A muscle that causes motion.


A muscle that can move the joint opposite to the movement produced by the agonist.


The primary muscle intended for exercise.


A muscle that assists another muscle to accomplish a movement.


A muscle that contracts with no significant movement to maintain a posture or fixate a joint.

Dynamic Stabilizer

A biarticulate muscle that simultaneously shortens at the target joint and lengthens at the adjacent joint with no appreciable difference in length. Dynamic stabilization occurs during many compound movements. The dynamic stabilizer assists in joint stabilization by countering the rotator force of an agonist. See example diagram: Hamstring weakness (during squat or leg press)

Antagonist Stabilizer

A muscle that contracts to maintain the tension potential of a biarticulate muscle at the adjacent joint. The antagonist stabilizer may be contracted throughout or at only one extreme of the movement. Also see active insufficiency. The Antagonist Stabilizer are activated during many isolated exercises when biarticulate muscles are utilized. The Antagonist Stabilizer may assist in joint stabilization by countering the rotator force of an agonist. For example, the Rectus Femoris contracts during lying leg curl to counter dislocating forces of Hamstrings. See knee flexion abduction force vector diagram (Rectus Femoris and Tibialis Anterior).