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An agonist muscle is the primary muscle responsible for generating a specific movement. It is also known as the prime mover. When a muscle contracts, the agonist muscle is the one that is tensed or strained. The muscle that is relaxing or lengthening is called the antagonist muscle. In an antagonistic muscle pair, the agonist and antagonist muscles work together to create contractions and extensions, allowing for free movement of our joints and muscles.
| Characteristics | Values |
|---|---|
| Definition | A muscle that is controlled by the action of an antagonist with which it is paired |
| Other names | Prime mover |
| Function | The agonist muscle contracts to produce force and create a specific movement |
| Example | When doing a bicep curl, the bicep brachii does most of the work and is the agonist |
| Antagonist muscle | The muscle with the opposite action of the agonist |
| Synergist muscle | Assists the agonist muscle by producing additional force or promoting stability |
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What You'll Learn
Agonist muscles are the primary movers
The agonist and antagonist muscles work in tandem to allow the joints and limbs to perform complex movements. These muscles move in opposite directions to each other, and the antagonist offsets the force exerted by the agonist so that the joints are not damaged. In other words, the antagonist provides the necessary resistance for the movement that the agonist undertakes, allowing just the right amount of force to be used.
In any pair of agonist and antagonist muscles, one muscle will contract (the agonist) and the other will relax (the antagonist) during each movement. For example, the bicep is the agonist during a bicep curl because it is the prime mover, while the tricep is the antagonist since it functions opposite to the bicep and lengthens during arm flexion. The roles reverse when performing a tricep exercise, such as a rope extension. In this case, the tricep becomes the agonist because it is the prime mover, and the bicep is the antagonist.
Agonist and antagonist muscles are essential for everyday tasks such as walking up stairs, reaching for something, or holding the arms in a natural position. They are also important in sports, such as when a footballer prepares to kick a ball. During the preparation phase, the hamstrings contract to flex the knee while the quadriceps lengthen to allow the movement, making the hamstrings the agonist and the quadriceps the antagonist. During the contact and recovery phase, the roles reverse, with the quadriceps becoming the agonist and the hamstrings the antagonist.
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Antagonists are the opposing muscles
In an antagonistic muscle pair, as one muscle contracts, the other muscle relaxes or lengthens. The muscle that is contracting is the agonist, and the muscle that is relaxing or lengthening is the antagonist. For example, when performing a bicep curl, the biceps contract to produce the movement, making them the agonists, while the triceps relax to allow the movement to occur, making them the antagonists. When we bring our arm back to a natural position, the bicep is relaxed and becomes the antagonist, while the tricep is contracted and becomes the agonist.
Antagonists play two important roles in muscle function. Firstly, they maintain body or limb position, such as holding the arm out or standing erect. Secondly, they control rapid movement, such as shadow boxing without landing a punch or the ability to check the motion of a limb.
The roles of agonists and antagonists can reverse depending on the movement. For example, during a tricep exercise such as a rope extension, the tricep becomes the agonist as it serves as the prime mover, and the bicep becomes the antagonist. Similarly, when flexing the leg at the knee, the hamstrings are the agonists, and the quadriceps femoris are the antagonists. However, during the contact and recovery phase, when the quadriceps contract to extend the knee, the hamstrings lengthen to allow the movement, making the quadriceps the agonists and the hamstrings the antagonists.
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Synergists assist agonist muscles
An agonist muscle is the primary muscle involved in an action, also known as the prime mover. For example, when doing a bicep curl, the bicep brachii does most of the work and is the agonist. The brachialis and brachioradialis assist the bicep in this movement, making them synergist muscles.
In the case of forearm flexion, such as lifting a cup, the biceps brachii is the prime mover. The brachialis and brachioradialis, located in the forearm and upper arm respectively, are synergists that aid in this movement. They assist the agonist muscle by producing additional force or promoting stability.
During a hip flexion, the iliopsoas is the agonist or prime mover. The iliacus, psoas major, and rectus femoris act as synergists by working together to flex the hip joint. The anterior fibres of the gluteus minimus and gluteus medius can also assist with hip flexion, depending on the position of the hip. These muscles work together to ensure the hip joint can flex efficiently.
Synergists are essential in assisting agonist muscles to perform their functions optimally. They provide additional force, stability, and support, allowing for smooth and controlled movements.
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Fixators stabilise the body
An agonist is the primary muscle involved in an action, also known as the prime mover. For example, when doing a bicep curl, the bicep brachii does most of the work and is the agonist. The brachialis and brachioradialis assist the bicep in this movement, making them synergist muscles.
A fixator muscle is a stabiliser that acts to eliminate the unnecessary movement of an agonist or prime mover. Fixators stabilise the body by preventing or reducing movement at another joint, thereby stabilising the origin of the agonist. In other words, fixators allow the agonist muscle to work effectively by stabilising the origin of the agonist muscle so that the latter can pull against the bone without it moving, thus achieving an effective contraction. For example, the trapezius and rhomboids work isometrically to ensure that the scapula does not move on the torso.
The abdominals can act as fixators to stabilise the body for hip and knee movements. For instance, when a footballer prepares to kick a football, their hamstrings contract to flex the knee while the quadriceps lengthen to allow the movement. The hamstrings are the agonist, and the quadriceps are the antagonist. In the next phase, the roles reverse: the quadriceps contract to extend the knee while the hamstrings lengthen to allow the movement. The abdominals would be acting as fixators in this phase.
The trapezius muscle can also act as a fixator when the biceps are flexing the elbow joint.
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Agonist and antagonist muscles work in tandem
An agonist muscle is the primary muscle involved in an action, also known as the prime mover. For example, when lifting a cup, the biceps brachii is the prime mover. The brachialis and brachioradialis assist the bicep in this movement, making them synergist muscles.
The agonist is the muscle that contracts to produce force and create a specific movement. In a bicep curl, the bicep is the agonist, contracting to produce the movement, while the tricep is the antagonist, relaxing to allow the movement to occur.
The roles can reverse depending on the action. In a tricep exercise, such as a rope extension, the tricep becomes the agonist, and the bicep is the antagonist.
The agonist and antagonist muscles work together with synergist muscles, which assist the agonist muscle. Synergist muscles can also be fixators that stabilize the origin of the agonist, preventing or reducing movement at another joint.
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Frequently asked questions
An agonist muscle is the primary muscle responsible for generating a specific movement at a joint by contracting, opposing the action of the antagonist muscle.
An antagonist muscle is the muscle with the opposite function to the agonist. In any pair, the agonist muscle contracts, while the antagonist muscle relaxes or lengthens, allowing for the free movement of our joints and muscles.
When we flex our arm (with a bicep curl, for instance), the bicep is contracted, making it the agonist muscle, and the tricep is relaxed, and therefore the antagonist muscle in this scenario.
