A systems-level perspective of the flexion-relaxation phenomenon in the lumbar spine

The current study presents a new musculoskeletal model of the active spinal stability system that includes the local system (e.g., multifidus muscles) and global system (e.g., lateral erector spinae, rectus abdominis muscles etc.) as proposed by Bergmark (1989), but then adds a super global system that considers the influence of the lower extremity tissues on the responses of the lumbar region. This innovative model was verified throughout in vivo experiments involving human subjects that included three different physical exertion tasks that stressed the low back and the lower extremities in different ways to explore these important interactions.;The empirical work in this dissertation focused on gathering data from the local, global and super global biomechanical systems before and after three 10 minute exercise protocols and then during a 40 minute recovery session. Twelve participants performed three separate experiments (three protocols) on different days: Protocol A- alternately perform 25 seconds of full trunk flexion and 5 seconds upright, relaxed posture; Protocol B- alternately perform 25 seconds of isometric exertion in a 45 degree trunk flexion posture and 5 seconds upright, relaxed posture; and Protocol C- consecutively perform 25 seconds of full trunk flexion followed by 5 seconds of upright, relaxed posture followed by 25 seconds of isometric exertion in a 45 degree trunk flexion posture and 5 seconds upright, relaxed posture. Kinematic and physiological measures were recorded before during after these protocols as well as during the recovery period.;The results of in vivo experiments, focused on the role of the pelvis/lower extremities in trunk flexion-extension, showed a 6.4% greater lumbar flexion angle (36° vs. 38.3°), a 10.2% greater (or later) EMG-off angle in multifidus (31.6° vs. 34.8°), and a 8% greater EMG-off angle in the iliocostalis (30.6° vs. 33°) in the restricted stooping posture than in the free stooping posture. Collectively, these results suggest that additional passive moments about the lumbar spine are generated in the restricted stooping posture because of the relative fixation of the pelvis that is seen during the restricted stooping condition. Consistent with these results, 22% greater lower extremity activation (10.5% MVC vs. 8.2% MVC) was observed in the free stooping posture, as compared to the restricted stooping posture. To summarize, these results indicate a significant role of the tissues of the larger super global system as a trunk stabilizer by immobilizing the pelvis during trunk flexion-extension motions and increasing the stiffness of the trunk systems by enhancing tension of the lumbodorsal fascia.;Regarding the effects of the 10 minute protocols on the biomechanical responses, results showed greater full lumbar flexion and deeper biomechanical equilibrium point between passive tissues and external moment (i.e., EMG-off angles) than the baseline (initial measure) after Protocol A: full lumbar flexion increased 7%; EMG-off angle increased 7.2% in multifidus and increased 7.8% in iliocostalis. In Protocol B the trends in the dependent variables were opposite to those seen in Protocols A: full lumbar flexion angle decreased by 4% and the EMG-off angles decreased by 4.9% in the multifidus and by 6.3% in iliocostalis. Protocol C (the mixed protocol) generated similar, but less pronounced results as compared to Protocol A: full lumbar flexion increased by 3.7%; EMG-off angles increased by 3.7% in multifidus and by 5.9% in iliocostalis.;In terms of the recovery process, the in vivo experiment, comparing characteristics of the recovery phase in three protocols, showed longer recovery time after the passive tissue elongation protocol (not fully recovered until 40 minutes of rest in all variables) than the muscle fatigue protocol (recovered after 5 minutes of resting in all variables) and the combined protocol (not fully recovered until 40 minutes of resting for the full lumbar flexion angle and the EMG-off angle; fully recovered in agonist muscle activation after 40 minutes of resting; and fully recovered in the synergist muscles after 5 minutes of resting).;The results of the theoretical modeling and experimental validation components of the current study indicate that a new musculoskeletal model with a more "systems-level" perspective is necessary to fully understand the biomechanical response of the lumbar spine during full flexion and near full flexion exertion. The results of this new systems-level biomechanical model can be used to develop a new EMG-assisted model of spinal loading and spinal stability as well as guidelines for designing safer working environments that can lower the risks of musculoskeletal injury to the low back. (Abstract shortened by UMI.)...