Antagonist (muscle)
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An antagonist is a kind of muscle that acts in opposition to the movement generated by the agonist and is responsible for returning a limb to its initial position.
These antagonistic muscles are found in pairs called antagonistic pairs. These consist of an extensor muscle, which "opens" the joint (i.e. increasing the angle between the two bones), and a flexor muscle, which does the opposite to an extensor muscle.
The reason why antagonistic pairs are needed in the body is because muscles can only exert a pulling force, and can't push themselves back into their original positions. An example of this kind of muscle pairing is the bicep and tricep.
When the bicep is contracting, the tricep is relaxed, and is able to be stretched back to its original position. This is the opposite when the tricep is contracting.
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