Viscoelastic Responses of the Lumbar spine during Prolonged Stooping

Abstract

There is considerable evidence that awkward postures of the low back are related to the incidence of low back disorders (LBDs). Specifically, the stooped or fully flexed posture maintained over a prolonged period of time has been known to lead to LBDs in many industrials tasks but the specific biomechanics/physiology of this link is not fully developed. This study combined empirical work with finite element analyses to explore this relationship.

The empirical work focused on quantifying the time-dependent responses of the lumbar spine during a prolonged stooped posture by assessing the changes in the sagittal plane range of lumbar flexion and the electromyographic activity of the back extensor musculature in the isokinetic lifts during and after prolonged stooping. Ten healthy participants performed a regimen of a 10-minute stooping period followed by a 10-minute upright standing recovery period, with an isokinetic lift every 2.5 minutes. Results showed significant creep effects of the flexion angle and the increased activity of extensor muscles during stooping to compensate for the reduced extensor moment producing capability of the passive tissues. The 10-minute upright standing did not produce a full recovery of the lumbar spine tissues but a 30-second rest break in the middle of the stooping period moderated these viscoelastic responses.

A three-dimensional finite element (FE) model of the lumbar spine was developed to predict the responses of the passive and active tissues of the low back during the prolonged stooping and recovery period. This model employed a nonlinear stress-strain relationship describing the viscoelastic material properties of individual components of the lumbar spine. The trunk flexion tasks that were performed in the in vivo empirical work were simulated in the FE model and the predicted results (range of motion, muscle activation levels, etc.) were compared with experimental results to validate the model. The predicted results by the FE model showed high correlation (R>0.9) with the in vivo experimental results, confirming the capability of the FE model as a potential tool for risk assessment of the prolonged stooping tasks.

Results of the in vivo experiment suggested the importance of proper duty cycles in reducing LBD risks due to repetitive prolonged stooping in work-related tasks. The FE model of this study showed potential to simulate various prolonged stooped postures in occupational tasks and predict time-dependent stress/strain of individual spinal tissues. The data from these simulations can be used to design better work postures and duty cycles that can reduce the risks for LBDs, without sacrificing work productivity.