Localised biodynamic responses of the seated human body during excitation by vertical vibration

Abstract

Dynamic force has been measured previously at the seat interface beneath the sitting human body exposed to vertical vibration. However, how the distribution of forces over the interface contributes to the total force has not been identified. This study seeks to understand biodynamic responses to vertical vibration at the ischial tuberosities, the middle thighs, and the front thighs, so as to allow the development of a biodynamic model representing the overall and localised responses. In this thesis, for convenience, the transfer function between the dynamic force at a location and the acceleration at the same location is referred to as the ‘localised apparent mass’.

How localised apparent masses at the ischial tuberosities, the middle thighs, and the front thighs contribute to the overall apparent mass of the body, and how thigh contact affects the distribution, were investigated initially. The vertical apparent mass at the ischial tuberosities dominated the overall apparent mass around 5 Hz for all postures, but that at the thighs dominated the apparent mass around 8 Hz when the feet were not supported. For all postures, the fore-and-aft cross-axis forces showed a slightly lower principal resonance frequency at the middle thighs and front thighs than at the ischial tuberosities. It is suggested that more than one mode contributes to the resonance in the fore-and-aft cross-axis apparent mass. When the feet were unsupported, the nonlinearities in the biodynamic responses were greater at the thighs than that at the ischial tuberosities.

How support from both rigid and soft backrests affect biodynamic responses measured at the seat pan were then investigated. For both rigid and soft backrests inclined from the vertical by 30 degrees, the vertical forces were greater at the ischial tuberosities than at the thighs at all frequencies less than 15 Hz. With increasing inclination of the rigid backrest, the resonance in the overall vertical in-line apparent mass at the seat pan broadened, and the frequency of the principal resonance in the overall fore-and-aft cross-axis apparent mass at the seat pan decreased. Irrespective of the backrest stiffness and backrest inclination, the frequency of the resonance in the overall vertical apparent mass at the seat pan was not correlated with the frequency of the resonance in the forces measured normal to the backrest.

The effect of thigh contact on the apparent mass of the body sitting on a foam cushion was then investigated. The frequency of the resonance in the vertical transmissibility of the foam was around 4 Hz at the ischial tuberosities but around 6 to 8 Hz at the front thighs. When sitting on the foam, the localised apparent masses at the middle thighs and the front thighs showed a resonance around 4 Hz, correlated with the principal resonance frequency in the vertical transmissibility of the foam measured at the ischial tuberosities. It is suggested that the forces at the thighs were affected by motions of the whole body at the ischial tuberosities. Differences between the overall vertical apparent mass of the body measured with the foam and with a rigid seat decreased with decreasing thigh contact.

A finite-element model was developed to reproduce the overall vertical in-line apparent mass and the overall fore-and-aft cross-axis apparent mass of the body sitting on a rigid seat while exposed to vertical vibration. The model has the upper-body represented by rigid bodies interconnected by revolute joints, and the buttocks and thighs represented by deformable parts covering the pelvis and femur bones. The shape of the thighs and buttocks was adjusted to the anthropometry of a subject, and viscoelastic material with different properties was assigned to the soft tissues of the buttocks, middle thighs, and front thighs. The model is shown to represent both the static pressure and the localised biodynamic responses over a rigid seat with different thigh contact conditions.

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Identifiers

ORCID for Michael J. Griffin: orcid.org/0000-0003-0743-9502

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