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Agonist Muscle Contraction

Dan Washmuth, Joanne Abramson
  • Author
    Dan Washmuth

    Dan has taught college Nutrition and Anatomy courses for several years. He has a B.S. in Exercise Physiology from Furman University and a M.S. in Dietetics & Nutrition from Florida International University. He is a Registered Dietitian (RD) and a Certified Exercise Physiologist (EP-C)

  • Instructor
    Joanne Abramson

    Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Explore agonist muscles contractions. Learn about agonist vs antagonist muscles, understand how opposing groups of muscles work, and see some some examples of agonist muscles. Updated: 02/09/2022

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Musculoskeletal System

The human musculoskeletal system consists of all the bones, muscles, cartilage, tendons, and ligaments of the body, and these structures work together primarily to create movement. Muscles are connected to bones by tendons. When a muscle contracts, it will shorten and pull on a bone. When the muscle pulls on a bone, it will move the bone and the body part it supports.

For example, the biceps brachii is the large muscle located at the front of the upper arm, and it is connected to the radius (long bone of the thumb side of the forearm). When the biceps brachii contracts, it will shorten and pull the radius upward toward the upper arm. This action causes the arm to bend at the elbow which is a movement known as elbow flexion.


The musculoskeletal system consists of the bones, muscles, cartilage, tendons, and ligaments and functions primarily to produce movement in the body.

musculoskeletal system


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  • 0:02 How Your Body Moves
  • 0:36 Anatgonistic Muscle Pairs
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Agonist Muscle Contraction

Define agonist: An agonist (also known as a prime mover) is a muscle that contracts to provide the main force to move or rotate a bone through its joint. An agonist muscle contraction refers to the contraction and shortening of the agonist muscle.

For example, the biceps brachii is the agonist muscle for elbow flexion, since this muscle is the primary muscle responsible for a person bending their elbow. There are other muscles that assist in this movement, such as the brachialis, but the biceps brachii is the prime mover agonist of elbow flexion. Since the brachialis assists in elbow flexion, this muscle is known as a synergist. A synergist is a muscle that assists an agonist in moving a specific part of the body.

Agonist vs Antagonist Muscles

How do opposing groups of muscles work? An antagonist muscle refers to a muscle that produces the opposite action of an agonist. For example, the biceps brachii functions to produce the movement of elbow flexion. The opposite movement of bending the elbow would be straightening the elbow. Straightening the elbow is a movement known as elbow extension. The muscle that functions to straighten the elbow is the triceps brachii which is the large muscle located at the back of the upper arm. Therefore, the triceps brachii is the antagonist of the biceps brachii, since the triceps brachii produces the opposite movement as the biceps brachii. It should be noted that agonists and antagonists are usually located on opposite sides of a body part. For example, the biceps brachii and triceps brachii are located on opposite sides of the upper arm.


The biceps brachii and triceps brachii function as agonists and antagonists during elbow flexion and elbow extension.

biceps and triceps


Some opposite movements of the body include:

  • Flexion vs Extension: Flexion involves decreasing the angle of a joint, while extension involves increasing the angle of a joint.
  • Lateral Rotation vs Medial Rotation: Lateral rotation involves rotating a body part away from the midline of the body, while medial rotation involves rotating a body part toward the midline of the body
  • Abduction vs Adduction: Abduction involves moving a body part away from the midline of the body, while adduction involves moving a body part toward the midline of the body. For example, when doing jumping jacks, raising the arms up to the sides of the body is abduction, while moving the arms back down to the sides is adduction.

Agonists and antagonists work as pairs. What this means it that when one muscle contracts and shortens, the other muscle will relax and lengthen. For example, while the biceps brachii is contracting and shortening to flex the elbow, the triceps brachii is relaxing and lengthening to allow elbow flexion to occur. Then, during elbow extension, the biceps brachii will relax and lengthen while the triceps brachii will contract and shorten.

Agonist Muscle Example

In addition to the biceps brachii, there are several other examples of agonist muscles in the human body. The following sections describe a couple of these agonists.

Hamstrings

The hamstring is a collection of three muscles located at the back of the upper leg. The three muscles of the hamstrings include the biceps femoris, semimembranosus, and semitendinosus, and they are attached to the bones of the lower leg. When these muscles contract, they pull the lower leg up towards the back of the body to bend the knee. Bending the knee is known as knee flexion, and the hamstrings are the agonists of knee flexion.

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Frequently Asked Questions

What is the role of an antagonist muscle?

An antagonist is a muscle that produces the opposite action of an agonist. When an agonist muscle contracts and shortens to move a body part, an antagonist will relax and lengthen to allow the movement to occur.

What is an agonist muscle and antagonist muscle?

An agonist muscle is a muscle that contracts to provide the main force to move or rotate a bone through its joint. An antagonist muscle is a muscle that produces the opposite action of an agonist.

What muscles are agonist?

There are many muscles that are classified as agonists in the human body. A few specific examples include the biceps brachii (during elbow flexion), hamstring (during knee flexion), and tibialis anterior (during dorsiflexion).

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