A review of literature shows that the driving-point mechanical impedance and apparent mass have been well established experimentally for the seated body and show a principal resonance at about 5 Hz. However, the causes of the principal resonance have not been fully understood. The standing body driving-point response has been determined in a few studies and the findings have varied. This thesis presents a study of the principal resonance of standing and seated bodies by inspection of experimental data and development of mathematical models.
The driving-point apparent masses of standing subjects were obtained in three experiments. Thirty two subjects were exposed to random vibration between 0.5 and 50 Hz at vibration magnitudes from 0.125 to 2.0 ms-2 r.m.s. A principal peak of the apparent mass of subjects standing normally was found in the 4 to 6 Hz frequency range. The resonance frequency tended to be higher in a normal standing posture than in a normal sitting posture, although the difference was generally within 1 Hz. The resonance frequency of the apparent mass decreased by about 1.5 Hz with increases in the vibration magnitude from 0.125 to 2.0 ms-2 r.m.s. in both the standing posture and the sitting posture. It was thought likely that common dynamic mechanisms in the upper-body contributed to the principal resonances of both standing and seated bodies.
The transmission of vibration to nine body locations was determined in the 0.5 to 20 Hz frequency range with twenty subjects in two experiments. A multi-axis measurement method was developed to determine the effect of pitch motion on translational motions along the spine in the sagittal plane. The movement of the upper-bodies of standing and seated subjects at the principal resonance consisted of bending of the spine, particularly in the lumbar region, pitching of the thoracic spine and rib cage and pitching of the pelvis. These motions might be coupled with each other due to the heavy damping of the human body. For the seated body, deformation of the buttocks tissue was also involved in the movement at the resonance. For the standing body, axial motion might be coupled with bending motion in the lower spine. A combination of rotational motions at the leg joints and deformation of the tissue at the sole of the foot occurred at the principal resonance.
Lumped parameter models were developed to interpret the experimental results and investigate dynamic mechanisms involved in the principal resonance. The inclusion of rotational degrees of freedom improved the representation of the transmissibilities. It is concluded from the experiments and the models that the principal resonance in the apparent mass of the seated body is mainly caused by deformation of the tissue beneath the pelvis in phase with vertical motion of the viscera. The deformation of the buttocks tissue causes vertical, fore-and-aft and pitch motions of the pelvis. The principal resonance of the standing body is most influenced by the dynamic response of the viscera and also influenced by rotational motions at the leg joints and deformation of the tissue of the foot sole. Bending motion of the spine, significant in the lumbar spine, occur at the principal resonance frequency but makes a minor contribution to the apparent mass resonance in both standing and seated postures.
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