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Antagonist (muscle)

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Title: Antagonist (muscle)  
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Subject: Digestion, Rolfing, Thoracic diaphragm, Thermogenesis, Muscular hydrostat, Reciprocal innervation, Sphincter ani externus muscle, Pneumatic artificial muscles, Synergist, Agonist (muscle)
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Antagonist (muscle)

This article is about the type of muscle. For other uses, see Antagonist.

An "antagonist" is a muscle that acts opposite to the specific movement generated by the agonist and is responsible for controlling the motion, slowing it down and returning a limb to its initial position.

Antagonistic pairs

Muscles can only exert a pulling force and cannot push themselves back into their original positions. Therefore performing "return motions" may need a pair of muscles with opposite effect. As one muscles contracts, the other relaxes. This pair is called an "antagonistic pair". An example of exception is Sphincter ani externus muscle.

Flexor-extensor pairs

"Reverse motions" need Antagonistic pairs located in opposite sides of a joint or bone. Including abductor-adductor pairs and flexor-extensor 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.

Antagonism is not an intrinsi property. It's a role, played depending on the motion. If the motion is reversed, agonist and antagonist swich roles. A flexor muscle is always flexor. But in flexion, it's always agonist and in extension, it's always antagonist. An extensor muscle is agonist in extension and antagonist in flexion.

Examples include:

When the biceps are contracting, the triceps relax, and stretch back to their original position. The opposite happens when the triceps contract.

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