Estimation of disc compression during transient whole-body vibration

This study was performed to examine health effects of transient whole-body vibrations on the lumbar spine. The aim was to detect extremes in the time course of compressive load acting on the disc L3-4 in order to estimate the health risk which depends on the amplitude of peak values of compression. Five healthy males were repeatedly exposed to various transient displacements with nearly sinusoidal or half-sinusoidal waveforms, different durations, and peak accelerations between about 1.4 and 4.1 ms(-2). Accelerations in the z direction were measured on the skin over the spinous processes of L3-4 in five subjects and averaged individually. Complete time series of dynamic compressive forces were calculated by means of a biomechanical model using the calculated effective mass of the human body above the disc L3-4 and relative accelerations between the vertebrae L3-4 for the first time. The amplitudes of the absolute peak values of the compressive forces were influenced only by the interaction between the initial direction and the duration of the waveform. Direct comparisons with the results of other authors are impossible due to methodical differences and missing data in the time domain. The nearly constant peak compressive forces with a shorter duration of transients connected with a higher-frequency content support the proposal to put more weight on vibrations above 8 Hz in a revised International Standard ISO 2631. The comparison of the calculated internal forces with results of in-vitro studies indicates a possible health risk for persons with a low vertebral strength during repetitive exposures to moderate transient whole-body vibrations.