<P> Agonist muscles and antagonist muscles refer to muscles that cause or inhibit a movement . </P> <P> Agonist muscles cause a movement to occur through their own activation . For example, the triceps brachii contracts, producing a shortening contraction, during the up phase of a push - up (elbow extension). During the down phase of a push - up, the same triceps brachii actively controls elbow flexion while producing a lengthening contraction . It is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action . Agonists are also interchangeably referred to as "prime movers," since they are the muscles considered primarily responsible for generating or controlling a specific movement . </P> <P> Another example is the dumbbell curl at the elbow . The "elbow flexor" group is the agonist, shortening during the lifting phase (elbow flexion). During the lowering phase the "elbow flexor" muscles lengthen, remaining the agonists because they are controlling the load and the movement (elbow extension). For both the lifting and lowering phase, the "elbow extensor" muscles are the antagonists (see below). They lengthen during the dumbbell lifting phase and shorten during the dumbbell lowering phase . Here it is important to understand that it is common practice to give a name to a muscle group (e.g. elbow flexors) based on the joint action they produce during a shortening (concentric) contraction . However, this naming convention does not mean they are only agonists during shortening . This term typically describes the function of skeletal muscles . </P> <P> Antagonist muscles are simply the muscles that produce an opposing joint torque to the agonist muscles . This torque can aid in controlling a motion . The opposing torque can slow movement down - especially in the case of a ballistic movement . For example, during a very rapid (ballistic) discrete movement of the elbow, such as throwing a dart, the triceps muscles will be activated very briefly and strongly (in a "burst") to rapidly accelerate the extension movement at the elbow, followed almost immediately by a "burst" of activation to the elbow flexor muscles that decelerates the elbow movement to arrive at a quick stop . To use an automotive analogy, this would be similar to pressing your gas pedal rapidly and then immediately pressing the brake . Antagonism is not an intrinsic property of a particular muscle or muscle group; it is a role that a muscle plays depending on which muscle is currently the agonist . During slower joint actions that involve gravity, just as with the agonist muscle (mentioned above), the antagonist muscle can shorten and lengthen . Using the example above of the triceps brachii during a push - up, the elbow flexor muscles are the antagonists at the elbow during both the up phase and down phase of the movement . During the dumbbell curl, the elbow extensors are the antagonists for both the lifting and lowering phases . </P>

Which muscle is considered the primary agonist of elbow extension