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How Muscles Work
Muscles usually work in pairs. A good example of a pair of muscles working together can be seen in the human arm. The muscle at the front of the arm is called the biceps muscle. The muscle at the back of the arm is called the triceps muscle. When the biceps muscle is working (contracted) the triceps muscle is relaxed. When the triceps muscle is contracted the biceps muscle is relaxed. This is called antagonistic action of muscles because they are always working in opposite ways.
Contraction of the biceps muscle bends the arm. Contraction of the triceps muscle straightens the arm.
There are muscles in the legs which work the same way to bend the knees, ankles and toes.
The lion and the antelope both have well developed muscles in their limbs. They need to use them often. For both it is a matter of life and death.
Arthropods also have muscles but we cannot see them because they are inside of their hard body-covering. These muscles also work in pairs as vertebrate muscles do.
If you look at a locust or a grasshopper you can see that their back legs are much thicker than the others. This is because they use them for hopping. The muscles inside the back legs are much larger than the muscles in the other legs.