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Abstract: Previous studies have quantified the power absorbed in the seated human body during exposure to vibration but have not investigated the effects of body posture or the power absorbed at the back and the feet. This study investigated the effects of support for the feet and back and the magnitude of vibration on the power absorbed during whole-body vertical vibration. Twelve subjects were exposed to four magnitudes (0.125, 0.25, 0.625, and 1.25ms−2 rms) of random vertical vibration (0.25–20Hz) while sitting on a rigid seat in four postures (feet hanging, maximum thigh contact, average thigh contact, and minimum thigh contact) both with and without a rigid vertical backrest. Force and acceleration were measured at the seat, the feet, and the backrest to calculate the power absorbed at these three locations. At all three interfaces (seat, feet, and back) the absorbed power increased in proportion to the square of the magnitude of vibration, with most power absorbed from vibration at the seat. Supporting the back with the backrest decreased the power absorbed at the seat at low frequencies but increased the power absorbed at high frequencies. Supporting the feet with the footrest reduced the total absorbed power at the seat, with greater reductions with higher footrests. It is concluded that contact between the thighs and the seat increases the power absorbed at the seat whereas a backrest can either increase or decrease the power absorbed at the seat. [Copyright &y& Elsevier]

Abstract: Vibration at the feet can contribute to discomfort in many forms of transport and in some buildings. Knowledge of the frequency-dependence of discomfort caused by foot vibration, and how this varies with vibration magnitude, will assist the prediction of discomfort caused by vibration. With groups of 12 seated subjects, this experimental study determined absolute thresholds for the perception of foot vibration and quantified the discomfort caused by vibration at the foot. The study investigated a wide range of magnitudes (from the threshold of perception to levels associated with severe discomfort) over a wide range of frequencies (from 8 to 315Hz in one-third octave steps) in each of the three orthogonal translational axes (fore-and-aft, lateral, and vertical). The effects of gender and shoes on absolute thresholds for the perception of vertical vibration at the foot were also investigated. Within each of the three axes, the vibration acceleration corresponding to the absolute thresholds for the perception of vibration, and also all contours showing conditions producing equivalent discomfort, were highly frequency-dependent at frequencies greater than about 40Hz. The acceleration threshold contours were U-shaped at frequencies greater than 80Hz in all three axes of excitation, suggesting the involvement of the Pacinian channel in vibration perception. At supra-threshold levels, the frequency-dependence of the equivalent comfort contours in each of the three axes was highly dependent on vibration magnitude. With increasing vibration magnitude, the conditions causing similar discomfort across the frequency range approximated towards constant velocity. Thresholds were not greatly affected by wearing shoes or subject gender. The derived frequency weightings imply that no single linear frequency weighting can provide accurate predictions of discomfort caused by a wide range of magnitudes of foot vibration. [Copyright &y& Elsevier]

Resonance frequencies evident in the apparent mass and the transmissibility of the human body decrease with increasing vibration magnitude, but the mechanisms responsible for this nonlinearity have not been established. This experiment was designed to explore the effects of body location on the nonlinearity of the body in supine postures. In a group of 12 male subjects, the apparent mass and transmissibility to the sternum, upper abdomen, and lower abdomen were measured in three postures (relaxed semi-supine, flat supine and constrained semi-supine) with vertical random vibration (0.25¿20Hz) at seven vibration magnitudes (nominally 0.0313, 0.0625, 0.125, 0.25, 0.5, 0.75, and 1.0ms¿2 rms). In all three postures, the apparent mass resonance frequencies and the primary peak frequencies in the transmissibilities to the upper and lower abdomen decreased with increases in vibration magnitude from 0.25 to 1.0ms¿2 rms. Nonlinearity generally apparent in transmissibility to the abdomen was less evident in transmissibility to the sternum and less evident in transmissibilities to the abdomen at vibration magnitudes less than 0.125ms¿2 rms. The nonlinearity was more apparent in the flat supine posture than in the semi-supine postures. The findings are consistent with the nonlinearity being associated with the response of soft tissues, more likely a consequence of passive thixotropy than muscle activity. [Copyright &y& Elsevier]

