Muscles are in pairs called "antagonistic muscles." Let's take the human elbow joint as an example from the diagram. The elbow joint has 2 muscles; triceps and biceps. These 2 muscles help raise your forearm up or down. In the diagram, it shows an arrow going up which represents that your forearm is being raised. To raise, your biceps contract and your triceps relax. Its the opposite when you lower yopur forearm. To lower, your biceps relax and your triceps contract.
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