Bones cannot move on their own. Muscles move them, and muscles move blood and food inside you too. This builds on Bones and the Skeleton.
Key idea
There are three kinds of muscle. Skeletal muscle moves bones, and you control it. Smooth muscle lines organs such as the stomach, intestines and blood vessels and works without your thinking. Cardiac muscle makes up the heart. Every muscle can only pull, by shortening, so skeletal muscles work in pairs.
Why it works
A muscle cell is packed with protein fibers that slide past each other and shorten the cell. They cannot push it longer. So to bend your elbow, the biceps on the front of the upper arm shortens and pulls the forearm up. To straighten it again, the triceps on the back shortens and pulls the other way while the biceps relaxes and is stretched.
Worked example: Which muscle pulls?
You kick a ball by straightening your knee. The quadriceps are on the front of the thigh and the hamstrings on the back. Which muscle group shortens to straighten the knee?
- Straightening the knee swings the lower leg forward.
- The quadriceps, on the front, attach below the knee and pull the lower leg forward.
- The hamstrings relax and are stretched.
Answer: The quadriceps.
Worked example: Name the kind
Food is squeezed along your intestines while you sleep. What kind of muscle does that?
- It works without your control.
- It lines an organ, not a bone.
Answer: Smooth muscle.
Common mistake
Students think a muscle pushes to straighten a joint. Muscles only shorten and pull. When a joint straightens, look for the muscle on the opposite side doing the pulling: the triceps straightens the elbow while the biceps relaxes.