What muscles are commonly referred to as the "flex in the mirror" muscles, responsible for lower arm movement?

The muscles known as the "flex in the mirror" muscles, particularly associated with lower arm movement, refer to the biceps. When you flex your arm, the biceps brachii is prominently displayed, and this muscle is responsible for the flexion of the elbow and the lifting of the forearm.

The biceps are located on the front part of the upper arm and are well-known for their role in pulling movements, such as lifting and curling weights. Their prominence makes them the most visible muscle when you flex your arm, which is why they are commonly highlighted during displays of strength or fitness, particularly in a mirror.

While the forearm flexors also play a role in lower arm movement, particularly in gripping and wrist flexion, they are not as visually prominent when considering the simple action of flexing the arm at the elbow compared to the biceps. The triceps are involved in extending the arm and are located on the opposite side of the arm from the biceps, and the term "main biceps" is somewhat redundant as the primary biceps muscle is typically what is referred to. Thus, the most appropriate and widely recognized answer to the question is indeed the biceps.