| In the present study, an iterative dynamic kinematics-based method was developed to alleviate limitations in earlier biomechanical models. Measured kinematics data were input into a nonlinear finite element model and differential equations of motion were numerically solved to calculate required joint moments and forces, subject to gravitational and inertial loading. In this manner, while satisfying equilibrium at all spinal levels and direction, calculated muscle forces, spinal loads, and system stability were in full accordance with prescribed kinematics and nonlinear passive properties. The method was employed to evaluate the effect on trunk biodynamic of some loading conditions and mechanical parameters such as different lifting techniques (i.e. stoop and squat), velocity of trunk movement (slow, medium and fast velocities), whole body vibration and shock, wrapping of global extensor muscles, antagonistic trunk muscle activities, changes in posture, abdominal co-activity and alterations in passive properties of the spine and buttocks.;Estimated spinal loads and muscle forces during free flexion-extension movements were significantly larger in fastest pace as compared to slower ones indicating the effect of inertial forces. Spinal stability was improved in larger trunk flexion angles and fastest movement. Partial or full flexion relaxation of global extensor muscles occurred only in slower movements. Some local lumbar muscles, especially in subjects with larger lumbar flexion and at slower paces, also demonstrated flexion relaxation. Results confirmed the crucial role of movement velocity on spinal biomechanics. Predictions also demonstrated the important role on response of the magnitude of peak lumbar rotation and its temporal variation.;Finally, our study of human response to a whole body vibration showed that, the input base excitation, via inertial and muscle forces, substantially influenced spinal loads and system stability. The flexed posture in sitting increased the net moment, muscle forces and passive spinal loads while improving the trunk stability. Similarly, the introduction of low to moderate antagonistic co-activity in abdominal muscles increased the passive spinal loads and improved the spinal stability. A trade-off, hence, exists between lower muscle forces and spinal loads on one hand and more stable spine on the other. Base excitations with larger acceleration contents substantially increase muscle forces/spinal loads and, hence, the risk of injury.;Predictions agreed well with measured base reaction forces and accelerations at different spinal levels. Moreover, qualitative agreement with recoded electromyography activity at different superficial muscles was also found. The kinematics-based approach was demonstrated to yield reliable data under various occupational tasks. Such data are crucial for effective prevention and treatment of spinal disorders. Future applications of kinematics-based approach to investigate manual material handling task with twisting and lateral bending of the trunk, whole body vibration with much larger acceleration contents will certainly shed light on the biomechanics of the trunk under circumstances with high risk of back injury. Moreover, development and integration of a nonlinear finite element model of the cervical spine to the current model would provide a better ground for future investigation on the biodynamic of the human spine. (Abstract shortened by UMI.);Our investigation on trunk biomechanics under different lifting techniques showed that net moments, muscle forces, passive (muscle or ligamentous) forces and internal compression/shear forces were larger in stoop lifts than in squat ones. For the relatively slow lifting tasks performed in this study with the lowering and lifting phases each lasting ∼2s, the effect of inertia and damping was not, in general, important. Moreover, posterior shift in the position of the external load in stoop lift reaching the same lever arm with respect to the S1 as that in squat lift did not influence the conclusions of this study on the merits of squat lifts over stoop ones. Results, for the tasks considered, advocate squat lifting over stoop lifting as the technique of choice in reducing net moments, muscle forces and internal spinal loads (i.e., moment, compression and shear force). Alterations in passive properties of spine substantially influenced muscle forces, spinal loads and system stability in both lifting techniques, though more so in stoop than in squat. Stability of spine substantially improved with greater passive properties, trunk flexion and load. Simulation of global extensor muscles with curved rather than straight courses considerably diminished loads on spine and increased stability throughout the task. |
