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Abstract
Work related musculoskeletal disorders (WMSD) have been identified as one of the leading work-related injuries. Low-back injuries constitute a substantial portion of WMSD and affect the population at large resulting annually in billions of dollars in direct workers compensation costs. The heavy burden of the low-back injuries on the society has motivated the search to understand the parameters affecting low-back injuries and to develop new methods for prévention as well as treatment of associated disorders.
The anatomical and physiological studies have demonstrated the relative contribution of the passive system (vertebrae, intervertébral dises and ligaments), the active system (group of muscles to transfer forces to the vertébral column) and the central nervous system in the opération of the trunk during various tasks. Previous studies have attempted to investigate biomechanics of the lumbar spine accounting for the active and passive Systems. During the daily occupational and recreational activities, the adaptive changes in the lumbar lordosis and pelvic rotation have been observed.
The response of the lumbar spine under large compression loads and sagittal flexion rotations similar to those experienced in occupational industrial lifting tasks remains yet unclear. Studies on the response of the entire lumbar spine in compression should inevitably be coupled with the considération of the lumbar lordosis and pelvic rotation. The objectives of this work are, hence, set as follows:
• Study of the synergy of active-passive lumbar spine under an axial compression load o f2800 N in quasi-neutral position. • Development of an optimal posture accounting for the adaptive capacity of the vertébral column in order to withstand the large compression loads of 2800 N in quasi-neutral position.
• Evaluation of muscular forces in the active system in order to maintain the equilibrium of the human body in such optimal posture.
To perform this study, a simplified finite element model, using deformable beam and rigid elements, is developed to model the nonlinear behavior of the passive system of the human lumbar spine. This is done to preserve both the accuracy in prédictions and the cost-effîciency in subséquent analyze. The nonlinear properties of the beam elements are determined based on the computed response under large loads of a 3D detailed model developed by Shirazi-Adl (1994).
The concept of optimal posture is subsequently exploited to examine the potential of the active and passive Systems to sustain the load in the quasi-neutral position. For this purpose, the sagittal moments required to equilibrate the posture subjected to an axial compression load of 2800 N are minimized and the corresponding changes in the posture and pelvic rotation are determined.
The originality of this work is to develop the muscle force évaluation algorithm considering the optimal posture configuration. This algorithm is based on the hybrid optimization-finite element approach to predict the equilibrium of active-passive system by taking into account a muscle architecture with 46 muscles. The hybrid fïnite element force-displacement method is used to ensure the compatibility between the applied loads and inter-segmental rotations obtained for the optimal posture.
The observation of the effect of variation in lordosis and pelvic rotation in reducing the required moments for equilibrium are in agreement with others (Shirazi-Adl and Pamianpour, 1996; Kiefer et al., 1997). In fact, a lordosis flattening of 20° reduces significantly the sum of the sagittal moments required for the equilibrium of the lumbar spine from 54,5 N.m to 1,5 N.m. Pamianpour et al. (1994) observed negligible muscle activities to maintain the equilibrium of the human body in quasi-neutral position with or without loads up to 445N in hands. The current work is curried out using four différent cost functions: a) sum of muscle forces, b) sum of muscle stresses, c) sum o f cubed muscle stresses and d) sum of the absolute values of dise shear forces for the whole lumbar spine. Among these functions, the formulation with the sum of cubed muscle stresses has been reported to predict the EMG data more accurately (Hughes et al., 1994). The results obtained by this objective function show the activation in almost ail muscles in each level of the lumbar spine in agreement with its interprétation to minimize the muscle fatigue (Han et al., 1991) and to maximize the active system endurance (van Dieën, 1997). The other 3 objective functions (i.e. a, b and d) provide the results in which only one muscle is activated per each lumbar level.
The results demonstrate the significant rôle of flattening of the lumbar lordosis and posterior pelvic tilt in diminishing the lumbar muscular activities required for equilibrium in neutral postures. These fïndings may have bearing in the design of réhabilitation exercises and therapeutic opérations for treatment of low-back disorders.