Which anatomical motion is an example of movement around the X axis?

Movement around the X axis refers to actions that occur in the sagittal plane, which divides the body into left and right portions. Trunk flexion and extension are prime examples of this motion, as they involve bending forward (flexion) and bending backward (extension) about the X axis. This means that as the trunk flexes forward, it moves in such a way that the angle between the front of the body and the thighs decreases, and during extension, that angle increases.

The other choices involve movements that occur in different planes or around different axes. Trunk rotation involves movement around a vertical axis and occurs in the transverse plane, while lateral flexion describes movement side to side, which occurs in the frontal plane. Shoulder internal rotation also takes place around a different axis, related to movements of the arm in the horizontal plane. Thus, the movement of trunk flexion and extension specifically embodies motion around the X axis, clearly illustrating its definition and relevance in biomechanics.