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 applie d at the lumbar spine and across the pelvis was investigated to help i dentify the sources of antagonistic lower torso muscle recruitment. Fi ve subjects in the neutral upright posture were loaded with L(4-5) mom ent and anterior shear forces under constrained and free pelvic rotati on conditions. The activity states of the erector spinae, latissimus d orsi, external oblique, rectus abdominis, gluteus maximus, and rectus femoris muscles were monitored using surface electromyography. Regress ion analysis was used to test if (1) lumbar shear loads influence the activity of the torso muscles, and (2) moments about the pelvis signif icantly influence the torso muscle activities. The pelvic muscles (glu teus maximus and rectus femoris) were monitored for their role in stab ilizing pelvic rotation. The statistical analysis did not support the hypothesis that torso muscles are recruited in response to shear loadi ng. Also, the data did not provide conclusive evidence that moments ab out the pelvis relative to the lumbar spine significantly influence to rso muscle recruitment. For the subjects that exhibited elevated antag onistic muscle activity, no explanation of this recruitment strategy c ould be statistically related to lumbar shear or pelvic moments. Relev ance-Omission of pelvic moments in biomechanical modelling of the lumb ar torso does not seem to be introducing significant error in the pred iction of torso muscle activity. The results of this study indicate th at the source of lumbar torso muscle antagonistic activity under sagit tal plane flexion loading seems to be a result of yet unidentified fac tors.