Lesson 1
Three kinds of muscle
Muscle cells are long and skinny, like stretchy threads. They're so long that scientists call them muscle fibers. Your body has three kinds:
- Skeletal muscle is attached to your bones. You control it: kick, wave, jump!
- Heart muscle is only in your heart. It squeezes all by itself, day and night, without you thinking about it.
- Smooth muscle lines your stomach, intestines, and blood tubes. It slowly squeezes food along and helps control your blood flow.
Wow!
Your heart muscle cells squeeze about 100,000 times a day, and they never take a break!
Lesson 2
How a muscle cell squeezes
Inside every muscle cell are millions of tiny threads lined up side by side. When a nerve cell sends the message "Move!", the threads grab onto each other and slide together, like pulling a rope hand over hand. That makes the whole cell shorter. When millions of cells get shorter at once, the muscle pulls!
Here's the big secret: muscles can only pull. They can't push. So they work in pairs, like a team taking turns:
- Your biceps (front of your upper arm) pulls to bend your arm.
- Your triceps (back of your upper arm) pulls to straighten it.
Try this
Put your hand on the front of your other arm, above the elbow. Slowly bend your arm. Feel the biceps get hard and bunch up? Now move your hand to the back of your arm and straighten it. Now the triceps is doing the work!
Lesson 3
Hungry for energy
Moving takes a LOT of energy. That's why muscle cells are packed with mitochondria, the cell's power plants. They turn food and oxygen into energy for squeezing.
- When you run, you breathe faster and your heart pumps harder. That sends more oxygen to your hungry muscle cells.
- Making energy also makes heat. That's why you get warm when you play hard.
- When you're cold, your muscles shiver. Lots of tiny, fast squeezes make heat to warm you up!
Wow!
Even when you sit still, muscles are working. Tiny muscles help you keep your head up, blink, and breathe.
Lesson 4
How muscles get stronger
When you exercise, your muscle cells get a little bit tired and worn. Then, while you rest, your body repairs them and makes them bigger and stronger than before. That's how practice and play make you stronger!
- Move every day. Running, swimming, climbing, and dancing all count.
- Eat protein. Foods like eggs, beans, fish, milk, and nuts give your body the building blocks for muscle.
- Rest and sleep. That's when the fixing and growing happens.
- Drink water. Muscles are mostly water!
Wow!
You have more than 600 muscles. The biggest is in your bottom, and the smallest is deep inside your ear. It's smaller than a grain of rice!