FORCES AND MOMENTS AT THE L4/L5 VERTEBRAL LEVEL WHILE FORWARD BENDING IN A SUPPORTED POSTURE

Abstract

INTRODUCTION Chronic lower back pain has been associated with improper body mechanics while lifting. Forces and moments imposed on the lumbar spine have been investigated while performing a variety of lifting tasks to better understand the mechanical nature of injury to this region. Research focused on the injured patient has been sparse. The purpose of this line of study is to determine the mechanical effects at the L4/L5 vertebral level while using a supported lifting posture. METHODS Nine males (mean age 49.6± 9.7 yrs) participating in a chronic back pain program volunteered as subjects. Each subject performed two styles of lift while removing a wash cloth from a support structure constructed to simulate the dimensions of a washing machine. The lifting postures were(1) a common bowed-back lift performed with both feet in a stationary position, and (2) a rotation about the hip joint of the weight-bearing leg while the contra-lateral leg was allowed to rotate posteriorly to maintain the standing curvature of the spine ("golfer's lift"). Data was recorded using a 60 Hz S-VHS video camcorder and a Bertec force platform (500 Hz). A sagittal plane inverse dynamic model was developed and used to calculate forces and moments imparted on the L4/L5 vertebral level. Self-reported subjective ratings of lower back pain were recorded on an ordinal scale from 0 to 10. RESULTS Two profiles of lifts were identified based on the direction of the net moment at L4/L5. Negative moments, requiring a contraction of the spinal extensors to maintain equilibrium, were found in the majority of subjects while performing the bowed-back posture, while only in one subject performing the golfer's lift posture. Positive moments, resulting in an unloading of the spinal extensors, were found in the majority of subjects while performing the golfer's lift posture. Significant differences F(1.68) = 64.74, p < .001] were found between the peak L4/L5 moments for the two lifting postures. A Wilcoxon matched-pairs signed-ranks test revealed a significant increase (p <. 01) in perceived back pain for the bowed-back posture. The majority of the total compressive force was found to be produced by muscular contraction. Load shear force was not statistically different between the two lifting postures. CONCLUSION For the sample of chronic back pain patients studied, net lumbar moments created during supported lifting were significantly related to posture. Negative net lumbar moments (spinal extensor loading) were found to increase in relation to significant increases in perceived lower back pain. It is recommended that the role of compressive forces in the spine in injury production be re-evaluated.