Flexion and extension movements are typically described using which notation?

Flexion and extension are movements that occur primarily around the sagittal plane, which separates the body into left and right halves. In biomechanical notation, movements along the axes of a three-dimensional Cartesian coordinate system are represented by their corresponding angular motion.

The notation for movements generally follows a standard convention where each axis is defined: the X-axis typically represents the anterior-posterior axis (forward and backward motions), the Y-axis represents the medial-lateral axis (side to side motions), and the Z-axis represents the vertical axis (up and down motions).

Since flexion and extension occur along the sagittal plane, they are associated with rotation around the X-axis. The use of ±θX indicates that the movement can be either flexion (a negative angle) or extension (a positive angle) about this axis. Thus, this notation accurately captures the essence of how flexion and extension are quantified in a biomechanical context, justifying why this choice is the correct answer.