Use stressed to describe emotional strain, like your stressed feeling before taking a test you know will be hard, or the overuse of your body, like a stressed tendon in your leg that hurts when you run.
You probably know the adjective stressed as it is used to describe feelings of worry, pressure, and anxiety, like feeling stressed by all the work you have to do. But stressed isn't always so negative — it also means "emphasized" or "accented," like the stressed syllable in a long word or the stressed themes of a politician's speech. The word comes from a Latin root, strictus, "tight or compressed."