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• W64761127 abstract "When the human body is exposed to mechanical vibration, the resonance frequencies of thefrequency response functions, such as apparent mass and transmissibility, decrease withincreasing magnitude of excitation. For the past two decades, this biodynamic ‘nonlinearity’ hasbeen reported with vertical and horizontal excitation of the body in a wide variety of static sittingand 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 thenon-linearity. A review of the literature suggested that either active muscular activity or passivethixotropy of soft tissues is the primary cause of the nonlinearity. The principal objective of thisthesis is to identify, and provide experimental evidence of, the primary causal mechanism forthe biodynamic nonlinearity.With 0.5 to 20 Hz broadband random vertical vibration at 0.25 and 2.0 ms-2 r.m.s., the firstexperiment investigated the effect of voluntary periodic upper-body movement and vibrationmagnitude on the apparent masses of 14 seated subjects. Some movements of the body, suchas ‘back-abdomen bending’, significantly reduced the difference in resonance frequency at thetwo vibration magnitudes compared with the difference during upright static sitting. Withoutvoluntary periodic movement, the median apparent mass resonance frequency was 5.47 Hz atthe low vibration magnitude and 4.39 Hz at the high vibration magnitude. With voluntary periodicmovement (e.g. back-abdomen bending), the resonance frequency was 4.69 Hz at the lowvibration magnitude and 4.59 Hz at the high vibration magnitude. It was concluded thatvoluntary or involuntary muscular activity, or passive thixotropy of soft tissues, or both muscleactivity and thixotropy, could explain the reduction in nonlinearity evident during voluntaryperiodic movement.The effect of shear history and vibration magnitude on the apparent mass was investigatedusing 12 subjects in a relaxed semi-supine posture assumed to involve less muscle activity thanstatic sitting and standing. The semi-supine subjects were exposed to two types of vertical (inthe x-axis of the semi-supine body) and longitudinal horizontal (z-axis) vibration: (i) continuousrandom 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. Withcontinuous random vibration, the dominant primary resonance frequency in the mediannormalised apparent mass decreased from 10.35 to 7.32 Hz as the magnitude of verticalvibration increased from 0.125 to 1.0 ms-2 r.m.s., and from 3.66 to 2.44 Hz as the magnitude ofhorizontal vibration increased from 0.125 to 1.0 ms-2 r.m.s. With the intermittent vibration, theresonance frequency was higher at the higher magnitude (1.0 ms-2 r.m.s.) and lower at thelower 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 sinusoidalexcitation showed similar dependence on the frequency and magnitude of vibration aspreviously reported for seated subjects, again suggesting thixotropy as a primary cause of thenonlinearity.In a group of 12 subjects, the apparent mass and transmissibility to the sternum, upperabdomen, 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) atseven vibration magnitudes (nominally 0.0313, 0.0625, 0.125, 0.25, 0.5, 0.75 and 1.0 ms-2r.m.s.). The motion transmission path that included more soft tissues exhibited a greaternonlinear response. The substantial nonlinearities found in transmissibilities to both the sternumand the abdomen of supine subjects, and previously reported for the transmissibilities of seatedand standing subjects, imply that soft tissues at the excitation-subject interface contribute to thenonlinearity.It is concluded that the thixotropy of soft tissues, rather than voluntary or involuntary muscularactivity, is the primary cause of the biodynamic nonlinearity seen with varying magnitudes ofexcitation." @default.
• W64761127 created "2016-06-24" @default.
• W64761127 creator A5060304566 @default.
• W64761127 date "2008-05-01" @default.
• W64761127 modified "2023-09-23" @default.
• W64761127 title "Mechanism of nonlinear biodynamic response of the human body exposed to whole-body vibration" @default.
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