Biceps + Triceps二頭肌＋三頭肌
The biceps on the front bends the elbow; the triceps on the back straightens it.
前面的二頭肌讓手肘彎曲，後面的三頭肌讓手肘伸直。
Try it · 試試看Do a slow push-up against a wall and feel the back of your arm work.對著牆慢慢做伏地挺身，摸摸上臂後面在用力。
The Human Body · Lesson 2 · 人體探索 第二課
How Do Muscles Move Your Bones?肌肉怎麼拉動骨頭？
Muscles can only pull, never push, so they work in pairs. Lift a 3D arm and watch the biceps and triceps take turns.
肌肉只能拉、不能推，所以總是成對工作；抬起 3D 手臂，看二頭肌和三頭肌輪流用力。
Big idea · 一句話看懂
Muscles can only pull. To bend and straighten a joint, two muscles take turns, one on each side.
肌肉只能拉。要讓關節彎曲又伸直，兩條肌肉在兩側輪流用力。
3D Model · 3D 模型
Biceps and Triceps Take Turns 二頭肌和三頭肌輪流用力
The bones are the real ones from Lesson 1; the two muscles are drawn to show how they work. Press Lift, Lower slowly, or Push, or drag the slider to bend the elbow yourself. Put something in the hand to see how hard the biceps has to pull, and turn on Lever to see why.
骨頭是第一課那副真實骨架；兩條肌肉是為了說明原理畫上去的。按「舉起」「慢慢放下」「推出」，或拖動滑桿自己彎手肘。在手上放個東西，看二頭肌要出多少力；打開「槓桿」看看為什麼。
Loading the arm… · 手臂載入中…
Drag to turn · 拖曳旋轉 Scroll or pinch to zoom · 滾輪／雙指縮放
This 3D model needs WebGL, which this browser does not support. The cards below still explain everything.
這個瀏覽器不支援 WebGL，無法顯示 3D 模型；下方的卡片一樣能看懂。
- Red: a muscle that is pulling (getting shorter)
紅色：正在用力收縮（變短）的肌肉 - Orange: a muscle working while it gets longer, like a brake
橘色：一邊用力一邊變長的肌肉，像煞車 - White ends: tendons, the tough cords that tie muscle to bone
白色兩端：肌腱，把肌肉綁在骨頭上的強韌繩索 - Gold arrows: the direction each muscle pulls. There are no pushing arrows.
金色箭頭：肌肉拉的方向——沒有「推」的箭頭
The bones are real, from the same BodyParts3D skeleton as Lesson 1. The two muscles are simplified shapes: in a real arm, the biceps starts at two points on the shoulder blade, and the triceps has three heads. The pulling force is a rough estimate for an adult arm.
骨頭是真實的，和第一課同一副 BodyParts3D 骨架。兩條肌肉是簡化的形狀：真實的二頭肌從肩胛骨的兩個地方出發，三頭肌則有三個頭。拉力是以成人手臂粗估的數字。
Bone models: BodyParts3D, © The Database Center for Life Science, licensed under CC BY 4.0. We selected the bones, simplified the shapes, and turned the model upright; the muscles are our own drawing. · 骨骼模型來源：BodyParts3D（日本生命科學資料庫中心，CC BY 4.0 授權）；本站挑選骨頭、簡化形狀並轉正方向，肌肉為本站自繪示意。
Reading · 英文閱讀
2
Unit 2: How Do Muscles Move Your Bones?
Make a fist and slowly bend your arm, bringing your hand up toward your shoulder. A bump rises on the front of your upper arm. That bump is your biceps, one of more than 600 muscles in your body. Together, your muscles make up about a third or more of your body weight. Every move you make, from blinking to kicking a ball, happens because a muscle pulls on something.
握緊拳頭，慢慢彎起手臂，把手往肩膀的方向抬。上臂前面會鼓起一塊——那就是二頭肌，你全身六百多塊肌肉中的一塊。肌肉加起來大約占體重的三分之一以上。你做的每一個動作，從眨眼到踢球，都是因為有一塊肌肉在拉著某個東西。
A muscle is a bundle of long, thin cells called muscle fibers. When your brain sends a signal down a nerve, the fibers contract, which means they get shorter and fatter. At each end, the muscle turns into a tendon, a tough cord that is tied to a bone. As the muscle shortens, the tendon pulls the bone along with it. But here is the surprising part: a muscle can only pull. It cannot push. When the signal stops, the muscle relaxes, but it does not stretch itself back out.
