Muscles can contract and relax but they cannot stretch themselves, so they must be stretched by the contraction of their antagonistic partners. All muscles work in antagonistic pairs.
Use the animation above to see how the biceps and triceps work together to make the elbow joint move. Hover over a muscle or bone to see its name. Click on a muscle to see its name and what happens when it contracts Click on a bone to see its common name.
The elbow is a hinge joint; it can open and close like a door. Biologists use the word “extend” to mean straighten the joint and the word “flex” to mean bend the joint. The triceps is an extensor muscle because it extends the elbow. The biceps is a flexor muscle because it flexes the elbow.