Mechanism of nonlinear biodynamic response of the human body exposed to whole-body vibration

Abstract

When the human body is exposed to mechanical vibration, the resonance frequencies of the frequency response functions, such as apparent mass and transmissibility, decrease with increasing magnitude of excitation. For the past two decades, this biodynamic ‘nonlinearity’ has been reported with vertical and horizontal excitation of the body in a wide variety of static sitting and standing postures that require activity from muscles to maintain the stability of the body. There has been speculation, but no experimental evidence, as to the mechanism causing the non-linearity. A review of the literature suggested that either active muscular activity or passive thixotropy of soft tissues is the primary cause of the nonlinearity. The principal objective of this thesis is to identify, and provide experimental evidence of, the primary causal mechanism for the biodynamic nonlinearity. With 0.5 to 20 Hz broadband random vertical vibration at 0.25 and 2.0 ms-2 r.m.s., the first experiment investigated the effect of voluntary periodic upper-body movement and vibration magnitude on the apparent masses of 14 seated subjects. Some movements of the body, such as ‘back-abdomen bending’, significantly reduced the difference in resonance frequency at the two vibration magnitudes compared with the difference during upright static sitting. Without voluntary periodic movement, the median apparent mass resonance frequency was 5.47 Hz at the low vibration magnitude and 4.39 Hz at the high vibration magnitude. With voluntary periodic movement (e.g. back-abdomen bending), the resonance frequency was 4.69 Hz at the low vibration magnitude and 4.59 Hz at the high vibration magnitude. It was concluded that voluntary or involuntary muscular activity, or passive thixotropy of soft tissues, or both muscle activity and thixotropy, could explain the reduction in nonlinearity evident during voluntary periodic movement. The effect of shear history and vibration magnitude on the apparent mass was investigated using 12 subjects in a relaxed semi-supine posture assumed to involve less muscle activity than static sitting and standing. The semi-supine subjects were exposed to two types of vertical (in the x-axis of the semi-supine body) and longitudinal horizontal (z-axis) vibration: (i) continuous random vibration (0.25–20 Hz) at five magnitudes (0.125, 0.25, 0.5, 0.75, and 1.0 ms-2 r.m.s.); (ii) intermittent random vibration (0.25–20 Hz) alternating between 1.0 and 0.25 ms-2 r.m.s. With continuous random vibration, the dominant primary resonance frequency in the median normalised apparent mass decreased from 10.35 to 7.32 Hz as the magnitude of vertical vibration increased from 0.125 to 1.0 ms-2 r.m.s., and from 3.66 to 2.44 Hz as the magnitude of horizontal vibration increased from 0.125 to 1.0 ms-2 r.m.s. With the intermittent vibration, the resonance frequency was higher at the higher magnitude (1.0 ms-2 r.m.s.) and lower at the lower magnitude (0.25 ms-2 r.m.s.) than during continuous vibration at the same magnitudes. The response was typical of thixotropy being the primary cause of the nonlinearity. Harmonic distortions in the dynamic force of semi-supine subjects exposed to sinusoidal excitation showed similar dependence on the frequency and magnitude of vibration as previously reported for seated subjects, again suggesting thixotropy as a primary cause of the nonlinearity. In a group of 12 subjects, the apparent mass and transmissibility to the sternum, upper abdomen, and lower abdomen were measured in three supine postures (relaxed semi-supine, lying flat, and constrained semi-supine) during vertical random vibration (0.25 to 20 Hz) at seven vibration magnitudes (nominally 0.0313, 0.0625, 0.125, 0.25, 0.5, 0.75 and 1.0 ms-2 r.m.s.). The motion transmission path that included more soft tissues exhibited a greater nonlinear response. The substantial nonlinearities found in transmissibilities to both the sternum and the abdomen of supine subjects, and previously reported for the transmissibilities of seated and standing subjects, imply that soft tissues at the excitation-subject interface contribute to the nonlinearity. It is concluded that the thixotropy of soft tissues, rather than voluntary or involuntary muscular activity, is the primary cause of the biodynamic nonlinearity seen with varying magnitudes of excitation.

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

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