Muscles work in pairs to produce movement at the joints. As one muscle, the prime mover, contracts, an opposing muscle, the antagonist, must relax. For example, when the biceps brachii on the anterior surface of the upper arm contracts to flex the arm, the triceps brachii on the posterior surface must relax. When the arm is extended, these actions are reversed. In a given movement, the point where the muscle is attached to a stable part of the skeleton is the origin; the point where a muscle is attached to a moving part of the skeleton is the insertion.

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