Agonist Muscle: Definition & Example

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  • 0:02 How Your Body Moves
  • 0:36 Anatgonistic Muscle Pairs
  • 1:44 Agonist vs Antagonist
  • 2:57 Examples of Antagonist…
  • 3:40 Lesson Summary
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Lesson Transcript
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.

This lesson discusses how your bones and muscles work together to move your body. It particularly focuses on antagonistic muscle pairs and agonist and antagonist muscle groups.

How Your Body Moves

The next time you play fetch with your dog, notice how her muscles move as she runs. Like us, your pup's muscles and bones work together so she can move. The bones are connected at moveable joints, and the muscles pull on the bones, bending the joints and moving her limbs and body in all directions.

This is one of the primary functions of our (and your dog's) musculoskeletal system. The musculoskeletal system contains all of the bones and muscles in our body, as well as the cartilage, ligaments and tendons that hold them all together.

Antagonistic Muscle Pairs

Let's go back to our furry friend. While watching her run, you may notice that her muscles ripple in a rhythmic pattern. In her haunches and shoulders, for example, the muscles contract and relax as the legs move back and forth. What you are watching is the motion of antagonistic muscle pairs. Muscles contract, or shorten, and pull on bones to move them at the joint. Your biceps, for example, contract to fold your forearm in towards your body. But muscles can only contract in one direction. So, is your arm forever stuck in the folded position? We know this isn't true.

This is where the second muscle group in the antagonistic muscle pair, the triceps, come in. After the biceps bend your arm at the elbow, they relax, and the triceps take over, contracting to move your arm back to the straight position. Just as you can see this movement on your dog, you can feel it on yourself. Take a moment to try it out. Flex and straighten your arm a few times with your other hand on your biceps. You will feel your biceps contracting and relaxing as your arm moves. Try the motion again this time with your other hand resting on your triceps.

Agonist vs Antagonist Muscle

Antagonistic muscle pairs are made up of agonist and antagonist muscle groups. The agonist muscle group is also referred to as the prime mover because it is the muscle group that provides the main pull to create a movement. It is the group of muscles that contract to move a joint. The antagonist muscle group opposes the agonist. The antagonist muscle relaxes so that the prime mover can contract and rotate the bone at its joint.

Let's go over that again:

  • Agonist muscles contract
  • Antagonist muscles relax

You can remember this by thinking of how much work the agonist muscles do, it must be 'agony'! It's important to remember that a muscle group can be either an agonist or an antagonist based on its role at a particular moment. For example, when you are bending your forearm towards your body, your biceps are contracting and bending your arm. Thus, your biceps are the agonist muscle group. At the same time, your triceps are relaxing, so your triceps are the antagonist muscle group. When you straighten your arm back out, your triceps contract and become the prime movers. In this case, your triceps are the agonist muscle group; and, since your biceps are relaxing, your biceps are the antagonist group.

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