The 5 Primary Roles of Muscles


In my previous post there was brief mention of agonist versus stabilizing muscles. When referring to agonist and stabilizing muscles we are referring to the type of action the muscle performs. When it comes to this level of functionality, there are five primary roles that are referred to:

  • Agonist
  • Accessory
  • Antagonist
  • Stabilizer
  • Neutralizer


Agonists are the primary muscles responsible for movement via isotonic or isometric concentric contractions. Isotonic contractions create a movement, isometric contractions maintain a position. A concentric contractions refers to a muscle that is producing power as it’s fibers shorten versus eccentric contractions where a muscle produces power as it’s fibers lengthen.

Example: The hamstring isotonically and concentrically contracts to flex the knee joint.


Accessory muscles assist agonist muscles.

Example: The quadricep is an agonist knee extensor and an accessory hip flexor.


Antagonist perform the opposite action of the agonist.

Example: If the hamstring is the agonist for knee flexion, it acts antagonistically to the quadricep as it eccentrically slow down knee extension. Think about what it feels like to slowly rise up from a squat.


Stabilizers prevent movement of a joint, usually via an isometric contraction

Example: the quadriceps may stabilize the knee in an extended position of permit plantar flexion of the ankle. While standing, rise up on to your tippy toes, do you feel how your quadriceps work to stabilize your knees.


Neutralizers prevent a motion, so another specific motion can occur. Unlike stabilizing muscles which act on joints, neutralizers act of other muscles.

The example found at is fundamentally important: The biceps can flex the elbow and supinate the forearm. If only elbow flexion is wanted, the supination component must be ruled out. Therefore, the pronator teres, which pronates the forearm, would contract to counteract the supination component of the biceps, and only elbow flexion would occur. Neutralizers act to cancel out an unwanted movement. Try doing some biceps curls in your palms facing up versus facing down, notice the difference.

Stabilizers and Neutralizers both use co-contraction to prevent motion and have an antagonistic relationship.


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