A systems-level perspective of the biomechanics of the trunk flexion-extension movement: Part I – Normal low back condition

•We examined the role of pelvis and lower extremity during trunk flexion-extension.•Greater lumbar flexion was observed in the lower extremity constrained condition.•Later flexion-relaxation was observed in the lower extremity constrained condition.•Greater EMG in low back was observed in the lower extremity constraints condition.•A complementary interaction between trunk and lower extremity exists.

Most of the previous studies of the lumbar region have not considered the influence of pelvic and lower extremity characteristics on the performance of the lumbar region. The goal of the current study was to explore these more systems-level effects by assessing the effects of a pelvic/lower extremity constraint on the biomechanical response of the lumbar spine in an in-vivo experiment. Twelve participants performed full range of motion, sagittal-plane trunk flexion-extension movements under two conditions: unconstrained stoop movement and pelvic/lower extremity constrained stoop movement (six repetitions in each condition over three days). Kinematics and muscle activities of the trunk and lower extremity muscles were monitored. Results showed a significant effect of pelvic/lower-extremity constraint on a number of lumbar performance measures. Trunk flexion angle was, as expected, significantly reduced with the lower extremity constraints (81° (free stoop) vs. 56° (lower extremity constrained)). At a more local level, there was a 6.4% greater peak lumbar flexion angle and a 9.1% increase in the lumbar angle at which the trunk extensor musculature demonstrated flexion-relaxation in the constrained stooping condition as compared to the unconstrained stooping condition. Also, the EMG of the L3/L4 paraspinals was greater in the restricted stooping as compared to the free stooping (16.3% MVC vs. 15.1% MVC).Relevance to industryLow back injuries are a significant challenge to many industries and developing accurate models of spinal stress at full stooping postures can help in the development of appropriate interventions to reduce prevalence.