Abstract: The apparent mass of the human body reflects gross movements caused by whole-body vibration and can be used to predict the influence of body dynamics on seat transmissibility. With vertical excitation, various models fit the measured vertical apparent mass of the human body, but experiments also show high fore-and-aft forces on the seat (the fore-and-aft cross-axis apparent mass) that have not influenced current models. This paper defines a model that predicts the vertical apparent mass and the fore-and-aft cross-axis apparent mass of the seated human body during vertical excitation. A three degree-of-freedom model with vertical, fore-and-aft and rotational (i.e. pitch) degrees of freedom has been developed with twelve model parameters (representing inertia, stiffness, damping, and geometry) optimised to the measured vertical apparent mass and the measured fore-and-aft cross-axis apparent mass of the body. The model provides close fits to the moduli and phases for both median data and the responses of 12 individual subjects. The optimum model parameters found by fitting to the median apparent mass of 12 subjects were similar to the medians of the same parameters found by fitting to the individual apparent masses of the same 12 subjects. The model suggests the seated human body undergoes fore-and-aft motion on a seat when exposed to vertical excitation, with the primary resonance frequency of the apparent mass arising from vertical motion of the body. According to the model, changes in the vertical, fore-and-aft, or rotational degree of freedom have an effect on the resonance in the fore-and-aft cross-axis apparent mass. [Copyright &y& Elsevier]

Abstract: The principal resonance frequency in the driving-point impedance of the human body decreases with increasing vibration magnitude—a nonlinear response. An understanding of the nonlinearities may advance understanding of the mechanisms controlling body movement and improve anthropodynamic modelling of responses to vibration at various magnitudes. This study investigated the effects of vibration magnitude and voluntary periodic muscle activity on the apparent mass resonance frequency using vertical random vibration in the frequency range 0.5–20Hz. Each of 14 subjects was exposed to 14 combinations of two vibration magnitudes (0.25 and 2.0ms−2 root-mean square (rms)) in seven sitting conditions: two without voluntary periodic movement (A: upright; B: upper-body tensed), and five with voluntary periodic movement (C: back-abdomen bending; D: folding-stretching arms from back to front; E: stretching arms from rest to front; F: folding arms from elbow; G: deep breathing). Three conditions with voluntary periodic movement significantly reduced the difference in resonance frequency at the two vibration magnitudes compared with the difference in a static sitting condition. Without voluntary periodic movement (condition A: upright), the median apparent mass resonance frequency was 5.47Hz at the low vibration magnitude and 4.39Hz at the high vibration magnitude. With voluntary periodic movement (C: back-abdomen bending), the resonance frequency was 4.69Hz at the low vibration magnitude and 4.59Hz at the high vibration magnitude. It is concluded that back muscles, or other muscles or tissues in the upper body, influence biodynamic responses of the human body to vibration and that voluntary muscular activity or involuntary movement of these parts can alter their equivalent stiffness. [Copyright &y& Elsevier]

Abstract: Oscillatory motions can cause injury in transport when standing passengers or crew lose balance and fall. To predict the loss of balance of standing people, a model is required of the relationship between the input motion and the stability of the human body. This experimental study investigated the effect of frequency, magnitude and direction of oscillation on the postural stability of standing subjects and whether response to rotational oscillation can be predicted from knowledge of response to translational oscillation. Twelve male subjects stood on a floor that oscillated in either horizontal (fore-and-aft or lateral) or rotational (pitch or roll) directions. The oscillations were one-third octave bands of random motion centred on five preferred octave centre frequencies (0.125, 0.25, 0.5, 1.0, and 2.0Hz). The horizontal motions were presented at each of four velocities (0.04, 0.062, 0.099, and 0.16ms−1 rms) and the rotational motions were presented at each of four rotational angles (0.73, 1.46, 2.92, and 5.85°rms) corresponding to four accelerations (0.125, 0.25, 0.5, and 1.0ms−2 rms), where the acceleration is that caused by rotation through the gravitational vector. Postural stability was determined by subjective methods and by measuring the displacement of the centre of pressure at the feet during horizontal oscillation. During horizontal oscillation, increases in motion magnitude increased instability and, with the same velocity at all frequencies from 0.125 to 2.0Hz, most instability occurred in the region of 0.5Hz. Fore-and-aft oscillation produced more instability than lateral oscillation, although displacements of the centre of pressure were similar in both directions. With the same angular displacement at all frequencies from 0.125 to 2.0Hz, pitch oscillation caused more instability than roll oscillation, but in both directions instability increased with increased frequency of oscillation. Frequency weightings for acceleration in the plane of the floor during translational and rotational excitation show the significance of low-frequency translational oscillation and high-frequency rotational motion, and show that it is necessary to know whether the measured acceleration is caused by translation or rotation through gravity. [Copyright &y& Elsevier]