Passive lumbar tissue loading during trunk bending at three speeds: An in vivo study

Low back disorders are closely related with the magnitude of mechanical loading on human spine. However, spinal loading contributed by the lumbar passive tissues is still not well understood. In this study, the effect of motion speed on lumbar passive moment output was investigated. In addition, the increase of lumbar passive moment during trunk bending was modeled.

Twelve volunteers performed trunk-bending motions at three different speeds. Trunk kinematics and muscle activities were collected and used to estimate instantaneous spinal loading and the corresponding lumbar passive moment. The lumbar passive moments at different ranges of trunk motion were compared at different speed levels and the relationship between lumbar passive moment lumbar flexion was modeled.

A non-linear, two-stage pattern of increase in lumbar passive moment was evident during trunk flexion. However, the effect of motion speed was not significant on lumbar passive moments or any of the model parameters.

As reported previously, distinct lumbar ligaments may begin to generate tension at differing extents of trunk flexion, and this could be the cause of the observed two-stage increasing pattern of lumbar passive moment. The current results also suggest that changes in tissue strain rate may not have a significant impact on the total passive moment output at the relatively slow trunk motions examined here.