肌肉是一束又長又細的細胞，叫做肌纖維。大腦沿著神經送出訊號時，肌纖維會收縮，也就是變短、變粗。肌肉的兩端各變成一條肌腱——一條綁在骨頭上的強韌繩索。肌肉一縮短，肌腱就把骨頭跟著拉過去。但令人意外的是：肌肉只能拉，不能推。訊號停止時，肌肉會放鬆，但它不會自己伸展回原來的長度。
So how does your arm straighten again? Muscles work in pairs, one on each side of a joint. When the biceps on the front of your upper arm contracts, it pulls your forearm up and bends the elbow. To straighten the elbow with force, as when you push a door open or do a push-up, the triceps on the back of your upper arm contracts and pulls the other way. While one muscle pulls, its partner relaxes and gets stretched. Your knee works the same way: the big muscles on the front of your thigh straighten it, and the ones on the back bend it.
那手臂要怎麼再伸直呢？肌肉是成對工作的，關節兩側各一條。上臂前面的二頭肌收縮時，把前臂往上拉，手肘就彎起來。要用力把手肘伸直，例如推開一扇門或做伏地挺身，就換上臂後面的三頭肌收縮，往另一個方向拉。一條肌肉在拉的時候，它的搭檔就放鬆、被拉長。膝蓋也是一樣：大腿前面的大肌肉把膝蓋伸直，後面的肌肉把它彎起來。
Your bones and muscles work like levers, with each joint as the pivot. The biceps is attached to the forearm only about four centimeters from the elbow, but your hand is more than thirty centimeters away. That means the biceps has to pull about eight times harder than the weight in your hand. Holding a three-kilogram dumbbell with your elbow bent at a right angle, your biceps pulls roughly as hard as lifting almost thirty kilograms. Why would the body be built this way? Because a short pull close to the joint moves the hand a long way, and quickly, which is just what you need to throw a ball or wave hello.
骨頭和肌肉就像槓桿，每個關節就是支點。二頭肌接在前臂上離手肘只有大約四公分的地方，手卻在三十多公分外。這表示二頭肌要出的力，大約是手上重量的八倍。手肘彎成直角拿著一個三公斤的啞鈴時，二頭肌出的力大概等於舉起將近三十公斤。身體為什麼要這樣設計？因為在靠近關節的地方拉一小段，手就能移動很遠、而且很快——丟球、揮手打招呼需要的正是這個。
Not all of your muscles move bones. The muscles you control on purpose, like your biceps, are called skeletal muscles. Your heart is made of cardiac muscle, which beats about 100,000 times a day without ever taking a break or waiting for you to think about it. The walls of your stomach, intestines, and blood vessels contain smooth muscle, which squeezes slowly to move food along and to control the flow of blood. You cannot tell these muscles what to do, and they keep working even while you sleep.
不是所有肌肉都拉骨頭。像二頭肌這種你可以自己控制的肌肉，叫做骨骼肌。心臟由心肌構成，一天大約跳十萬下，從不休息，也不用等你想到它。胃、腸和血管的管壁裡有平滑肌，它們慢慢地收縮，把食物往前推送、控制血液的流動。這些肌肉你無法指揮，連睡覺時它們也照樣工作。
Muscles also do a job you might not expect: they keep you warm. Working muscles produce heat, which is why you get hot when you run, and why you shiver on a cold day. Shivering is thousands of tiny, fast contractions whose only purpose is to make heat. Like bones, muscles respond to how you use them. Regular exercise makes muscle fibers thicker and stronger, while weeks of rest in bed make them smaller. Now try it yourself: rest one hand on your upper arm, then lift a bag and press down on a desk, and feel your two muscles take turns.
