Keywords: seated person, coupling, 2DOF, 3DOF, lumped mass model.

Prolonged exposure to vibration, is prone to cause more damage to human health in sitting posture compared to standing posture. Continuous compressive loading when combined with vibration may impair the metabolism in the tissues of the intervertebral discs thus giving start to an irreversible process. Lumped mass models have been proposed to gain some insight over the human behaviour when subjected to transient loading in sitting posture [1,2,3]. Even though continuous and finite element models can also be used, lumped mass models are preferred due to their simplicity. The seat is modelled as a single degree of freedom, SDOF, system. Whereas the human body in sitting posture can be modelled not only as SDOF but also two, 2DOF, or three, 3DOF, of freedom system. In this study seated person is modelized as two and three degrees of freedom model. Human body can be subjected to low (ex. in cars, buses and trains) and or high frequency vibrations (ex. in helicopters). The coupling imposed in the mathematical models between the masses corresponding to various parts of human body, however, do not allow the prediction of the main features of human response to high frequency vibrations. It is claimed that [4,5] 2DOF human body predicts better the seat transmissibility.

In this study, sensitivity analysis of 3DOF (models LM1 and LM2) and 4DOF, (models LM3 and LM4) linear seat-person models are performed. The models (LM1, LM3) and (LM2, LM4) are thought to be representative of coupled and partially coupled models. The model parameters are chosen to be stiffness, , damping coefficient, and mass, . Two types of input namely continuous and shock loading are applied to seat-person models. The continuous loading is chosen to be the floor accelerations measured in a car (km/hr, rd gear, rpm) driven on a smooth and rough road. The shock type of loading corresponds to the floor accelerations measured in a car(km/hr, rd gear, rpm) crossing over a hump on an asphalt road. The chosen models are insensitive to the changes in the vibration spectra. When the stiffness of the seat is increased by 80% (while keeping the other model parameters as constant) the peak values of TR1( ) and TR2( ) of models LM1 and LM2 are decreased respectively by 25% and 18%. A shift of 0.3 Hz (towards high frequencies) in values has also resulted. An increase of 10% in value increases the peak TR2 value of model LM1 (LM3) by 7% (4%). This result verifies the adverse effect of low back (i.e. waist) support present in some of the cars

Unlike seat damping, a 20% increase in value seems to be sufficient to reduce the peak TR2 value of model LM1 (LM3) by 11% (7%). Compared to model LM1 (LM3) model LM2 (LM4) is less sensitive to changes occurring both in and . Model LM2 (LM4) displays well the response at high frequencies.

1

R.R. Coermann, "The Mechanical Impedance of the Human Body in Sitting and Standing Position at Low Frequencies", Human Factors, 227-253, 1962.

2

P.É. Boileau, S. Rakheja, X.Yang, I. Stiharu, "Comparison of Biodynamic Response Characteristics of Various Human Body Models as Applied to Seated Vehicle Drivers", UK Informal Group Meeting on Human Response to Vibration, Mira, 1-13, 1996.

3

X. Wu, S. Rakheja, P.É. Boileau, "Study of Biodynamic Functions of Human Response to Whole Body Vertical Vibration", UK Group Meeting on Human Response to Vibration, Southampton, 195-208, 1997.

4

L. Wei, J. Griffin, "The Prediction of Seat Transmissibility from Measures of Seat Impedance", Journal of Sound and Vibration, 214, 121-137, 1998. doi:10.1006/jsvi.1998.1540

5

F.M.L. Amirouche, S.K. Ider, "Simulation and Analysis of a Biodynamic Human Model Subjected to Low Accelerations; A correlation Study", Journal of Sound and Vibration, 124, 281-292, 1998. doi:10.1016/S0022-460X(88)80111-1

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