Abstract
This work aimed to evaluate trunk muscle forces, internal loads and stability margin under some simulated standing postures, with and without external loads, using a nonlinear finite element model of the T1–S1 spine with realistic nonlinear load-displacement properties. A novel kinematics-based algorithm was applied that exploited a set of spinal sagittal rotations, initially calculated to minimize balancing moments, to solve the redundant active–passive system. The loads consisted of upper body gravity distributed along the spine with or without 200 N held in the hands, either in the front of the body or on the sides. Nonlinear and linear stability/perturbation analyses at deformed, stressed configurations with a linear stiffness-force relationship for muscles identified the system stability and critical muscle stiffness coefficient. Predictions were in good agreement with reported measurements of posture, muscle EMG and intradiscal pressure. Minimal changes in posture (posterior pelvic tilt and lumbar flattening) substantially influenced muscle forces, internal loads and stability margin. Addition of 200 N load in front of the body markedly increased the system stability, global muscle forces, and internal loads, which reached anterior shear and compression forces of ~500 N and ~1,200 N, respectively, at lower lumbar levels. Co-activation in abdominal muscles (up to 3% maximum force) substantially increased extensor muscle forces, internal loads and stability margin, allowing a smaller critical muscle coefficient. A tradeoff existed between lower internal loads in passive tissues and higher stability margins, as both increased with greater muscle activation. The strength of the proposed model is in accounting for the synergy by simultaneous consideration of passive structure and muscle forces under applied postures and loads.




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Acknowledgements
This work is supported by grants from the Institut de recherche en santé et en sécurité du travail du Québec (IRSST-Québec) and the Natural Sciences and Engineering Research Council of Canada (NSERC-Canada). The assistance of N. Arjmand in development of optimization routines is gratefully appreciated.
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Shirazi-Adl, A., El-Rich, M., Pop, D.G. et al. Spinal muscle forces, internal loads and stability in standing under various postures and loads—application of kinematics-based algorithm. Eur Spine J 14, 381–392 (2005). https://doi.org/10.1007/s00586-004-0779-0
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DOI: https://doi.org/10.1007/s00586-004-0779-0