Ask Dr. Universe: Muscles are made of cells that grow as they get used

Washington State University

Dr. Universe: How do we get muscles? – Jalane, 10, Ohio

Dear Jalane,

I’ve always been bookish. I spend most of my time in the lab rather than running, jumping or climbing.

But I still have super strong muscles. They’re way bigger than when I was a young.

I asked my friend Edward Johnson how I got so buff. He teaches classes about the human body in the School of Biological Sciences at Washington State University.

He told me that muscles are made of muscle cells. They’re also called muscle fibers or myocytes.

We usually think cells are way too small to see with our eyes. But muscle cells can be some of the longest cells in your body. They’re shaped like long tubes or hot dogs. Each muscle cell is as long as the muscle it’s part of. So a muscle cell in an adult’s thigh muscle could be 14 inches long!

A muscle is a bundle of those long, hot dog-shaped muscle cells. Once you’re an adult, you don’t grow new muscle cells. But the muscle cells you have can get bigger or smaller.

“Each individual muscle cell responds to exercise and being used by getting bigger,” Johnson said. “That’s called hypertrophy.”

When you run, jump or pick up something heavy, that work tells your muscle cells to get bigger.

That’s because each muscle cell is made of proteins called myofilaments. As you flex and release your muscles, those proteins slide over each other. That’s how muscles work.

When you use your muscles, that movement tells your body to make more myofilaments for your muscle cells. More myofilaments packed into your muscle cells means bigger muscle cells – and that means bigger muscles.

On the other hand, not using your muscles can make them smaller. That’s called atrophy.

You may have seen an atrophied muscle if you’ve seen an arm or leg that’s been in a cast for a while. The cast keeps the muscles still while a broken bone heals. When the cast comes off, that arm or leg probably looks like it shrunk a little bit. The muscle cells have become smaller because they didn’t exercise for a long time.

Johnson told me that muscle atrophy is a problem for astronauts, too. On Earth, gravity always pushes on your body. Your muscles work harder because they have to move against gravity.

“In space, the effect of gravity is not there,” Johnson said. “So, NASA has spent a lot of money with muscle physiologists, trying to come up with ways to use the muscles so that they don’t atrophy.”

Maybe that’s why I’m so strong even though I mostly lift books. My muscles work hard because of the gravity of your excellent questions.

Sincerely,

Dr. Universe

Adults can help kids submit a question at askdruniverse.wsu.edu/ask.