Passive stiffness of the human neck in flexion, extension, and lateral bending.

The passive bending stiffness, tolerable bending moment levels, and elastic energy storage capabilities of the cervical spine were evaluated in 40 male and 19 female volunteer subjects from a university student population, about the flexion, extension, and lateral bending axes. Bending moments were applied to the neck while subjects lay on a frictionless jig and simultaneous angular displacements of the skull relative to the rib cage were recorded. Myoelectric signals from the sternocleidomastoid and splenius capitis muscles were amplified over audio speakers to train subjects to relax their musculature. Tolerable levels of bending moments were quite low (less than 10 N m on average for all conditions) given the very large magnitude of bending moment applied to the neck estimated from impact models. Subjects tolerated greater moment and elastic energy storage in the cervical passive tissues during flexion than in both extension and in lateral bending. In addition males were able to tolerate larger applied moments, were stiffer, and stored more energy than females in all loading conditions. The data derived from this study of loading at very slow rates provide a lower limit for bending moments and for cervical spine stiffness, and yield a first approximation to dynamic bending stiffness values that may be included in models of the cervical spine.