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.

Clinical Relevance:​ Epidemiological surveys have suggested an increased risk of low-back pain in drivers of tractors, trucks, buses, and airplanes. Furthermore, automobile commuters have been found to have an increased incidence of herniated nucleus pulposus compared with a population that drives less frequently. Although confounding factors cannot be excluded, cyclical loading due to vibration may be responsible. Knowledge of the resonant frequencies of the spine may allow appropriate reduction of vibration input and a change to less harmful frequencies. Our data suggest that, for the unsupported sitting posture, frequencies between four and five hertz should be avoided.