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Antagonists counter the movements of agonists to allow your joints to move.
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When you perform any exercise, the muscle that makes the movement happen - that is, the one that does the work and gets tired - is called the agonist. That's easy to remember, because once you "feel the burn," it can seem like a sort of agony. But no muscle works in total isolation, and for every muscle that produces a movement, there's another muscle that produces the opposite movement at the same joint. This muscle is known as the antagonist. If it contracted at the same time as the agonist, your joint would never move. Instead, the antagonist muscle usually lengthens to allow motion as the opposing agonist muscle contracts.
When a muscle contracts to move a joint - such as the biceps bending your elbow - it is called the agonist. The antagonist is the muscle group opposite the active muscle. In this example, the antagonist is the triceps.
Consider the Triceps and Biceps
Your triceps and biceps are some of the best examples for understanding agonist/antagonist muscles. Each is the primary mover for opposing motions of the elbow: Your biceps muscle flexes your arm at the elbow, while your triceps extends the arm at the elbow. So when you do biceps curls, your biceps is the agonist muscle, because it's powering the primary movement. Your triceps is the antagonist muscle.
If you reverse the motion, doing triceps extensions or press-downs against resistance, your triceps becomes the agonist muscle because it powers the motion, while your biceps becomes the antagonist muscle that passively lengthens so that your triceps can move the elbow joint.
ID Other Muscle Pairings
Every skeletal muscle in your body has an antagonist that performs the opposite motion at the same joint. For example, your pectoral or chest muscles swing your arms together in front of you; your back muscles are the antagonists to this movement because if given the opportunity, they'd pull your arms back. Your quadriceps, in the front of your thigh, extend your leg at the knee; in which case your hamstrings are the antagonists that passively go along to allow this motion. If the muscle roles were reversed and your hamstrings were the agonists being used to bend your leg at the knee, your quadriceps would become the antagonist muscles.
When the Antagonist Flexes
Although, as a general rule, the antagonist muscle passively lengthens to let the agonist muscle do its job, there are a few notable exceptions. The first is in isometric contractions, in which your muscles contract but don't produce any joint movement. Some examples of this include wall sits, holding a high or low plank position, or standing in the "up" position of a calf raise. During isometric contractions, both of the muscles in an agonist/antagonist pairing contract at once; in fact, it's these opposing contractions from both muscles that allow you to hold that joint position.
The other time your antagonist muscle will kick in is to help the agonist muscle with suddenly slowing down or stopping a movement; the shorter and faster the movement you're stopping, the more the antagonist muscle kicks in to help the agonist muscle control movement at the joint in question.