Dynamic properties of the biomechanical model of the human body-influence of posture and direction of vibration stress

Previous studies have shown that exposure to whole-body vibration can interfere with comfort, activities, and health. In analogy to materials handling it is assumed that the elevated spinal forces are a crucial component in the pathogenesis of the health impairment. To estimate the forces a biomechanical model was developed. In the model the human trunk, neck and head, the legs, and the arms are represented by 27 rigid bodies. An additional body simulates the vibrating vehicle or machinery. The bodies are connected by visco-elastic joint elements. In total 106 force elements imitate the trunk, neck, and leg muscles. The motion equations were derived by means of the dynamics of systems of rigid bodies. Motions were simulated in different standing and sitting postures and in three vibration directions. The transfer functions between the accelerations of the surface or the seat and the spinal forces were computed. By means of these functions it can be shown that under the conditions investigated the compressive forces seem to be the dominant stressor between the forces transmitted in the lumbar spine. However, it cannot be stated that under horizontal vibration the health risk is only dependent from the comressive forces. Here the relationship with the shear strength of the spine being much lower than the compressive strength must be regarded.