What Is an Antagonist Muscle?

The triceps branchii relaxes when the biceps branchii contracts.
The human muscular system.
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  • Written By: Christina Hall
  • Edited By: Lauren Fritsky
  • Last Modified Date: 16 December 2014
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An antagonist muscle is one that works in opposition to the movement initiated by an agonist muscle. The antagonist muscle in a muscle set brings a limb or other anatomical part back to its initial position of rest. These muscle sets are referred to as antagonistic pairs, which are needed by the body because muscles can only inherently exert a force that pulls on ligaments and bones; it needs the opposite muscle in the pair to bring it back to its original position. The muscle responsible for moving the body part away from its position shortens or contracts, and the antagonist muscle responds by stretching, which then enables the second movement.

The majority of the 600 skeletal muscles that make up the human anatomy exist in agonist/antagonist pairs. One example of this pairing includes the biceps brachii and triceps brachii. When the biceps brachii contracts, the triceps brachii relaxes, which enables it to stretch back to its resting position; the opposite occurs when the triceps brachii contracts. The agonist/antagonist pairing can also be identified as a flexor/extensor pairing. The flexor moves to open a joint, whereas the extensor does the opposite, decreasing the joint angle. The antagonist muscle is especially important when a person extends or contracts limbs, holds objects against gravity, and tries to maintain balance while standing erect.

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The movement of agonist/antagonist muscle groups is coordinated by the central and peripheral nervous system. The motor cortex inside the brain sends a message through the spinal cord and peripheral nerve system to the agonist muscle. The motor cortex in the right hemisphere controls movements on the left side of the body and the one on the left controls the right. This signal begins the complex movement, which eventually leads to antagonist muscles bringing a body part back to its place of rest. The antagonist muscle helps the body maintain a state of relaxation.

All muscles contain receptors, or muscle spindles, that receive the specific messages from the motor cortex. Muscle spindles located in an antagonist muscle receive signals which inform them of a muscle that is in a state of contraction. It begins a stretch in response to this information. Sometimes muscle signaling can work the other way around; the agonist muscle receives a message that the other muscle is in a state of stretch and then contracts or shortens to reverse the stretch. Most muscle systems in the body work in a way based on this basic principle.

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anon315442
Post 5

What is the difference between an antagonist muscle and an antagonistic muscle?

bythewell
Post 3

I guess this is why people feel better when they stretch after sitting in one place for a while.

One muscle in the antagonist muscle pairs must always be stretched out and the other is contracted, so even if you feel quite relaxed, it still feels good to switch them over by stretching out.

One of the reasons you should try to do some stretching exercises every hour or so. So that your muscles don't get made to sit in one position for a long time.

browncoat
Post 2

@umbra21 - If it makes it easier, it's not like the agonist muscles are constantly working to keep the limbs (or whatever) in the resting state.

They just pull them back to that position and then relax. So, for example if you lift up your arm, the agonist muscles do the lifting, and the antagonist muscles do the movement back to the side of the body, at which point both muscles relax.

In some cases gravity could just do the work, but not always. Having antagonistic muscle pairs ensures that we have a full choice of movement and force in our arms and legs, rather than just in one direction.

umbra21
Post 1

I understand that the muscle is called the antagonist muscle because it works in opposition to the muscle that is doing the "work" or the agonist. It makes logical sense.

But it also seems counter intuitive to me, just because generally I think that something that is antagonistic sounds aggressive, and usually these muscles are working to put the limb, or whatever back into its normal "resting" position.

I know that's just because I have a culturally influenced definition of the word "antagonist" but even knowing it, I still find it difficult to remember which muscles are the antagonistic muscles and which ones are the agonist muscles.

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