7) Muscles work to lift a bone in your arm by contracts and relaxes to

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By contracting, muscles pull on bones and allow the body to move. When you want to bend your elbow, your biceps muscle contracts, and, at the same time, the triceps muscle relaxes. The biceps is the flexor, and the triceps is the extensor of your elbow joint.

Explanation:

yeah, hope it helps a little. :)