ON THE HEALTH RISK OF THE LUMBAR SPINE DUE TO WHOLE-BODY VIBRATION - THEORETICAL APPROACH, EXPERIMENTAL-DATA AND EVALUATION OF WHOLE-BODY VIBRATION

The guidance on the effects of vibration on health in standards for wh ole-body vibration (WBV) does not provide quantitative relationships b etween WBV and health risk. The paper aims at the elucidation of expos ure-response relationships. An analysis of published data on the stati c and dynamic strength of vertebrae and bone, loaded with various freq uencies under different conditions, provided the basis for a theoretic al approach to evaluate repetitive loads on the lumbar spine (''intern al loads''). The approach enabled the calculation of ''equivalent''-wi th respect to cumulative fatigue failure-combinations of amplitudes an d numbers of internal cyclic stress. In order to discover the relation between external peak accelerations at the seat and internal peak loa ds, biodynamic data of experiments (36 subjects, three somatotypes, tw o different postures-relaxed and bent forward; random WBV, a,,, r.m.s. 1.4 ms(-2), containing high transients) were used as input to a biome chanical model. Internal pressure changes were calculated using indivi dual areas of vertebral endplates. The assessment of WBV was based on the quantitative relations between peak accelerations at the seat and pressures predicted for the disk L5/S1. For identical exposures clearl y higher rates of pressure rise in the bent forward compared to the re laxed posture were predicted. The risk assessment for internal forces considered the combined internal static and dynamic loads, in relation to the predicted individual strength, and Miner's hypothesis. For exp osure durations between 1 min and 8 h, energy equivalent vibration mag nitudes (formula B.1, ISO 2631-1, 1997) and equivalent vibration magni tudes according to formula B.2 (time dependence over-energetic) were c ompared with equivalent combinations of upward peak accelerations and exposure durations according to predicted cumulative fatigue failures of lumbar vertebrae. Formula B.1 seems to underestimate the health ris k caused by high magnitudes, formula B.2 is recommended for the evalua tion of such conditions. (C) 1998 Academic Press.