Laws of Physiologic Spinal Motion

Harrison Fryette proposed three laws of physiologic spine motion that help clinicians interpret asymmetrical motion patterns found during an examination. Fryette’s Laws can be summarized as follows:

Law I. If the anterior-posterior (AP) curve of the spine is in a neutral or mid-range position, side bending and rotation occur to opposite sides.

Law II. If the spinal segment is in a non-neutral (flexed or extended) position, side bending and rotation occur to the same sides. This coupling of movement is due to the influence of the zygapophyseal joints upon segmental motion.

Law III. If a segment is moved into one of the cardinal planes of motion, the available motion in the other two planes is reduced.