How Muscles Move Bones
Describe how muscles pull on bones at joints, how the biceps and triceps help bend and straighten the elbow, and how chosen movements differ from automatic muscle activity in the heart and digestive system.
What a learner can do afterwards
- Describe how the biceps and triceps help bend and straighten the elbow, and explain that muscles can also pull while holding a joint still
- Explain voluntary control and involuntary muscle activity, including that skeletal muscles can also act without a conscious choice
- Name the heart and muscles of the digestive system as examples of involuntary muscles
1 · Read
Bones meet at joints. Many muscles attach to bones through tough bands called tendons. Muscles move bones by pulling, not pushing. When a muscle contracts, it tightens and pulls. It often gets shorter, but it can also pull while staying the same length. That is how a muscle can work while you hold something still.
The biceps is at the front of your upper arm. The triceps is at the back. To actively bend your elbow, the biceps pulls more and the triceps relaxes more. To straighten it, the triceps pulls more and the biceps relaxes more. Both can pull together to steady the joint. Different joints allow different moves, such as bending, turning or swinging.
These boxes compare movements. Both muscles can also pull together to hold the joint steady.
You can choose to move skeletal muscles in your arms and legs. We call this voluntary control. These muscles can also act without a choice, such as when you shiver. Smooth muscle in your stomach and intestines squeezes and moves food. Cardiac muscle in your heart pumps blood. Smooth and cardiac muscles are involuntary: they work without you deciding, even while you sleep.
Muscles pull to move bones at joints or hold them still. You can choose many movements, while heart and digestive muscles work without that choice.
Original lesson, questions and flashcards written for this learning dataset.
Original readings and flashcards written for this learning dataset. Anatomy references informed the facts; their text and illustrations are not reproduced.
More to read
Muscles for different jobs
When you wave, skeletal muscles pull on the bones in your arm. You can choose that movement. Skeletal muscles also help you keep your balance and hold a position. Some of their actions happen without a conscious choice, such as shivering.
Your heart contains cardiac muscle. Its contractions pump blood. The heart keeps beating while you sleep, without you deciding when each beat should happen.
Smooth muscle works in the walls of your stomach and intestines. Its squeezes help mix and move food. You do not choose each squeeze.
Skeletal, cardiac and smooth name three kinds of muscle tissue. Voluntary control means choosing an action. A muscle used for chosen movements can also take part in automatic actions.
From a muscle to a bone
A muscle's pull can reach a bone through a tendon. A tendon is a strong band of tissue joining muscle to bone. When the muscle pulls, the tendon carries that force to the bone.
The bone may move at a joint. It does not become shorter when your elbow bends. Instead, the bones change position where they meet.
A muscle can also pull without moving the bone. Think of holding an object still. Your muscles can keep working even though your arm stays in one place.
Muscles cannot push bones away. Pulls in different directions allow bones to move one way and then back again.
Where bones can move
A joint is a place where bones meet. Many joints allow movement, but they do not all allow the same movements.
Your elbow lets you bend and straighten your arm. Your shoulder allows more directions. You can move your arm forward and out to the side. The way a joint is built helps determine which movements it allows.
Muscles provide the pulls that move bones at these joints. The joint allows the movement, and the muscles supply the pulling force.
If you try these movements, move gently. Stop if it hurts, and never force a joint farther.
Moving an elbow and holding it still
The front of your upper arm contains the biceps. The triceps is at the back. These muscles help move the bones at your elbow.
For active bending, the biceps provides more pull while the triceps relaxes more. For active straightening, the triceps provides more pull while the biceps relaxes more.
This does not mean the muscles must always take separate turns. They can pull together to help keep the joint steady.
A contraction is a muscle producing a pull. The muscle often gets shorter, but it can also keep the same length while pulling. Holding still can involve muscle work as well as moving.
2 · Watch
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