ (The surprising reason our muscles get tired - Christian Moro)

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    shuting1215 に公開 2021 年 01 月 14 日
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