Models of European drivers - new ways of the assessment of health risks caused by long-term whole-body vibration

Modelling the response of the human body to occupational whole-body vibration provides the possibility of predicting the forces acting on the disc and/or endplates of lumbar vertebrae. Due to the complex structure of the human body, complex dynamic models based on human anatomy are required to adequately reflect the dynamic properties of the body. Based on experimental results the influence of posture and stature on the biodynamic behaviour of human subjects is described. To reflect the biodynamic response of different occupational groups of workers exposed to whole-body vibration, an existing model was adapted to five typical different postures and ten representative statures registered of European drivers of heavy machines. The resulting fifty models were tested with white noise signals in three directions as input to the four interfaces buttock, back, feet, and hands. The results of the static and dynamic shares of the predicted spinal forces at six spinal levels show strong influences of the factors stature and posture. Calculated risk factors to estimate the probability of injuries reflect the influence of these factors. The complex relationships between whole-body vibration exposure (WBV) and health risk have been discussed recently [3]. Variable postures, anthropometric characteristics, spinal geometry, and individual spinal tolerance were identified as important factors that co-determine the effects of occupational WBV on health. These factors are not sufficiently considered by current evaluation procedures. ISO 2631-1 [1] provides a Health Guidance Caution Zone without quantitative information that would permit an allocation of a certain exposure condition within that zone, considering such associated factors as posture and/or age. The width of the zone seems to be questionable in view of the extent of the biological variability of tolerance and predicted effects of posture and/or anthropometry as discussed by Seidel [3] and Seidel et al. [4,5]. ISO 2631-5 [2] gives exclusive consideration to age through a quantitative correction of the predicted ultimate strength of lumbar vertebrae. Neither standard provides quantitative guidance concerning effects on different levels of the lumbar spine and the relevance of vibration input via backrest, feet and hand support for health. Further critical comments were reported earlier [3,6]. The European Directive [7] states an extremely high limit value for WBV in z direction without any limitation of an energy-equivalent evaluation. The consequence is a very dubious assessment of health effects, especially for WBV containing high peak values and/or short daily exposure times [8]. The prediction of intraspinal forces by means of suitable dynamic finite element (FE) models can bypass the difficulties linked with the complicated separate quantifications of many different relationships and/or effects of factors by simulating the whole chain between vibration inputs in different directions to parts of a specific human body and the outputs of compressive and shear forces acting on different levels of the lumbar spine. The range of applicability depends on the ability of the FE-models to reflect different postures and statures. A sufficient solution requires anatomy-based dynamic FE models, whereas phenomenological models are not suitable for this purpose [3,9,10]. With the processing of predicted compressive forces in the time domain it is possible to calculate dose measures which characterise the probability of fatigue failure [2,11]. The aims of this study were (1) to use FE models for the prediction of intraspinal forces caused by real exposure conditions measured in European countries, (2) to use these predictions for an evaluation procedure reflecting the risk of fatigue failure taking into account anthropometric characteristics of European drivers, and (3) to compare the results of this new assessment method with existing evaluation procedures of ISO 2631-1 [1], VDI 2057 [21], ISO 2631-5 [2] and the Directive 2002/44/EC [7]. The comparisons reveal significant differences.