How do you determine muscle agonists, antagonists, and synergists?
In this example, biceps brachii is the agonist or prime mover. Triceps brachii is the antagonist and brachialis is a synergist with biceps brachii.
As we begin to study muscles and their actions, it’s important that we don’t forget that our body functions as a whole organism. Although we learn the actions of individual muscles, in real movement, no muscle works alone. While we often have one main muscle to do an action, it is nearly always assisted in that action by other muscles. To keep things in balance in the body we also nearly always have a muscle that is assisting, resisting, or opposing any action. Let’s take a look at how we describe these relationships between muscles.
Muscle agonists
We describe the main muscle that does an action as the agonist. It is sometimes also called the “prime mover”. Many actions in the body do have one muscle that is responsible for more of the work in that action than any other muscle. For example, the agonist, or prime mover, for hip flexion would be the iliopsoas. Although it does not work alone, iliopsoas does more of the work in hip flexion than the other muscles that assist in that action.
Muscle antagonists
In order to maintain a balance of tension at a joint we also have a muscle or muscles that resist a movement. The main muscle that resists a movement is called the antagonist. We could also say that the antagonist is the main muscle that does the opposite of the action that it is resisting. For example, we could say that gluteus maximus is an antagonist of the primary hip flexor, iliopsoas because gluteus maximus is a hip extensor. Gluteus maximus is an antagonist of iliopsoas, which does hip flexion, because gluteus maximus, which does extension of the hip, resists or opposes hip flexion.
Muscle synergists
We describe muscles that work together to create a movement as synergists. For example, iliacus, psoas major, and rectus femoris all can act to flex the hip joint. There are some sections within other muscles that can also assist with flexion of the hip joint, for example, the anterior fibers of gluteus minimus and gluteus medius can assist with flexion of the hip joint, depending on the position of the hip when it’s being flexed. All of these muscles together could be referred to as synergists for flexion of the hip joint.
In real life, outside of anatomical position, we move our body in all kinds of creative and interesting ways. While we need the main muscle, or agonist, that does an action, our body has a good support system for each action by using muscle synergists. Likewise, our body has a system for maintaining the right amount of tension at a joint by balancing the work of a muscle agonist with its antagonist. Balance between a muscle agonist, its synergists, and its antagonist(s) is important for healthy movement and avoiding pain and injury.
Want to learn more about terminology and the language of kinesiology? Check out our articles: What Is Anatomical Position? and What Is Muscle Origin, Insertion, and Action?
