The Changes of Trunk Motion Rhythm and Spinal Loading During Trunk Flexion and Extension Motions Caused by Lumbar Muscle Fatigue.

Previous studies indicated that lumbar extensor muscle fatigue could potentially affect lumbar-pelvic rhythm and influence spinal loading during trunk motions. In this study, the effects of lumbar extensor muscle fatigue on the normalized lumbar-pelvic rotation rhythm and the associated L5/S1 joint loading during weight lifting and lowering tasks were investigated. Thirteen volunteers performed lifting and lowering of a 20-lbs box both before and after lumbar extensor muscle fatigue, which was generated through a static weight holding task. The normalized lumbar-pelvic motion ratio (L/P ratio) and the external moment on the L5/S1 joint were calculated and compared. Results showed that subjects demonstrated significantly larger normalized L/P ratios during both weight lifting and lowering tasks with the influence of fatigue. In addition, although the spinal loadings remain unchanged at the beginning and ending of both lifting and lowering motions, significantly larger L5/S1 joint moments were observed during both motions after fatigue. Such changes indicate potentially elevated risk of back injury. In a clinical setting, the current results demonstrated that lumbar muscle fatigue could cause transient changes in lumbar-pelvic motion rhythm. Therefore, lumbar muscle fatigue must be avoided when using lumbar-pelvic motion rhythms for patient diagnosis or rehabilitation assessment.