Biomechanical analysis and assessment of lumbar stress during load lifting using a dynamic 19-segment human model

A dynamic biomechanical human model is presented which allows the quantification of mechanical parameters such as torque, compressive and shear forces, and pressure at the lumbar intervertebral discs. The human model comprises a total of 19 body segments. Various trunk flexions can be analysed due to the provision of 5 joints at the level of the 5 lumbar intervertebral discs. The influence of intraabdominal pressure on spinal load is considered. The inclusion of the influences of gravity and inertia permits the analysis of both static body postures and dynamic body movements. Since the model is 3-dimensional, the lumbar stress can be calculated during both symmetrical tasks in the median sagittal plane as well as during non-symmetrical ones. The influences on spinal stress of trunk inclination and the position of an external load relative to the body are quantified for various load weights up to 50 kg. The torque at the lumbo-sacral joint L5-S1 lies, dependent on posture and load lever-arm, within the range between 0 and 500 Nm; the compressive force on L5-S1 lies within the range between 0.4 and 10 kN, and the shear force at L5-S1 between 0.2 and 0.9 kN. The influences of lift velocity and jerky movement on lumbar stress are quantified. Simulated humpback and hollow-back postures are studied. The compressive forces at the 5 lumbar intervertebral discs are compared. The validity of the model is examined by comparing the model calculations with the intradiscal pressure measurements taken from the literature. Strength tests on lumbar intervertebral discs and vertebrae are collated from the literature in order to assess the lumbar stress during load lifting. The lumbar ultimate compression strength varies within a wide range. The mean value for a total of 307 lumbar segments amounts to 4.4 kN, the standard deviation to 1.9 kN. In conclusion, lumbar compressive force values during lifting fall within the same range as the strength values for the human lumbar spine.