Modelling the dynamic mechanisms associated with the principal resonance of the seated human body.

OBJECTIVES Simple mathematical models have been developed to obtain insights into resonance phenomena observed at about 5 Hz in the dynamic responses of the seated human body exposed to vertical whole-body vibration. DESIGN Alternative lumped parameter models with a few degrees-of-freedom have been investigated. Rotational degrees-of-freedom, with eccentricity of the centre of gravity of the mass elements, represented responses in the fore-and-aft and pitch axes caused by vertical vibration. BACKGROUND The causes of body resonance are not fully understood, but this information is required to develop cause-effect relationships between vibration exposures and effects on human health, comfort and performance.Method. The inertial and geometric parameters for models were based on published anatomical data. Other mechanical parameters were determined by comparing model responses to experimental data. RESULTS Two models, with four and five degrees-of-freedom, gave more reasonable representations than other models. Mechanical parameters obtained with median and individual experimental data were consistent for vertical degrees-of-freedom but varied for rotational degrees-of-freedom. CONCLUSIONS The resonance of the apparent mass at about 5 Hz may be attributed to a vibration mode consisting of vertical motion of the pelvis and legs and a pitch motion of the pelvis, both of which cause vertical motion of the upper-body above the pelvis, a bending motion of the spine, and vertical motion of the viscera. RELEVANCE The mathematical models developed in this study may assist understanding of the dynamic mechanisms responsible for resonances in the seated human body. The information is required to represent mechanical responses of the body and assist the development of models for specific effects of vibration.