The Difference between Agonist and Antagonist Muscles

The Difference between Agonist and Antagonist Muscles

Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.


An agonistic and antagonistic muscle pair involve one muscle contracting while the other relaxes. A agonist is the muscle that contracts and lengthens, while an antagonist is the muscle that relaxes. During a muscle movement, the agonist is the one doing all the work. For example, when performing a bicep curl, the biceps will be contracted while the triceps will be in 'agony'.


An agonist and an antagonist are complementary muscle groups that work together to complete a specific action. They perform the same movement but cancel out any extra motion produced by the agonist. A strong synergist helps keep the body in place during movement. It is often called a stabilizer or fixator, and its primary function is to aid in flexing and extending the biceps.

During movement, opposing muscles work together to keep the body balanced. The quadriceps femoris and biceps act as an agonist and antagonist. The quadriceps femoris, which is in the anterior compartment of the thigh, opposes the quadriceps and hamstrings, which act together during knee extension and flexion. In addition, the biceps and triceps work together to extend the forearm and flex the wrist.

A muscle that is complementary to an agonist and antagonistic is known as a synergist. A synergist is a group of muscles that perform opposite actions at the same joint. They do this by coordinating their actions. In many instances, this is true. One example is the hamstrings, which work together to stabilize a knee joint. Another example is the semitendinosus, which performs knee flexion and internal rotation. The biceps femoris and semitendinosus muscles contract during knee flexion.

A synergist is a muscle that works in conjunction with a prime mover. Both muscles contract at the same time to provide a joint with the motion it requires. The synergist stabilizes the joint by holding it in position while the prime mover generates the movement. The prime mover is the agonist, while the antagonist is responsible for returning the limb to its initial position.


Neutralizers between agonistic and anti-agonistic muscles are muscles that act to prevent the unwanted movement caused by the underlying limb muscle or joint action. In particular, they cancel out the line of pull caused by a dominant agonist. Since many muscles are capable of generating pulling forces in two directions, they are often required to perform at least two separate actions in a joint. This is why it is essential to have neutralizers in your exercise routines.

In the elbow, for example, the biceps and triceps move the elbow joint in opposite directions. The agonists tend to flex the wrist while the antagonists neutralize the action. To compensate for this, stabilizing muscles surround the proximal joint and contract to permit smooth distal joint movement. In turn, a stabilizer muscle helps the agonist by providing additional tension. These muscles can be antagonistic or synergist.

The role of neutralizers between agonistic and antagonistic muscle pairs is vital for efficient action. These muscles must be complementary to each other to be effective. Reciprocal inhibition is one method to induce antagonists to relax while the agonists contract. In other words, when the hamstrings and quadriceps are relaxed, the antagonists relax. This makes the limb move easier.

An agonist is the primary muscle involved in movement. It contracts to move the bone. Its antagonists relax in opposite directions, opposing unwanted planes of motion. The result is movement. The agonist is the primary muscle responsible for certain joint motion, while the antagonists act in the opposite direction. In contrast, the antagonistic is a supporting muscle that resists movement carried out by the prime movers.

Independent descending control

In an experiment to examine the role of the Ia afferents in posture and movement, we altered the deafferenting to model abnormal afferent control. By deafferenting the Ia-PN, we set all related gains to zero while keeping all other commands intact. In addition, we analyzed the effect of deafferentation on the final posture. Furthermore, we tested whether the Ia-PN's deafferentation had any effect on the final posture.

In this study, the PN mediates motor commands in reaching movements. The descending commands were categorized into static and dynamic modules, depending on the type of movement. A common approach is to segment descending commands into a posture module and a movement module. As a result, we were able to measure the descending commands of each of these modules independently. Further, we also showed that descending commands are tuned to fit a variety of reach-and-hold movements.

The results from this study also showed that the difference between the initial and final joint angle was smaller than that found in the corresponding test with the passive antagonist muscle. This result indicates that mutual compensation may have the potential to minimize undesirable effects of hysteresis in the antagonistic muscle system. The result of this study was a model incorporating a functionally relevant model of the descending motor system.

The descending motor pathways in the spinal cord modulate reflex circuits. The descending pathways change the gain and threshold of a reflex. These changes depend on the behavioral context in which the flexor and antagonistic muscles are activated. A simple geometric model of the artificial joint was used to study the agonist-antagonist interaction. This simulation also showed how descending pathways influence the onset of a reflex.

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Isometric contraction

To maintain body balance and prevent injury, opposing muscles are essential. In fact, there are major and minor opposing muscles in the neck, wrists, ankles, and neck. These muscles include flexors and extensors. The latter are responsible for the movement of the fingers and the wrist. Extensors are responsible for the movement of the wrist and hamstrings for the ankles.

A study comparing voluntary isometric step contractions of elbow muscles in normal subjects and paretic patients determined the maximum isometric torque developed during the elbow's flexion and extension. The associated agonist and antagonist EMGs were recorded to assess the relationship between the two. All subjects, except for a paretic spastic patient, showed a linear relationship between the two variables.

The main difference between isometric and eccentric muscle contractions is that the isometric contraction produces more force than the concentric or eccentric contraction. This is because isometric contraction produces more force but is not shortening the muscle. Moreover, this type of muscle contraction produces a constant tension, and the length of the muscle remains constant. This is why it is widely used to compare the functional properties of different muscle types.

An agonist is a muscle that contracts, while the antagonist is a muscle that resists. This is the case in soccer, for example. The bicep is the agonist while the tricep relaxes. The other way around, the tricep is the antagonist when the knee is extended against gravity. When the agonist muscle is stretched, it creates tension in the antagonist.

Reciprocal activation

Reciprocal activation of agonistic and antagonistic muscle is an efficient mechanism for precise regulation of joint movement. The co-activation of antagonistic and agonistic muscles decreases hysteresis aftereffects and ensures a more accurate estimation of joint angle. However, the mechanism may not be appropriate for all joint movements. In order to investigate the effect of reciprocal activation, a more detailed study is needed.

Various studies have investigated the mechanisms underlying reciprocal inhibition. In SCI, the role of spinal interneurons in the process of disynaptic reciprocal inhibition of motoneurons is considered. Since the Ia inhibitory interneurons receive excitatory input from many descending systems, lesions in the spinal cord may decrease the excitability of these interneurons, resulting in decreased inhibition of antagonist muscle innervation.

This model can be used to study aging-related changes in coordination. In this study, seven healthy subjects performed arm extensions to 36 degrees of amplitude. Electromyograms of elbow extension showed that the agonist and antagonistic muscles were simultaneously activated during this movement. The coactivation of agonistic and antagonistic muscle activation was altered in the experiment because the subjects were instructed to fix their upper arms at the target before the movement, while the relaxation command reduced it. However, basic features of reciprocal activation remained the same.

We determined the relationship between agonist and antagonistic muscle activation by recording the reflex response to a muscle tap. This study supported the idea that the antagonist muscle reflex response is a genuine reflex response that is distinct in temporal and spatial parameters. Further, we showed that RFs of agonist muscles were significantly stronger than those of antagonistic muscles. This finding was further supported by the results of a previous study involving patients with SCI.

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