Combination of Motions in Multiple Anatomical Planes

By Janis Savlovskis, MD

A century ago, the American physiologist Harrison Fryette defined three laws of spine biomechanics. The Third Law applies to the non-coupled combination of the spine motions and sounds like: when motion is introduced in one plane, it will modify (reduce) motion in the other two planes (Greenman 1989). The quantitative estimation of how one type of motion restricts other movements is complex, and articles dedicated to this topic are not numerous. The methodology of available studies is so inhomogeneous that the pooling of the results seems not rational.

The available literature data about the combination of cervical spine motions follows Fryette's Third Law (Edmondston 2005, Walmsley 1996, Bergman 2005, Feipel 1999). In the thoracic spine, the situation is more ambiguous. There is some evidence about no restriction in the thoracic axial rotation when the movement is initiated from the extended posture and some restriction from the flexed posture (Edmondston 2007, Montgomery 2011). However, the range of thoracic extension in habitual standing position is small and poorly correlated with the magnitude of the standing thoracic kyphosis (Edmondston 2011). Our biomechanical model demonstrates a slightly greater thoracic extension range that corresponds better to the loaded spine (prone or 4-point kneeling). This is why we applied the Third Fryette's law to both – the cervical and thoracic spine.

The biomechanics of the lumbar spine is controversial in terms of the combination of motion. Multiple studies suggest the "paradoxical" increase of the axial rotation and lateral bending of the lumbar spine when the movement is initiated from the flexed position (Pearcy 1991, Panjabi 1989, Drake 2008). And the decrease of the amount of lateral bending when the movement is initiated from the extended position (Ebert 2014, Panjabi 1989). Other studies do not find the increase in combined motion range mentioned above (Gunzburg 1991, Burnett 2008, Haj 2019). Finally, some suggest that the motion in one segment of the spine (i.e., thoracic) may affect the mobility in the other segment (lumbar) (Nairn 2014, Montgomery 2011). Our biomechanical model represents the more cautious and conservative approach: the flexion of the lumbar spine doesn't restrict any motion in other anatomical planes; In contrast, the extension, lateral flexion, and axial rotation interfere mutually according to the Third Freyett's law.

Implemention of the Friyette's Third Law in the application
"Biomechanics of the Spine"

Cervical spine in neutral position. Screenshot from the app Biomechanics of the Spine The maximal extension of the cervical spine. Screenshot from the app Biomechanics of the Spine
First published: Jan/2022

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