What is the main function of biceps during a bicep curl?

The main function of the biceps during a bicep curl is to perform a concentric contraction to lift the weight. In this exercise, as the elbow flexes, the biceps muscle shortens, generating the force necessary to lift the weight against gravity. This contraction is the primary action that allows the lifter to raise the weight from a lowered position to a curled position.

While the other aspects mentioned have their relevance in various contexts—like stabilizing the shoulder joint which helps in maintaining overall arm position, or the possibility of lengthening or stretching during other exercises—the specific action during a bicep curl is focused on the biceps actively contracting to enable movement and lift the load.