Metrics of whole-body vibration and exposure-response relationship for low back pain in professional drivers: a prospective cohort study.

OBJECTIVES: The objective of this study was to investigate the relation between alternative measures of exposure to whole-body vibration (WBV) and low back pain (LBP) in professional drivers.
METHODS: The incidence of 12-month LBP, high pain intensity (numerical rating scale score > 5), and disability in the lower back (Roland and Morris disability scale score >or=12) was investigated in a cohort of 537 drivers over a 2-year follow-up period. LBP outcomes, individual characteristics, and work-related risk factors were investigated by direct interview using a structured questionnaire. Daily vibration exposure was expressed in terms of either equivalent acceleration over an 8-h reference period [A(8), root-mean-square (r.m.s.) method] or vibration dose value [VDV, root-mean-quad (r.m.q.) method]. From the vibration magnitudes measured on vehicles and total exposure duration, seven alternative measures of cumulative vibration exposure were calculated for each driver, using expressions of the form Sigma a (i)(m)t(i), where a (i) is the frequency-weighted r.m.s. or r.m.q. acceleration magnitude on vehicle i(a(ws) or a (wq), respectively), t(i) is the lifetime exposure duration for vehicle i, and m = 0, 1, 2, or 4.
RESULTS: In the drivers' cohort, the cumulative incidence of LBP outcomes were 36.3% for 12-month LBP, 24.6% for high pain intensity, and 19.2% for disability in the lower back. A transition model, which takes into account the temporal sequence of cause and effect and captures the longitudinal part of the relationship, showed that VDV performed better than A(8) for the prediction of LBP outcomes. After adjusting for potential confounders, test for trends suggested an increased risk for developing high pain intensity and disability over time (and, to a lesser extent, 12-month LBP) with the increases of cumulative vibration doses computed from lifetime exposure duration and r.m.q. vibration magnitude (i.e. Sigma[a (wqi)(m)t(i)]). Measures of exposure duration, either daily or lifetime, also provided good indications of risk for LBP outcomes over time. Physical work load, but not psychosocial environment, was significantly associated with the occurrence of LBP outcomes over time.
CONCLUSIONS: Measures of vibration exposure derived from exposure duration (daily or lifetime) and r.m.q. acceleration magnitude (VDV, Sigma[a(wqi)(m)t(i)]) were better predictors of LBP outcomes over time than measures of vibration exposure including r.m.s. acceleration (A(8), summation operator[a(wsi)(m)t(i)]). Patterns of exposure-response relationship were more evident for the outcomes high pain intensity or disability in the lower back than for the binary response 12-month LBP.