Trunk extension from anatomical position is movement around the X axis in:

In the anatomical position, the body is standing upright with the arms at the sides and the palms facing forward. Trunk extension refers to the action of moving the trunk backward away from the thigh, typically achieved by contracting the muscles that extend the spine, such as the erector spinae.

When considering trunk extension in relation to axes of rotation, it is important to understand the orientation of these axes. The X axis generally runs from front to back (anterior to posterior), and movement around this axis can be described in terms of angular displacement.

In this context, trunk extension is characterized by extending the spine backward, which corresponds to a decrease in the angle formed between the trunk and the horizontal plane. This action can be represented as a rotation about the X axis in the negative direction, indicated by −θX. Hence, this choice accurately captures the nature of the trunk's movement during extension.

Understanding the other axes assists in clarifying why this answer is appropriate. The Z axis runs vertically (up and down), and movements around it are typically associated with rotation, such as twisting motions. The Y axis generally runs from side to side (lateral), and movements around this axis are related to lateral flexion or bending to the sides.