Influence of the Posture of the Trunk on the Spine Forces During Whole-Body Vibration

Typically drivers of container-bridge cranes are forced to sit with a forward bent upper trunk to control the position and motion of the container. Fork-lift truck drivers incline the upper trunk to the side to look forward or they twist to one side during reversing. Assuming that these inclined postures result in a higher health risk than vibration exposure in the upright sitting posture the forces transmitted in the lumbar spine were assessed by means of a biomechanical model. Under realistic vibration stress the bent postures result in an increase of the compressive and the shear forces. By the enhanced shear forces a displacement of the upper motion segments is prevented. The quantitative relationship between the mean values of the forces and the chest inclination can be sufficiently approximated by regression functions. The increase of the spine forces is the result of the increased muscle forces stabilizing the inclined trunk. On container bridge cranes or fork-lift trucks the typical postures of the drivers result in enhanced spinal forces compared with the upright sitting posture. This effect must be considered in the risk analysis of workplaces with whole-body vibration.