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 below to see how the biceps and triceps work together to
make the elbow joint move.
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.
