Biodynamic models representing distributed human-seat interactions can assist seat design. This study sought to develop a finite element (FE) model representing the soft tissues of the body supported by seating and the vertical in-line apparent mass and the fore-and-aft cross-axis apparent mass of the seated human body during vertical vibration excitation. The model was developed with rigid parts representing the torso segments, skeletal structures (pelvis and femurs) and deformable parts representing the soft tissues of the buttocks and the thighs. The model had three vibration modes at frequencies less than 15 Hz and provided reasonable vertical in-line apparent mass and fore-and-aft cross-axis apparent mass. The model can be developed to represent dynamic interactions between the body and a seat over a seat surface (e.g. dynamic pressure distributions and variations in seat transmissibility over the seat surface). Practitioner Summary: The three-dimensional FE model of the human body represents the in-line apparent mass and cross-axis apparent mass measured on a seat. With deformable soft tissues it can assist seat design by representing dynamic human-seat interactions, such as pressure distributions and variations in seat transmissibility over a seat surface.

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The transmissibility of seat depends on the dynamics of both the seat and the human body, and shows how the amplification and attenuation of vibration varies with the frequency of vibration. A systematic methodology was developed for finite element (FE) modelling of the dynamic interaction between a seat and the human body and predicting the transmissibility of a seat. A seat model was developed to improve computational efficiency before models of the seat pan and backrest were calibrated separately using load–deflection and dynamic stiffness measurements, joined to form the complete seat model, and integrated with the model of a manikin for further calibration. The calibrated seat model was combined with a human body model to predict the transmissibility of the seat. By combining a calibrated seat model with a calibrated human body model, and defining appropriate contacts between the two models, the vibration transmissibility with a seat–occupant system can be predicted. Practitioner Summary: FE models are capable of reflecting complex dynamic characteristics of a seat–body system. A methodology for using FE methods to model a seat–body system to predict seat transmissibility has been demonstrated. The method can be developed to explore how seating dynamics interact with human biodynamics.

The biodynamic responses to the human body give an understanding of why human responses to vibration (changes in health, comfort, and performance) vary with the frequency and direction of vibration. Studies have shown that biodynamic responses also vary with the magnitude of vibration and that the backrests of seats influence the transmission of vibration to the seated human body. There has been little study of the nonlinearity in the biodynamic responses of the body to dual-axis excitation and no study of the influence of backrests during dual-axis excitation. This study investigated the apparent mass and cross-axis apparent mass of the human body exposed to random vibration (0.2 to 20 Hz) in all 15 possible combinations of four magnitudes (0, 0.25, 0.5 and 1.0 ms–2 r.m.s.) of fore-and-aft vibration and the same four magnitudes of vertical vibration. Nonlinearity was evident, with the body softening with increasing magnitude of vibration when using a fixed magnitude of vibration in one direction and varying the magnitude of vibration in the other direction. The fore-and-aft apparent mass on the seat was greater without a backrest at the lower frequencies but greater with a backrest at the higher frequencies. The vertical apparent mass on the seat was decreased by the backrest at low frequencies. Cross-axis coupling was evident, with excitation in one axis producing a response in the other axis. It is concluded that the nonlinearity of the body evident during single-axis and multi-axis vibration, and the influence of backrests, should be taken into account when determining frequency weightings for predicting human responses to vibration and when optimising the dynamics of seating to minimise exposure to vibration.

A coupled track-train-seat-human model with lateral, vertical and roll vibrations was developed to investigate the ride comfort of high-speed trains. The ride comfort was assessed by the total equivalent acceleration defined according to ISO 2631-1. It was found that the track flexibility had a minor influence on the lateral, vertical and roll floor vibrations below 20 Hz, while the total equivalent acceleration was overestimated by over 10% at high speed using a rigid track. The flexibility of the carbody generated dramatic floor vibration in lateral, vertical and roll directions above 7.5 Hz, so the rigid carbody model significantly underestimated the total equivalent acceleration. The total equivalent acceleration tended to increase with the increase of speed. The equivalent accelerations for two symmetric seat positions illustrated analogous trends with the increasing speed. The ride comfort was the worst close to the carbody ends, followed by the carbody centre at high speed. The vertical acceleration on the seat pan was the most severe whatever the seat position and speed are. The neighbouring subject brought about a reduction of the total equivalent acceleration for the subject under evaluation. The carbody damping can reduce the total equivalent acceleration effectively in the whole speed range.

A rigid-flexible coupled train-seat-human model was developed for studying ride comfort of rail vehicles. The weighted acceleration r.m.s. value at the seat pan was used for assessing ride comfort. The contribution of different modes of the carbody to ride comfort was defined from the perspective of power spectral density. The role of first bending mode was more important than other bending modes when bogie and seat positions are far from the nodes of first bending mode. Under this situation, the bogie spacing filter effect of first bending mode can be adopted to predict peaks and troughs of the equivalent acceleration on the floor or human-seat interface varying with speed or first bending frequency. The incorporation of human-seat system was necessary in order to accurately evaluate ride comfort. The ride comfort exhibited a global trend of improvement with increased bending rigidity or reduced speed. Increasing the damping ratio of bending modes improved ride comfort effectively. Increasing stiffness or decreasing damping of human-seat vertical contact on the seat pan worsened ride comfort. Equivalent accelerations at symmetrical seat positions showed analogous tendency as the speed. Ride comfort was worst at two ends whatever the speed was, followed by carbody centre at high speed.