Absorbed Power and Mechanical Impedance of the Seated Human Measured within a Real Vehicle Environment Compared with Single Axis Laboratory Data

It is known that exposure to whole-body vibration when driving different types of vehicles may cause health effects and discomfort. Absorbed power might be a good quantity for assessing the risk of injury due to such exposure. Mechanical impedance is a measure which describes biodynamics and can define human response to vibration. It can then also be used in the design of preventative measures. The mechanical impedance of the human body in the vertical direction is presented in the ISO-standard 5982 (1981). However, it does not show concordance with results from later studies. This study has investigated how to measure and quantify absorbed power and mechanical impedance in a real vehicle environment. The vehicle was an 8-seater minibus driven on a snow covered gravel road at a speed of approximately 50 kmh⁻¹. Three male subjects were exposed to vibration in the vehicle, sitting on a rigid seat mounted at the back of the minibus. Two postures were used, sitting erect and relaxed. Force and acceleration were measured using a specially designed seat plate. The results showed that absorbed power and mechanical impedance can be measured in a vehicle, but the results differ from other published single-axis exposure results. There seems to be a force cross-talk between the directions. This force distorts the results if the difference in vibration magnitude is as large as in this study. For instance, absorbed power negative results are shown around the frequency range 5–10 Hz in the x direction. Further research is needed in order to define the level and linearity of the force cross-talk. In addition, studies to investigate other conditions, such as back and arm rests, seat foam and suspension seats he of great interest.