肌肉還有一個你可能想不到的工作：幫你保暖。工作中的肌肉會產生熱，所以你跑步時會變熱，冷天時會發抖——發抖就是成千上萬次又小又快的收縮，唯一的目的就是產生熱。跟骨頭一樣，肌肉會回應你怎麼使用它：規律運動讓肌纖維變粗、變強壯；在床上躺好幾個星期，肌肉就會變小。現在自己試試看：把一隻手放在上臂上，提起書包、再用力按桌面，感覺兩條肌肉輪流用力。
Read aloud · 課文朗讀
Questions · 閱讀理解
Why do muscles have to work in pairs?
Because a muscle can only pull, never push. One muscle bends the joint, and its partner on the other side pulls it straight again.
What does a tendon do?
A tendon is a tough cord at the end of a muscle that ties the muscle to a bone, so when the muscle shortens, the tendon pulls the bone.
Why does the biceps have to pull much harder than the weight in your hand?
The biceps is attached only about four centimeters from the elbow, while the hand is more than thirty centimeters away, so, like a lever, it must pull about eight times harder than the weight.
Words & Phrases · 生字及片語
muscle(n.)肌肉You use more than forty muscles in your face to smile and frown.微笑和皺眉時，你用到臉上四十多塊肌肉。contract(v.)收縮Your heart contracts to push blood out into your body.心臟收縮，把血液送到全身。relax(v.)放鬆Take a deep breath and let your shoulders relax.深呼吸，讓肩膀放鬆。tendon(n.)肌腱The Achilles tendon connects your calf muscle to your heel.阿基里斯腱把小腿的肌肉連到腳跟。fiber(n.)纖維A single fiber of this rope is thin, but thousands together are very strong.這條繩子的一根纖維很細，但成千上萬根合在一起就非常結實。pair(n.)一對；一雙I need a new pair of running shoes.我需要一雙新的跑鞋。lever(n.)槓桿A seesaw is a lever, and the middle is its pivot.蹺蹺板是一種槓桿，中間就是它的支點。shiver(v.)發抖We shivered at the bus stop in the cold wind.我們在冷風中的公車站發抖。
Quick Check · 小測驗
1. What can a muscle do?
2. Which muscle bends your elbow?
3. What connects a muscle to a bone?
4. What kind of muscle is your heart made of?
5. According to the reading, why do we shiver when we are cold?
Muscle Pairs · 成對的肌肉
One pulls, the other lets go 一條拉、一條放
In every pair, the two muscles sit on opposite sides of a joint. Doctors call them antagonists: partners that pull in opposite directions.
每一對肌肉都位在關節的兩側，醫學上叫做拮抗肌：方向相反、互相搭配的搭檔。
Quadriceps + Hamstrings股四頭肌＋大腿後肌群
The quadriceps on the front of the thigh straightens the knee; the hamstrings on the back bend it.
大腿前面的股四頭肌把膝蓋伸直，後面的大腿後肌群把它彎起來。
Try it · 試試看Sit on a chair and straighten one leg. Feel the top of your thigh go hard.坐在椅子上把一條腿伸直，摸摸大腿上面變硬了。
Shin muscle + Calf muscle脛前肌＋小腿肌
The shin muscle lifts your toes; the calf muscle, pulling on the Achilles tendon, points them down.
脛前肌把腳尖抬起；小腿肌拉著阿基里斯腱，讓腳尖往下踩。
Try it · 試試看Stand on your tiptoes, then rock back onto your heels.踮起腳尖，再往後站到腳跟上。
Belly muscles + Back muscles腹肌＋背肌
Belly muscles curl you forward; back muscles straighten you up again.
腹肌讓你向前捲起身體，背肌再把你拉直。
Try it · 試試看Do one slow sit-up, then lie on your stomach and lift your chest a little.慢慢做一下仰臥起坐，再趴著把胸口微微抬起。
Three Kinds of Muscle · 三種肌肉
Not every muscle moves a bone 不是每一種肌肉都拉骨頭
Only skeletal muscle pulls on bones, and it is the only kind you can move just by deciding to.
只有骨骼肌會拉骨頭，也只有它是你「想動就能動」的。
Skeletal muscle骨骼肌
Moves your body, keeps you standing, and makes heat. It gets tired and needs rest.
讓身體動、讓你站得住，也會產生熱。它會累，需要休息。
Cardiac muscle心肌
Beats about 100,000 times a day for your whole life and never takes a break. Lesson 3 follows the blood it pumps.
