TED-ed~練習托福聽力的好素材

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先放中文，試試看自己能理解多少？

肌肉。我們有超過600塊肌肉。

大約佔了身體重量的三分之一到二分之一它們和結締組織一起組成我們的身體，使我們能夠站立和移動。

不管健身是不是你的興趣愛好，肌肉都需要你的持續關注，因為你每天的鍛煉情況會決定肌肉萎縮還是增長。

假想你正站在一扇門前，準備拉門。

你的大腦和肌肉會達成完美的平衡以幫助你達成目標。

首先，你的大腦給遙控你手臂的神經發射信號。

神經接受信號之後，開始行動引起肌肉收縮與放鬆牽引手臂的骨骼運動從而做出相應的動作。動作難度越大，大腦發出的信號越強協助完成動作所調動的遙控神經單位越多。

但是如果門是鐵制的呢？

現在，光靠你手臂的肌肉是不能產生足夠的拉力來開門的所以你的大腦會讓其他肌肉來幫忙。你紮好馬步，收緊小腹，拉緊脊背，產生足夠的力把門扯開。

你的神經系統剛剛借用了你所有的已有資源：別的肌肉來完成你的目標。

當這一切發生時你的肌肉纖維經歷了細胞層面的變化。

你拉伸細胞時，細胞經歷了細微的損傷在這裡，是一件好事。

作為回應，受損的細胞會釋放出被稱作細胞因子的炎性分子激活了免疫系統以修復損傷。

這就是肌肉變大的魔法。

對肌肉組織造成的損傷越大，你身體的自我修復就會越多。

損傷和修復的循環最終使肌肉變大變結實逐漸適應更大的需求。

因為我們的身體已經適應了日常的活動，這些就不能產生足夠的壓力來促進新的肌肉生長了。所以，要長肌肉，要經過一個叫過度增長的過程我們的細胞需要比它已經適應的更大的工作強度。

事實上，如果你不持續的讓肌肉鍛煉起來，它們會變小這被稱為肌肉萎縮。

相反的，讓肌肉承受更高的拉力，特別是肌肉在伸展時，又被稱作離心收縮，為肌肉提供有效的生長條件。

然而，肌肉生長還要更多條件。

沒有所需的營養，激素，和休息，你的身體永遠不能修復受損的肌肉組織。

我們飲食中的蛋白質通過以氨基酸的形式提供新組織的基礎來維持細胞的質量。

足量的蛋白質攝入，與自然產生的荷爾蒙一起，比如胰島素，比如生長因子和睾酮，協助組織的修復和生長。

這個至關重要的修復過程主要發生在我們休息時，尤其是晚上睡覺的時候。

性別和年齡也會影響修復機制，這就是為什麼有更多睾酮的青壯年，在肌肉增長上更有優勢。基因因素也會影響肌肉增長。

有些人對肌肉損傷有更強大的免疫反應，更容易修復和替換受損肌肉纖維，這都加大了肌肉增長的潛力。

身體會對你的需求做出回應。

如果你撕裂肌肉，補充營養，休息然後重複你就為肌肉變大變強壯創造了條件。

肌肉增長和生活一個道理：有效的成長需要挑戰和壓力。

再來英文，查缺補漏。

Muscles. We have over 600 of them. They make up between 1/3 and 1/2 of our body weight, and along with connective tissue, they bind us together, hold us up, and help us move. And whether or not body building is your hobby, muscles need your constant attention because the way you treat them on a daily basis determines whether they will wither or grow.

Say youre standing in front of a door, ready to pull it open. Your brain and muscles are perfectly poised to help you achieve this goal. First, your brain sends a signal to motor neurons inside your arm. When they receive this message, they fire, causing muscles to contract and relax, which pull on the bones in your arm and generate the needed movement. The bigger the challenge becomes, the bigger the brains signal grows, and the more motor units it rallies to help you achieve your task.

But what if the door is made of solid iron?

At this point, your arm muscles alone wont be able to generate enough tension to pull it open, so your brain appeals to other muscles for help. You plant your feet, tighten your belly, and tense your back, generating enough force to yank it open. Your nervous system has just leveraged the resources you already have, other muscles, to meet the demand.

While all this is happening, your muscle fibers undergo another kind of cellular change. As you expose them to stress, they experience microscopic damage, which, in this context, is a good thing. In response, the injured cells release inflammatory molecules called cytokines that activate the immune system to repair the injury. This is when the muscle-building magic happens.

The greater the damage to the muscle tissue, the more your body will need to repair itself. The resulting cycle of damage and repair eventually makes muscles bigger and stronger as they adapt to progressively greater demands. Since our bodies have already adapted to most everyday activities, those generally dont produce enough stress to stimulate new muscle growth. So, to build new muscle, a process called hypertrophy, our cells need to be exposed to higher workloads than they are used to. In fact, if you dont continuously expose your muscles to some resistance, they will shrink, a process known as muscular atrophy. In contrast, exposing the muscle to a high-degree of tension, especially while the muscle is lengthening, also called an eccentric contraction, generates effective conditions for new growth.

However, muscles rely on more than just activity to grow. Without proper nutrition, hormones, and rest, your body would never be able to repair damaged muscle fibers. Protein in our diet preserves muscle mass by providing the building blocks for new tissue in the form of amino acids. Adequate protein intake, along with naturally occurring hormones, like insulin-like growth factor and testosterone, help shift the body into a state where tissue is repaired and grown. This vital repair process mainly occurs when were resting, especially at night while sleeping.

Gender and age affect this repair mechanism, which is why young men with more testosterone have a leg up in the muscle building game. Genetic factors also play a role in ones ability to grow muscle. Some people have more robust immune reactions to muscle damage, and are better able to repair and replace damaged muscle fibers, increasing their muscle-building potential.

The body responds to the demands you place on it. If you tear your muscles up, eat right, rest and repeat, youll create the conditions to make your muscles as big and strong as possible. It is with muscles as it is with life: Meaningful growth requires challenge and stress.