Final answer:
When the agonist muscle performs an eccentric contraction, the antagonist muscle controls and decelerates the movement to maintain balance and precision, such as when lowering a hand weight with the triceps providing resistance.
Step-by-step explanation:
When the agonist muscle group is activated in an eccentric manner, as in the example of the biceps brachii muscle lengthening while lowering a hand weight, the antagonist muscle group usually acts to control the movement. This means that the antagonist muscles, in this case, the triceps brachii, provide resistance and help decelerate the movement. The purpose of the antagonist is twofold: (1) they maintain body or limb position, and (2) they control rapid movement, particularly in actions requiring fine control such as lowering a weight smoothly.