一輩子每天跳大約十萬下，從不休息。第三課會跟著它打出的血液走一趟。
Smooth muscle平滑肌
Squeezes slowly in waves to move food along and to widen or narrow your blood vessels.
像波浪一樣慢慢收縮，把食物往前推，也讓血管變寬或變窄。
Measure Yourself · 親身測量
Feel your muscles take turns 感覺肌肉輪流用力
Rest your left hand on your right upper arm, with your thumb on the front and your fingers on the back. Then try these three moves.
把左手放在右上臂上，拇指在前面、其他手指在後面，然後做下面三個動作。
- 1Lift · 舉起Lift a school bag by bending your elbow. The front (biceps) goes hard; the back stays soft.彎起手肘提起書包：前面（二頭肌）變硬，後面還是軟的。
- 2Push · 推Put your palm on a desk and press straight down. Now the back (triceps) goes hard, and the front goes soft.手掌放在桌上用力往下壓：這次換後面（三頭肌）變硬，前面變軟。
- 3Relax · 放鬆Let your arm hang loosely. Both muscles are soft, and you can wiggle them from side to side.讓手臂自然垂下：兩條肌肉都軟軟的，可以左右晃動它們。
Myth vs. Fact · 常見迷思
Four things people get wrong 四個常見的誤會
✗ Myth · 迷思Muscles can push as well as pull.肌肉可以拉，也可以推。
✓ Fact · 事實A muscle can only pull. Every push you make, like pushing a door, is really a muscle pulling on the other side of a joint. In the model, the arrows only ever point toward the middle of a muscle.肌肉只能拉。你做的每一個「推」（像推門），其實都是關節另一側的肌肉在拉。模型裡的箭頭，永遠指向肌肉的中間。
✗ Myth · 迷思When you lower a heavy bag, your triceps does the work.慢慢放下重物時，是三頭肌在出力。
✓ Fact · 事實Gravity pulls the bag down. Your biceps keeps working and lets itself get longer bit by bit, like a brake. Press Lower slowly in the model and watch it turn orange.把重物往下拉的是重力；二頭肌一直在用力，讓自己一點一點變長，就像煞車。在模型裡按「慢慢放下」，看它變成橘色。
✗ Myth · 迷思If you stop exercising, your muscles turn into fat.不運動，肌肉就會變成脂肪。
✓ Fact · 事實Muscle and fat are different tissues, and one cannot turn into the other. Unused muscles shrink, and if you eat more than you use, fat can build up, so it can look that way.肌肉和脂肪是兩種不同的組織，不會互相變來變去。不用的肌肉會變小；吃得比用掉的多，脂肪會增加——所以看起來像是變了。
✗ Myth · 迷思The biceps is the biggest muscle in your body.二頭肌是全身最大的肌肉。
✓ Fact · 事實The biggest is the gluteus maximus, the muscle you sit on. It straightens your hip every time you climb stairs or stand up from a chair.最大的是臀大肌，就是你坐著的那塊肌肉。每次爬樓梯或從椅子上站起來，都是它把髖部伸直。
Remember It · 記憶口訣
Muscles in a sentence 一句話記住肌肉
Muscles pull, bones move, joints bend肌肉拉、骨頭動、關節彎
Three parts, three jobs. If you remember the order, you can explain any move your body makes.
三個部分、三個工作。記住這個順序，就能解釋身體的任何一個動作。
Bi = two, tri = threebi 是二、tri 是三
A bicycle has two wheels and a tricycle has three. The biceps starts with two heads and the triceps with three.
bicycle 兩個輪子、tricycle 三個輪子；biceps 起點有兩個頭，triceps 有三個頭。
One pulls, one lets go一條拉、一條放
In every pair, one muscle contracts while its partner relaxes. If both pulled hard at once, the joint would lock in place.
每一對肌肉裡，一條收縮，它的搭檔就放鬆。如果兩條同時用力，關節就會卡住不動。
Short pull, long swing拉一小段，甩一大段
The biceps shortens only a few centimeters, but the hand swings through a big arc. That trade of force for speed is why you can throw a ball so fast.
