How is side-bending the trunk to the left categorized in terms of movement direction around the Z axis?

Side-bending the trunk to the left involves lateral flexion, which is a movement that occurs primarily in the frontal plane. When considering the movement in terms of rotational directions around the three anatomical axes, side-bending is classified as a diagonal or angular movement around the Z axis (which is oriented vertically).

In biomechanical terms, movements around the Z axis are generally described as positive or negative based on the right-hand rule; if the movement is performed to the left (in this case, bending the trunk to the left), it is categorized as negative rotation. This aligns with the conventions used in biomechanics where leftward movements around the Z axis are designated as negative because they reflect a counterclockwise rotation from the perspective of an observer facing the trunk.

Therefore, categorizing side-bending the trunk to the left as negative is consistent with the biomechanical definitions for movements around the Z axis.