Measures of internal lumbar load in professional drivers – dependence on posture, anthropometry, age, duration of exposure and type of machine

Introduction In European and International Standards and Directives, external loads in terms of daily vibration exposure A(8) or vibration dose value VDV are used as predictors of adverse health effects. These values only partly reflect the internal load acting on the lumbar spine. Particularly for shock containing acceleration time histories, a method for the calculation of internal spinal forces on the basis of a Finite-Element-Model (FEM) was developed within the EU VIBRISKS project [1]. The model can be adapted to the anthropometry (ten categories) and the sitting posture (six categories) of the driver. The method has been proposed in a working draft of the Standardisation Committee ISO/TC 108/SC 4/WG 15 [2]. It has already been standardised in the German Standard DIN SPEC 45697 [3]. The risk assessment is based on the daily compressive dose Sed (MPa) and the risk factor R (non-dimensional). Both values are determined by the compressive internal forces, by the daily and lifetime exposure duration and by the age at the start of exposure. For a re-analysis of the Italian part of an epidemiological study performed within the VIBRISKS project, the internal forces, Sed and R were calculated for the 537 participants of the study. Methods Typical acceleration time histories for various machine types and working tasks were selected. Impacts due to sitting down or loosing the contact to the seat were eliminated. At the end, 19 checked time histories with a duration of 200 s were available. All signals were shock containing [4]. Beside the calculation of Sed and R factor, r.m.s., r.m.q. and standard deviation of the time histories of the internal forces were assessed in all three axes separately and as a vector sum. Therefore, a modification of the program published on the CD in DIN SPEC 45697 had to be used, because it was necessary to get the interim results of the calculations in terms of internal forces. In the horizontal axes, the daily doses were calculated without relation to the spinal endplate areas. The daily doses for the internal forces acting in anterior and posterior directions were separately computed. In total, 21 different variables were calculated. Results The values of the forces, Sed and R factor varied over the lumbar spine levels from T12/L1 to L5/S1. The shapes of the spine-level dependent curves were determined by the posture and the type of machine (example for posture group1 in Fig. 1). The category of body mass and Body Mass Index (BMI) and the posture - in association with the covariates listed in Table 1 - significantly influenced the maximum R factor (statistical test: ANCOVA). The interaction between the body mass/BMI-category and the posture was not significant (F=1.3, p=0.11). When several machines in identical sitting posture were used, the type of machine showed a significant effect as well (Table 1, last three lines). In these cases, the effect of the yearly exposure duration was not longer calculated due to a lack of variability of the covariate. Instead of that, the last column of Table 1 contains the interaction between the body mass/BMI-category and the type of machine which was significant for all three postures. The p-value was p<0.001 in nearly all cases. The exceptions are indicated with (+) in Table 1. Similar results were found for the forces and the Sed values. Conclusions The results of this study showed that the measures of internal lumbar load in a population of professional drivers according to ISO/WD 2631-5 were significantly influenced by numerous individual and work related factors: anthropometry, age, duration of exposure, posture and type of machine. Measures of internal lumbar load might be well suited as predictors of adverse health effects on the lumbar spine of an individual for a working lifetime. References [1] Hinz B, Seidel H, Blüthner R, Menzel G, Hofmann J, Gericke L, Schust M (2007) Whole-body vibration experimental work and biodynamic modelling, Annex 18 to the final report on Task 6.1: Whole-body vibration laboratory studies and biodynamic modelling. VIBRISKS (EC FP5 Project No. QLK4-2002-02650) www.vibrisks.soton.ac.uk [2] ISO/WD 2631-5. Mechanical vibration and shock – Evaluation of human exposure to vibration – Part 5: Methods for evaluation of vibration containing multiple shocks. DIN, 2011. ISO/TC 108/SC 4/WG 15. [3] DIN Deutsches Institut für Normung (2012) Mechanische Schwingungen und Stöße – Verfahren zur Bewertung stoßhaltiger Ganzkörper-Vibrationen; mit CD-ROM (Mechanical vibration and shock – Method for evaluation of impulsive whole-body vibration; with CD-ROM), German National Standard, DIN SPEC 45697. [4] Schust M, Hinz B, Menzel G, Pinto I, Hofmann J, Bovenzi M (2012) Comparison of different methods for detecting multiple shocks in vibration time histories. Paper presented at the 47th United Kingdom Conference on Human Responses to Vibration, held at ISVR, University Southampton, 17 - 19 September 2012