PARTITIONING OF THE L4/L5 DYNAMIC MOMENT INTO MUSCULAR, LIGAMENTOUS AND DISC COMPONENTS FOR CALCULATION OF TISSUE LOADS DURING LIFTING
Abstract (summary)
The purpose of this work was to increase understanding of lumbar mechanics by means of the development and utilization of a sagittal plane, dynamic biomechanical model of the low back. The work was motivated by the observation that current, anatomically simple models, sometimes predict injurious levels of disc compression during human load handling when, in fact, no injury occurs. This author felt that the problem may be an overly simplistic representation of anatomical structure in current models and that a more realistic model would yield reasonable disc compression estimates. This model incorporated extensive anatomical detail of a three-dimensional musculoligamentous-skeletal system. The muscle part of the model was driven from measured surface EMG and thus permitted and accounted for antagonist cocontraction. The dynamic resultant moment, generated about an axis through the L4/L5 disc during sagittal plane lifts, was partitioned into restorative components provided by the disc in bending, 7 ligaments and 48 active muscles. Three subjects performed 6 to 8 lifts of loads ranging from 268 to 890 N. All three elected to lift with a "flatbacked" posture. In retrospect, this ultimately limited the extent of examination of model response that was possible, particularly regarding the role of the ligaments. Skeletal kinematics and the dynamic L4/L5 resultant moment were obtained from cine analysis of markers on the rib cage and pelvis input to a linked segment model. Estimations of L4/L5 disc compression and shear were, on average, 16.2% and 42.5% lower, respectively, for generation of moments ranging from 250 to 450 Nm than those calculated for a commonly implemented simple 5 cm erector tissue moment arm length. There was no need to invoke intra-abdominal pressure, lumbodorsal fascia (LDF) or other hypothesized compression reducing mechanisms. In fact, IAP and the LDF contributed no more than 4% to supporting the peak dynamic L4/L5 moment. Muscle activity, particularly that of the sacrospinalis, dominated the generation of the restorative moment. Due to the flat spine maintained through the heavy loading phase of the lifts, the ligaments played a very minor role because they were not significantly strained.