Each time you take a step, 200 muscles work in unison to lift your foot, propel it forward, and set it down.
你每走一步，都需要200块肌肉一起工作，你的脚被操控着抬起，向前推进，然后落下。
It's just one of the many thousands of tasks performed by the muscular system.
这只是数千个肌肉系统活动中的一个。
This network of over 650 muscles covers the body and is the reason we can blink, smile, run, jump, and stand upright.
这个由超过650个肌肉组成的网络覆盖了了整个身体，并成为了我们能够眨眼、微笑、奔跑、跳跃和站起来的理由。
It's even responsible for the heart's dependable thump.
它甚至还担负心脏稳定跳动的重任。
First, what exactly is the muscular system?
那么问题来了，肌肉系统究竟是怎么工作的呢？
It's made up of three main muscle types:
它由三种主要的肌肉种类构成：
skeletal muscle, which attaches via tendons to our bones,
骨架肌，也就是通过筋腱附着在我们骨头上的肌肉，
cardiac muscle, which is only found in the heart,
贲门肌，只存在于心脏周围，
and smooth muscle, which lines the blood vessels and certain organs, like the intestine and uterus.
还有平滑肌，存在于血管和一些特定的器官上，比如肠道和子宫。
All three types are made up of muscle cells, also known as fibers, bundled tightly together.
这三种肌肉都由肌肉细胞构成，以纤维的形式，紧紧地贴合在一起。
These bundles receive signals from the nervous system that contract the fibers, which in turn generates force and motion.
这些聚合体收到了神经系统的指示之后就会告知纤维，由纤维完成力度和动作的控制。
This produces almost all the movements we make.
这一过程完成了几乎我们所有的动作。
Some of the only parts of the body whose motions aren't governed by the muscular system are sperm cells,
而在我们身体的某些部分，动作不是由肌肉系统控制的，它们包括精细胞，
the hair-like cilia in our airways, and certain white blood cells.
我们气道里的丝状纤维，还有某些白细胞。
Muscle contraction can be split into three main types.
肌肉的收缩可以被分为三种主要类型。
The first two, shortening muscle fibers and lengthening them, generate opposing forces.
前两种包括收缩型肌肉纤维和伸长型肌肉纤维，彼此会产生相对的作用力。
So the biceps will shorten while the triceps will lengthen or relax, pulling up the arm and making it bend at the elbow.
所以在肱三头肌伸展或者放松时，肱二头肌就会收缩，完成胳膊举起，向着手肘弯曲的动作。
This allows us to, say, pick up a book, or if the muscle relationship is reversed, put it down.
这使得我们可以拿起东西，比如一本书，或者反过来，放下一本书。
This complementary partnership exists throughout the muscular system.
这种互补的伙伴关系贯穿于整个肌肉系统之中。
The third type of contraction creates a stabilizing force.
第三种收缩方式，创造出了一种稳定化的力。
In these cases, the muscle fibers don't change in length, but instead keep the muscles rigid.
在这种情况之下，肌肉纤维的长度保持不变，而整个肌肉处于一种僵硬的状态。
This allows us to grip a mug of coffee or lean against a wall.
这允许我们能够紧紧握住一杯咖啡，或者靠着墙。
It also maintains our posture by holding us upright.
这同时也让我们能够保持直立的姿势。

Skeletal muscles form the bulk of the muscular system, make up about 30-40% of the body's weight, and generate most of its motion.
肌肉系统中，骨架上的骨骼肌占据了人体30%--40%的重量，并管控着大部分的活动。
Some muscles are familiar to us, like the pectorals and the biceps.
有些肌肉对我们而言更为熟悉，比如胸肌和肱二头肌。
Others may be less so, like the buccinator, a muscle that attaches your cheek to your teeth,
其他的肌肉我们就比较陌生了，比如颊肌，这是一种把你的脸颊连接到牙齿上的肌肉，
or the body's tiniest skeletal muscle, a one-millimeter-long tissue fragment called the stapedius that's nestled deep inside the ear.
还有身体上最纤细的骨骼肌，一片一毫米长的组织材料，叫做镫骨肌，位于我们耳朵的深处。
Wherever they occur, skeletal muscles are connected to the somatic nervous system,
它们存在的地方，骨骼肌被连接到身体的神经系统，
which gives us almost complete control over their movements.
几乎给了我们完整的活动控制权。
This muscle group also contains two types of muscle fibers to refine our motions even further, slow-twitch and fast-twitch.
这种肌肉组也包含两种肌肉纤维，以慢转换和快转换的方式进一步改善我们的活动。
Fast-twitch fibers react instantly when triggered but quickly use up their energy and tire out.
快转换纤维在被激发的时候，会持续地向外作出回应，但是它们的能量会很快被消耗掉。
Slow-twitch fibers, on the other hand, are endurance cells.
相反，慢转换纤维细胞是具有持久性的细胞。
They react and use energy slowly so they can work for longer periods.
它们的作用过程和能量消耗十分缓慢，所以工作时间更久。
A sprinter will accumulate more fast-twitch muscles in her legs through continuous practice,
一个运动员可以通过持久的练习在腿部积攒更多的快速转换肌肉，
enabling her to quickly, if briefly, pick up the pace,
从而让她更快、更简单的提速，
whereas back muscles contain more slow-twitch muscles to maintain your posture all day.
然而背肌包括了更多的慢转换肌肉，来长久地保持你的姿势。
Unlike the skeletal muscles, the body's cardiac and smooth muscles are managed by the autonomic nervous system beyond our direct control.
与骨骼肌不同，身体的贲门肌和平滑肌是由自动神经系统控制，而非听从我们的直接控制。
That makes your heart thump roughly 3 billion times over the course of your life, which supplies the body with blood and oxygen.
它使你的心脏在一生之中大概能跳动30亿下，支撑了你身体中血液和氧气的供应。
Autonomic control also contracts and relaxes smooth muscle in a rhythmic cycle.
自动控制系统同时也在一个有节奏的循环中协调、放松平滑肌。
That pumps blood through the smooth internal walls of blood vessels,
血液通过血管内部平滑的壁管，
enables the intestine to constrict and push food through the digestive system,
使肠道能够控制并推动食物在消化系统中继续前进，
and allows the uterus to contract when a person is giving birth.
允许子宫在婴儿诞生时收缩。
As muscles work, they also use energy and produce an important byproduct, heat.
在肌肉工作时，它们同时也用能量来创造一个重要的副产品，热量。
In fact, muscle provides about 85% of your warmth,
事实上，肌肉提供了你身体中85%的热度，
which the heart and blood vessels then spread evenly across the body via the blood.
这样才使得心脏和血管通过血液的持续流动在身体中平稳运作。
Without that, we couldn't maintain the temperature necessary for our survival.
如果没有肌肉，我们无法保持所需的体温，从而也难以幸存。
The muscular system may be largely invisible to us,
肌肉系统的存在对我们而言似乎难以察觉，
but it leaves its mark on almost everything we do, whether it's the blink of an eye or a race to the finish line.
但它的效用却几乎体现在我们做的每一件事上，无论是眨眼，还是一场冲向终点线的比赛。