In vivo measurements of spinal column vibrations.

Recent epidemiological studies have indicated high risk factors for persons exposed to vibrations. We measured the in vivo responses of the lumbar vertebrae and sacrum of five volunteers who were subjected to pure sinusoidal vertical vibrations in the seated position. Two acceleration amplitudes were used, one and three meters per second squared, with frequencies ranging from two to fifteen hertz. Spinal vibration was measured for two lumbar vertebrae using a transducer that was attached directly to the spinous processes. Axial, horizontal, and rotatory accelerations in the sagittal plane were determined for each vertebra. Vertical acceleration at the sacrum was also measured. The amplitude ratios of the accelerations at the vertebrae and sacrum with respect to the acceleration of the seat were calculated. Our results show that the pure vertical sinusoidal input vibration at the seat produced vibrations of the lumbar vertebrae not only vertically but also horizontally, as well as a rotational vibration. The resonance frequency of the lumbar vertebrae in the vertical direction was an average of 4.4 hertz. Horizontal and rotatory resonance frequencies could not be determined.