Quantification and modeling of the lumbar erector spinae as a function of sagittal plane torso flexion

Forward bending of the torso, or torso flexion in the sagittal plane, has previously been associated with reports of low back disorders in the occupational environment. Sagittal plane torso flexion results in anterior rotation of the lumbar vertebral bodies, resulting in a flattening of the lumbar spine. The change in lumbar spine orientation results in lengthening of the lumbar erector spinae muscles, which alters their force producing capability, and would ultimately alter the loading on the lumbar spine that biomechanical models would predict. Prior studies have also indicated that the sagittal plane moment-arms at several lumbar levels decrease as full torso flexion is reached from neutral. However, the relationship between the erector spinae moment-arm and intermediate torso flexion angles between neutral and near full torso flexion is not known. Thus, this study was undertaken to describe, utilizing magnetic resonance imaging, the lumbar erector spinae musculature with respect to the lumbar spine in the sagittal plane as a function of quantified torso flexion, and investigate the impact on predicted spinal loading.; Magnetic resonance imaging scans from subjects in a lateral recumbent posture for four different torso flexion angles indicated that the moment-arms of the erector spinae at T₁₂/L₁, L₄/L ₅ and L₅/S₁ decrease as the torso was flexed from neutral to 45 degrees. The decrease in sagittal plane erector spinae moment-arms were able to be predicted from the quantified lumbar spinal curvature, torso flexion angle, and anthropometric measures such as the trunk circumference about the iliac crest and trunk width measures at the iliac crest.; Utilizing a three-dimensional dynamic electromyogram-driven biomechanical model of the torso that predicts spinal loading on the L₅/S ₁ intervertebral disc, the erector spinae vector spinae moment-arm from the L₅/S₁ intervertebral disc was allowed to vary as a function of measured sagittal plane torso flexion. The decrease in the erector spinae moment-arm as a function of torso flexion resulted in an increase in the predicted spinal loading over what would have been predicted from a biomechanical model that did not allow the erector spinae moment-arm to vary as a function of torso flexion.