ABSTRACT
It is apparent from whole body vibration data that the human spinal system has a characteristic response to vibrational inputs in a seated posture. The first resonance occurs within a band of 4.5 to 5.5 Hz. Similar, but less tightly defined resonances, are also identified in the 9.4 to 13.1 Hz range. Using direct measures, one can confirm the resonance at 4.5 to 5.5 Hz, but can show how it is markedly affected by the pelvis-buttocks system. The response of the human is due to a combination of a vertical subsystem and a rotational subsystem. The latter is characterised by rocking of the pelvis. Hence, pelvic rocking was shown to be an important factor in the first natural frequency response of the seated individual. Cushions have not been found to be very effective in attenuating the dynamic response and, in fact, increased the response. There are some limitations of these studies. The techniques are highly invasive, and by necessity, limited to a few subjects. The experiments are difficult, and the analyses quite complex.
