The first time feels easy,

第一次感覺輕而易舉

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

但每一次舉起都讓你覺得越來越費力，直到你無法繼續。

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

在你的手臂裡，負責抬升的肌肉變得無法收縮。

Why do our muscles get fatigued?

為什麼我們的肌肉會疲勞？

We often blame lactic acid or running out of energy,

我們經常責怪乳酸或是我們把精力用光了

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

但這些因素本身並不能解釋肌肉疲勞。

There's another major contributor:

還有另一個主因：

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

肌肉對大腦訊號的反應能力。

To understand the roots of muscle fatigue,

要了解肌肉疲勞的根源

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

了解肌肉接收神經訊號後如何收縮是有幫助的。

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

這些訊號經由又長又細的運動神經元細胞，在幾分之一秒內從大腦傳到肌肉。

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

運動神經元與肌肉細胞之間有個微小的間隙

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

並且透過這個間隙進行的粒子交換讓肌肉得以收縮。

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

在間隙的一側，運動神經元含有一種稱為乙醯膽鹼的神經傳遞物質。

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

在另一側，電荷粒子，或離子，排列在肌肉細胞薄膜上：

potassium on the inside, and sodium on the outside.

鉀離子在內部，鈉離子在外部。

In response to a signal from the brain,

為了回應來自大腦的訊號

the motor neuron releases acetylcholine,

運動神經元釋放出乙醯膽鹼

which triggers pores on the muscle cell membrane to open.

促使肌肉細胞膜上的孔洞打開。

Sodium flows in, and potassium flows out.

鈉離子向內流動，鉀離子向外流動。

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

這些電荷粒子的流動是肌肉收縮的重要步驟：

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

電荷的變化產生一種電波訊號，稱為動作電位

that spreads through the muscle cell,

它會傳過肌肉細胞

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

同時釋放儲存在內部的鈣離子。

This flood of calcium causes the muscle to contract

如此大量的鈣離子導致肌肉收縮

by enabling proteins buried in the muscle fibers to lock together

使藏在肌肉纖維中的蛋白質能夠鎖在一起

and ratchet towards each other, pulling the muscle tight.

並相互咬合，將肌肉拉緊。

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

用來驅動收縮的能量來自稱為三磷酸腺苷的分子 (adenosine triphosphate，簡稱 ATP)。

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

之後三磷酸腺苷也有助於將離子從薄膜打回去

resetting the balance of sodium and potassium on either side.

讓鈉離子與鉀離子回到兩側重新排列。

This whole process repeats every time a muscle contracts.

每次肌肉收縮時都會重複整個過程。

With each contraction,

隨著每一次收縮

energy in the form of ATP gets used up,

三磷酸腺苷的能量會消耗殆盡

waste products like lactic acid are generated,

像乳酸這樣的代謝廢物會產生

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

一些離子從肌肉的細胞薄膜散去

leaving a smaller and smaller group behind.

留下越來越小的離子群。

Though muscle cells use up ATP as they contract repeatedly,

雖然肌肉細胞反覆收縮時會消耗三磷酸腺苷

they are always making more,

但它們會製造更多

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

所以大多數時候即使肌肉疲憊不堪，也沒有耗盡這項能量來源。

And though many waste products are acidic,

而且雖然許多代謝廢物都是酸性的

fatigued muscles still maintain pH within normal limits,

疲勞的肌肉仍舊讓酸鹼值保持在正常範圍內

indicating that the tissue is effectively clearing these wastes.

表示這些組織正有效率地清除這些廢物。

But eventually, over the course of repeated contractions

但最終在這些重複收縮的過程中

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

可能沒有足夠濃度的鉀離子、鈉離子跟鈣離子

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

能立即讓肌肉細胞薄膜適當地重置系統。

So even if the brain sends a signal,

所以即使當大腦傳遞一個訊號

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

肌肉細胞也無法產生收縮所需的動作電位。

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

即使當肌肉細胞中或周遭的離子，像鈉離子、鉀離子或鈣離子被耗盡

these ions are plentiful elsewhere in the body.

這些離子在身體其他部位也非常豐富。

With a little time,

在很短的時間內

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

它們會流回需要它們的區域

sometimes with the help of active sodium and potassium pumps.

有時候會有活躍的鈉離子與鉀離子幫浦協助。

So if you pause and rest,

所以如果你停止動作或休息

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

當這些離子重新充滿整個肌肉時，肌肉疲勞就會消退。

The more regularly you exercise,

當你運動越頻繁

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

就要花費越多的時間來使肌肉疲勞。

That's because the stronger you are,

那是因為你越強壯

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.

為了提起一定重量而傳遞出的這種從大腦到肌肉收縮的神經訊號週期所需時間越短。

Fewer cycles means slower ion depletion,

越少的週期代表較慢的離子消耗

so as your physical fitness improves,

只要你增進體能狀況

you can exercise for longer at the same intensity.

你就能以相同強度鍛鍊更長時間。

Many muscles grow with exercise,

許多肌肉都在運動下成長

and larger muscles also have bigger stores of ATP

而越大的肌肉也有更多三磷酸腺苷儲存在內

and a higher capacity to clear waste,

還有清除代謝廢物的能力也更高

pushing fatigue even farther into the future.

使疲勞被推向更遠的未來。