Final answer:
During the downward part of a bicep curl, the biceps are eccentrically contracting and lengthening, while the triceps are relaxing and passive as they are not required to act.
Step-by-step explanation:
During the downward part of a bicep curl, the biceps are lengthening or eccentrically contracting, and the triceps are relaxing or staying passive. This is because during a bicep curl, the biceps and triceps act as opposing muscles. When performing the upward movement, or the concentric phase of the curl, the biceps contract to bend the elbow, and the triceps relax.
However, during the downward motion, which is also known as the eccentric phase, the biceps are still active, but they are lengthening while controlling the descent of the weight. This controlled motion requires the biceps to exert force while elongating, which is why it is considered an eccentric contraction. Meanwhile, the triceps remain passive because their main function, to straighten the elbow by contracting, is not needed in this phase of the exercise.