Spinal flexion and extension primarily occur around which axis?

In biomechanics, spinal flexion and extension primarily occur around the x-axis. This axis runs from side to side (medial to lateral) through the body. When the spine flexes, it bends forward, reducing the angle between the anterior surfaces of the vertebrae, and when it extends, it moves back to an upright position. The motion around this axis is critical in understanding movements related to posture, lifting mechanisms, and core stability during various activities.

Considering the other options, the y-axis typically corresponds to a vertical plane, which is involved in lateral flexion or side-bending motions of the spine. The z-axis runs from front to back (anterior to posterior) and is associated with rotation of the spine. The k-axis, while not a standard term in biomechanics, would not represent a primary axis of movement in the typical discussions of spinal motions. Therefore, focusing on the x-axis helps clarify how flexion and extension mechanics operate.