Trunk and hip muscle recruitment in response to external anterior lumbosacral shear and moment loads

The response of select lumbar torso and pelvic muscles to loads applied at the lumbar spine and across the pelvis was investigated to help identify the sources of antagonistic lower torso muscle recruitment. Five subjects in the neutral upright posture were loaded with L(4-5) moment and anterior shear forces under constrained and free pelvic rotation conditions. The activity states of the erector spinae, latissimus dorsi, external oblique, rectus abdominis, gluteus maximus, and rectus femoris muscles were monitored using surface electromyography. Regression analysis was used to test if (1) lumbar shear loads influence the activity of the torso muscles, and (2) moments about the pelvis significantly influence the torso muscle activities. The pelvic muscles (gluteus maximus and rectus femoris) were monitored for their role in stabilizing pelvic rotation. The statistical analysis did not support the hypothesis that torso muscles are recruited in response to shear loading. Also, the data did not provide conclusive evidence that moments about the pelvis relative to the lumbar spine significantly influence torso muscle recruitment. For the subjects that exhibited elevated antagonistic muscle activity, no explanation of this recruitment strategy could be statistically related to lumbar shear or pelvic moments. Relevance-Omission of pelvic moments in biomechanical modelling of the lumbar torso does not seem to be introducing significant error in the prediction of torso muscle activity. The results of this study indicate that the source of lumbar torso muscle antagonistic activity under sagittal plane flexion loading seems to be a result of yet unidentified factors.