Rocket ties a string to a toy car. He pulls it across the table. Easy!
Then he tries to push it back with the string. The string just bunches up.
"What do you notice?" Raven asks. "Can a string push?"
Nova hovers over the car. "Muscles are a bit like strings," she says. "So how could the car go back?"
A muscle can pull on a bone. But it cannot push the bone back.
So muscles work in pairs. One pulls one way. The other pulls back.
Your biceps is on the front of your upper arm.
Your triceps is on the back of your upper arm.
To bend your elbow, the biceps contracts and the triceps relaxes.
To straighten your elbow, the triceps contracts and the biceps relaxes.
| Move | Biceps | Triceps |
|---|---|---|
| bend the elbow | contracts (pulls) | relaxes |
| straighten the elbow | relaxes | contracts (pulls) |
| Statement | True or false? |
|---|---|
| Muscles can pull and push. | ? |
| The biceps is on the front of the upper arm. | ? |
| When the biceps contracts, the triceps relaxes. | ? |
| One muscle alone can bend and straighten the elbow. | ? |
Perfect pulling! Tomorrow we find the tough cords that hook muscles onto bones.