Flexing of the forearm by the biceps brachii
The biceps brachii is the agonist, or primer mover, responsible for flexing the forearm. The triceps brachii (not shown) acts as the antagonist. The brachioradialis and brachialis are synergist muscles, and the rotator cuff (not shown) fixes the shoulder joint allowing the biceps brachii to exert greater force.
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