The surprising reason our muscles get tired - Christian Moro

翻译人员: Hanlin Wang 校对人员: Liu Qi

You're lifting weights.

当你举重的时候，

The first time feels easy,

第一次感觉很容易，

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

但一次比一次费劲， 直到你再也无法继续。

Inside your arms,

在你的胳膊里，

the muscles responsible for the lifting have become unable to contract.

负责举重的肌肉已经无法收缩。

Why do our muscles get fatigued?

为什么我们的肌肉会感到疲劳？

We often blame lactic acid or running out of energy,

我们常把原因归咎于 乳酸或者能量耗尽，

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

但这些并不是肌肉疲劳的全部原因。

There’s another major contributor:

还有另外一个主要因素：

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

肌肉对大脑信号做出反应的能力。

To understand the roots of muscle fatigue,

为了理解肌肉疲劳的根本原因，

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

首先要了解在接到神经信号后， 肌肉是如何收缩的。

These signals travel from the brain to the muscles in a fraction of a second

这些信号在不到一秒钟的时间内

via long, thin cells called motor neurons.

通过细长的运动神经元 从大脑传递至肌肉。

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

运动神经元和肌肉细胞 被很小的间隙隔开，

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

粒子通过这个间隙交换， 从而实现肌肉收缩。

On one side of the gap,

在间隙的一侧，

the motor neuron contains a neurotransmitter called acetylcholine.

运动神经元含有一种叫做 乙酰胆碱的神经递质。

On the other side,

在间隙的另一侧，

charged particles, or ions,

带电粒子，或者离子，

line the muscle cell’s membrane:

排列在肌肉细胞膜上：

potassium on the inside, and sodium on the outside.

钾离子在内，钠离子在外。

In response to a signal from the brain,

接收到大脑传来的信号后，

the motor neuron releases acetylcholine,

运动神经元释放乙酰胆碱，

which triggers pores on the muscle cell membrane to open.

使肌肉细胞膜上的孔隙张开。

Sodium flows in, and potassium flows out.

钠离子流入，钾离子流出。

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

这些带电粒子的流动 是肌肉收缩至关重要的一步：

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

电荷的变化产生了一种 叫作动作电位的电信号，

that spreads through the muscle cell,

这种电信号通过肌肉细胞传播，

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

刺激储存在其中的钙离子的释放。

This flood of calcium causes the muscle to contract

大量地钙离子将使肌肉收缩，

by enabling proteins buried in the muscle fibers to lock together

通过让埋藏在肌肉纤维里的 蛋白质锁在一起，

and ratchet towards each other,

相互拉紧，

pulling the muscle tight.

从而使肌肉拉紧。

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

肌肉收缩的能量来自 一种名为ATP酶的分子。

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

之后，ATP酶也有助于 将离子泵回到膜上，

resetting the balance of sodium and potassium on either side.

重新恢复钾钠离子 在两侧的平衡。

This whole process repeats every time a muscle contracts.

每次肌肉收缩都要重复这个过程。

With each contraction,

肌肉每收缩一次，

energy in the form of ATP gets used up,

ATP酶形式的能量将被用尽，

waste products like lactic acid are generated,

产生乳酸等废物，

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

部分离子从肌肉细胞膜上流失，

leaving a smaller and smaller group behind.

残留的离子越来越少。

Though muscle cells use up ATP as they contract repeatedly,

尽管肌肉细胞反复收缩会耗尽ATP酶，

they are always making more,

但肌肉细胞一直不断产生新的ATP酶,

so most of the time

所以大部分情况下，

even heavily fatigued muscles still have not depleted this energy source.

即便是非常疲累的肌肉 也不会完全耗尽ATP酶。

And though many waste products are acidic,

即便产生的很多废物都是酸性的，

fatigued muscles still maintain pH within normal limits,

但疲劳的肌肉始终把酸碱度 保持在正常范围，

indicating that the tissue is effectively clearing these wastes.

这说明组织正在高效地清理这些废物。

But eventually, over the course of repeated contractions

但是最终，随着肌肉反复收缩，

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

肌肉细胞膜附近可用的

钾、钠、钙离子的浓度

immediately available near the muscle cell membrane

可能不足以使整个系统恢复正常。

to reset the system properly.

So even if the brain sends a signal,

所以即便大脑传来了信号，

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

肌肉细胞仍然因不能产生 动作电位而无法收缩。

Even when ions like sodium, potassium or calcium

即使肌肉细胞内部及其周围的

钾、钠、钙离子被耗光，

are depleted in or around the muscle cell,

these ions are plentiful elsewhere in the body.

在身体的其他部位 这些离子仍然十分丰富，

With a little time,

只需要一点点时间，

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

它们就可以流回到需要的地方，

sometimes with the help of active sodium and potassium pumps.

有时这个过程需要 活性钠钾泵的帮助。

So if you pause and rest,

所以如果你停下来歇一会儿的话，

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

当这些离子补充至整个肌肉， 肌肉疲劳会消退。

The more regularly you exercise,

锻炼越规律，

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

肌肉达到疲劳状态的时间便会越慢。

That’s because the stronger you are,

这是因为你越是强壮，

the fewer times this cycle of nerve signal from the brain

举重时，

神经信号从大脑至肌肉收缩的循环

to contraction in the muscle has to be repeated

所需重复的次数就越少。

to lift a certain amount of weight.

Fewer cycles means slower ion depletion,

次数越少意味着离子消耗越慢，

so as your physical fitness improves,

所以随着身体素质的提升，

you can exercise for longer at the same intensity.

同样强度的运动可以持续更长时间。

Many muscles grow with exercise,

很多肌肉在锻炼中增长，

and larger muscles also have bigger stores of ATP

而越大的肌肉拥有越大的ATP酶储备，

and a higher capacity to clear waste,

更强的废物清洁能力，

pushing fatigue even farther into the future.

使疲劳感的出现越来越晚。