Transverse-contour modeling of trunk muscle-distributed forces and spinal loads during lifting and twisting

Study Design: An electromyography-assisted biomechanical model was develope d using electromyographic (surface and in-dwelling) data collected during a symmetric lifting and twisting activities. Objectives. To develop a biomechanical model of the lumbar region that cons iders the ability of the broad, flat muscles of the trunk (external oblique s, internal obliques and latissimus dorsi) to activate different anatomic r egions at different intensity levels and the spine reaction forces that res ult during lifting and twisting tasks. Summary of Background Data. Many biomechanical models of the lumbar region use single-vector representations for the external oblique, internal obliqu e, and latissimus dorsi muscles. This simplification limits the description of the complexity of the resultant forces produced by these muscles and do es not consider their differential activation capacity. Methods. Human subjects performed lifting and twisting exertions while musc le electromyographic activities were sampled at one location on the rectus abdominis and erector spinae muscles and at multiple locations on the latis simus dorsi, external oblique, and internal oblique muscles. These data wer e used in conjunction with in vivo digitized muscle origin and insertion po ints to predict muscle forces and spine loads through the use of the electr omyography-assisted modeling method. Results. The measures of model performance such as percentage of error (6-2 1%) in the prediction of the external torques, correlations (0.83-0.98) bet ween internal and external torques and the values of predicted muscle force capacity were all similar to data collected in previous electromyography-a ssisted models, but the predictions of spinal loading, particularly shear f orces, were quite different. The results have shown that by modeling the br oad, flat muscles of the torso using multiple-force vectors, the calculated shear forces in the spine were reduced. Conclusions. The multivector, transverse-contour model developed in this re search illustrates the importance of realistic multipe-vector modeling and the importance of considering the selective-activation capacity of the abdo minal oblique musculature.