There is considerable evidence that awkward postures of the low back are related to the incidence of low back disorders (LBDs), 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 motion and the electromyographic activity of the back extensor musculature 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 at every 2.5 minutes. Results showed significant creep effects of the flexion angle and the increased activity of extensor muscles in 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 moderated these viscoelastic responses. A nonlinear viscoelastic 3D finite element (FE) model of the lumbar spine was developed to predict the responses of the passive and active tissues of the low back. Validation of the FE model by comparing its predicted results (range of motion, muscle activation levels, etc.) with experimental results indicated good agreement in terms of mechanical behaviors in stooping, confirming the capability of the FE model as a potential tool for risk assessment of the prolonged stooping tasks.
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