Response of the seated human body to whole-body vertical vibration: biodynamic responses to mechanical shocks

Abstract

The biodynamic response of the seated human body has been investigated with 20 males exposed to upward and downward shocks at 13 fundamental frequencies (1 to 16 Hz) and 18 magnitudes (up to ±8.3 ms -2 ). For 1-and 2-degree-of-freedom models, the stiffness and damping coefficients were obtained by fitting seat acceleration waveforms predicted from the measured force to the measured seat acceleration waveform. Stiffness and damping coefficients were also obtained in the frequency domain with random vibration. The optimum stiffness and damping coefficients varied with the magnitude and the frequency of shocks. With both upward and downward shocks, the resonance frequency of the models decreased from 6.3 to 4 Hz as the vibration dose values of the shocks increased from 0.05 to 2.0 ms -1.75 . The stiffness and damping obtained from responses to shocks were correlated with, and similar to, the stiffness and damping obtained with random vibration.Key words: biodynamics; mechanical shocks; apparent mass; force; nonlinearity. Practitioner summaryWhen modelling the dynamic response of the seated human body to vertical acceleration less than 1 g, the relation between force and acceleration can be well represented by a single degree-of-freedom model although the optimum stiffness and damping depend on the magnitude and frequency of sinusoidal, random, or shock motion.

Response of the seated human body to whole-body vertical vibration: discomfort caused by mechanical shocks

“…The apparent mass of each subject was calculated by fitting the mass, m, the damping, c, and stiffness, k, of a single degree-of-freedom model to the measured force and acceleration time histories (Zhou and Griffin, 2016). The association between subjective responses and biodynamic responses was investigated by calculating correlations between the ratio of the vertical apparent masses at two frequencies and the ratio of the subjective responses at the same two frequencies.…”

“…The biodynamic responses obtained from the acceleration and force time histories, and biodynamic models developed from these measures, are reported separately (Zhou and Griffin, 2016).…”

Frequency-dependence of discomfort caused by vibration and mechanical shocks

“…The nonlinearities in biodynamic responses to vertical vibration have been shown to vary with the frequency of vertical vibration, although only at frequencies greater than about 2.5 Hz (Zhou and Griffin, 2014a). The optimum stiffness of a simple biodynamic model representing the vertical apparent mass of the body exposed to vertical shocks of constant vibration dose value, increased for shocks having fundamental frequencies greater than 3.15 Hz, with the optimum stiffness more dependent on the fundamental frequency of a shock than the magnitude of a shock (Zhou and Griffin, 2017a). Considering the large differences in the physical characteristics of the stimuli (as shown in Table 1), these nonlinearities may partly explain why the equivalent contours for vibration and shock in Figure 6 differ over the range 2.5 to 16 Hz.…”

Effects of seating on the discomfort caused by mechanical shocks: Measurement and prediction of SEAT values

“…A 3-degree-of-freedom model of the coupled seat and human body system can predict both the modulus and the phase of vertical seat transmissibility for vibration in the range 1.25 Hz to 25 Hz (Wei and Griffin, 1998). A 1-degree-of-freedom model and a 2degree-of-freedom model can predict the vertical forces applied to the body when sitting on a rigid seat with vertical mechanical shocks having peak accelerations less than about 1 g (Zhou and Griffin, 2017b), but such models have not been used to predict the transmission of shocks though seats and their effects on discomfort.…”

“…With vertical mechanical shocks, linear 1and 2-degree-of freedom models can also provide good fits to the measured biodynamic response of the body, but the optimum stiffness and optimum damping of the models vary with both the nominal frequency of the shocks (1 to 16 Hz) and the magnitude of the shocks (up to ±8.3 ms -2 r.m.s.) (Zhou and Griffin, 2017b). There are no known investigations of linear models for predicting mechanical shocks experienced when sitting on soft seats and no known studies of the applicability of a linear model for predicting the SEAT value of a seat exposed to mechanical shock.…”