二頭肌只縮短幾公分，手卻劃過一大段弧線。用力氣換速度，所以你才能把球丟得那麼快。
In Chinese · 中文怎麼說
A Cramp Is a “Pulled Sinew” 「抽筋」是抽了哪條筋？
In Chinese, a muscle cramp is 抽筋 (chōujīn), literally “a sinew being pulled.” That is a good description: in a cramp, a muscle contracts on its own and will not relax. Its partner on the other side can help. For a calf cramp, gently pulling your toes toward you uses the shin side to stretch the calf. Another saying, 活動活動筋骨, “move your sinews and bones a little,” means to loosen up and warm up before exercise.
中文把肌肉痙攣叫「抽筋」，字面上是「一條筋被抽住」。這個說法很傳神：抽筋時，肌肉自己收縮、放不開。這時關節另一側的搭檔可以幫忙——小腿抽筋時，把腳尖輕輕往身體方向拉，就是用前側把小腿肌拉開。另一句「活動活動筋骨」，則是運動前暖身、鬆一鬆的意思。
Classroom Activity 1 · 課堂活動一
Build a Cardboard Arm with String Muscles 用紙板和棉線做一隻手臂
You need · 準備材料
- Two strips of stiff cardboard, about 3 × 20 centimeters (upper arm and forearm)兩條硬紙板，約 3 × 20 公分（上臂與前臂）
- A brass paper fastener (the elbow)一個雙腳釘（手肘）
- Two pieces of string, about 40 centimeters each (the muscles)兩條棉線，各約 40 公分（肌肉）
- A hole punch and tape打洞機和膠帶
Steps · 步驟
- Join the two strips at one end with the fastener so they can bend like an elbow.用雙腳釘把兩條紙板的一端接起來，讓它們可以像手肘一樣彎。
- Punch a hole in the forearm strip about 3 centimeters from the elbow. Tie one string there and run it along the front of the upper arm. This is the biceps.在前臂紙板離手肘約 3 公分處打一個洞，綁上一條棉線，沿著上臂的前面拉上去——這是二頭肌。
- Tape the second string to the short tab of the forearm behind the elbow and run it along the back of the upper arm. This is the triceps.把第二條棉線貼在前臂超出手肘後面的那一小段，沿著上臂的後面拉上去——這是三頭肌。
- Hold the upper arm still and pull each string. Then try to push with a string. What happens?固定上臂，分別拉兩條線；再試試看用線「推」。會發生什麼事？
Teaching tip: a string, like a muscle, goes slack when you try to push with it, so the arm needs one string on each side. Then move the biceps hole farther from the elbow: the pull gets easier, but the hand moves less.
教學提示：棉線和肌肉一樣，一推就軟掉，所以手臂兩側各需要一條。接著把二頭肌的洞打在離手肘更遠的地方：拉起來比較省力，但手移動的距離變短了。
Classroom Activity 2 · 課堂活動二
The Clothespin Squeeze: Do Muscles Get Tired? 捏曬衣夾：肌肉會累嗎？
You need · 準備材料
- A spring clothespin for each student每人一個有彈簧的曬衣夾
- A timer一個計時器
- Paper to record the counts記錄次數的紙
Steps · 步驟
- Hold the clothespin between your thumb and index finger. Count how many times you can squeeze it open in 30 seconds.用拇指和食指捏住曬衣夾，數一數 30 秒內能捏開幾次。
- Without resting, do it again two more times, and write down each count.不要休息，再連續做兩次，記下每次的次數。
- Rest for one minute, then try once more.休息一分鐘，再做一次。
- Compare the counts. What happened, and what did the rest do?比較這幾次的次數：發生了什麼事？休息帶來什麼改變？
Teaching tip: the counts usually drop as the small muscles in the hand tire, and they come back after a rest. That is why skeletal muscles need breaks, while the heart, made of a different kind of muscle, never gets tired this way.
教學提示：次數通常會越來越少，因為手上的小肌肉累了；休息之後又會恢復。這就是骨骼肌需要休息的原因——而由另一種肌肉構成的心臟，不會這樣累。
⚕ Health note · 健康提醒This lesson is for learning about the body. It is not medical advice. If a muscle hurts badly, swells, or a cramp will not go away, tell an adult and see a doctor.本課是認識身體的教材，不是醫療建議。如果肌肉非常痛、腫起來，或抽筋一直不好，請告訴大人並就醫。