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Nonlinearity in the localised apparent masses of the seated human body exposed to vertical vibration

机译：人体在垂直振动下的局部表观质量的非线性

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The apparent mass of the seated human body exposed to vertical whole-body vibration has shown nonlinear characteristics with the principal resonance frequency decreasing with increasing vibration magnitude. Experimental studies have been designed to investigate the cause of nonlinearity but there seems no consistent conclusion on the role of body parts in the nonlinearity of the apparent mass. Biodynamic modelling provides insight into effect of each model parameter on the dynamics of the system so that the contribution of individual body parts on the nonlinearity can be investigated. In this paper, a multi-body dynamic model of seated human body that can represent the dynamic forces over the local area of contact beneath the ischial tuberosities and thighs was developed. Model parameters were determined for 14 individuals who were exposed to vertical whole-body vibration at 0.25, 0.5, and 1.0m/s~2 r.m.s. in a previous study. With the model parameters determined for 14 individuals and 3 vibration magnitudes, statistical analysis was performed to confirm the effect of vibration magnitude on model parameters. The model developed in this study contained six rigid bodies representing upper-torso, lower-torso, pelvis, thighs, legs, and viscera of the body, interconnected by linear rotational springs and dampers. The geometry parameters of the model were initially derived from a finite-element model of spine and were later scaled to each of the 14 individuals. It was found as vibration magnitude increased, the vertical stiffness beneath the ischial tuberosities, the middle thighs, and the front thighs, the stiffness of viscera, the stiffness of the lower thoracic joint, and the vertical damping beneath the ischial tuberosities decreased significantly. For the vertical apparent mass, reducing the vertical stiffness beneath the ischial tuberosities and thighs and the stiffness of viscera reduced the resonance frequency, while reducing the other stiffness parameters did not see the similar effect. For the fore-and-aft cross-axis apparent mass, reducing the fore-and-aft stiffness beneath the ischial tuberosities and thighs reduced the resonance frequency, while reducing the other parameters of stiffness did not show the similar effect. It was concluded that the soft tissues beneath the ischial tuberosities and thighs and the viscera are the parts principally causing the nonlinearity observed in the apparent masses of the human body.
机译：人体在垂直垂直振动下的表观质量表现出非线性特征，其主共振频率随振动幅度的增加而减小。已经设计了实验研究来研究非线性的原因，但是关于身体部位在表观质量的非线性中的作用似乎没有一致的结论。生物动力学建模可以深入了解每个模型参数对系统动力学的影响，从而可以研究各个身体部位对非线性的影响。在本文中，一个坐着的人体的多体动力学模型可以代表坐骨结节和大腿下方接触局部区域的动力。确定了在0.25、0.5和1.0m / s〜2 r.m.s下暴露于垂直全身振动的14个人的模型参数。在以前的研究中。在为14个人和3个振动幅度确定模型参数的情况下，进行统计分析以确认振动幅度对模型参数的影响。本研究开发的模型包含六个刚体，分别代表上躯干，下躯干，骨盆，大腿，腿和内脏，并通过线性旋转弹簧和阻尼器互连。该模型的几何参数最初是从脊柱的有限元模型导出的，后来被缩放为14个人中的每一个。结果发现，随着振动幅度的增加，坐骨结节，大腿中部和前大腿下方的垂直刚度，内脏的刚度，胸下关节的刚度以及坐骨结节下方的垂直阻尼显着降低。对于垂直表观质量，降低坐骨结节和大腿下方的垂直刚度以及内脏的刚度会降低共振频率，而降低其他刚度参数则看不到类似的效果。对于前后横轴视在质量，降低坐骨结节和大腿下方的前后刚度会降低共振频率，而降低其他刚度参数则不会显示出相似的效果。结论是，坐骨结节和大腿下方的软组织以及内脏是造成人体表观质量非线性的主要部分。

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