Functional spinal unit

A functional spinal unit (FSU) (or motion segment) is the smallest physiological motion unit of the spine to exhibit biomechanical characteristics similar to those of the entire spine.

A FSU consists of two adjacent vertebrae, the intervertebral disc and all adjoining ligaments between them and excludes other connecting tissues such as muscles. The three-joint complex that results is sometimes referred to as the "articular triad."