Relationships of low back outcomes to internal spinal load: a prospective cohort study of professional drivers

Purpose – To investigate the relationships between low back symptoms and alternative measures of external dose and internal spinal dose in professional drivers exposed to whole body vibration (WBV). Methods – The occurrence of low back symptoms were investigated in a cohort of 537 drivers over a two-year follow up period. Low back pain (LBP), individual characteristics, and work-related risk factors were investigated with a structured questionnaire. Exposure to WBV was evaluated by means of measures of external dose (daily vibration exposure in terms of either equivalent continuous acceleration over an eight-hour period (A(8)) or vibration dose value according to the EU Directive on mechanical vibration) and measures of internal lumbar load (daily compressive dose Sed and risk factor R according to ISO/CD 2631-5 2014). Results – In the drivers’ cohort, the cumulative incidence of 12-month low back outcomes was 16.8% for LBP, 9.3% for chronic LBP, and 21.8% for sciatic pain. The measures of internal spinal load were better predictors of the occurrence of low back symptoms than the measures of daily vibration exposure. A two-fold increase in the risk estimates for low back outcomes was found in the upper quartile of the R factor (0.41-0.72 units) compared to the lower one (0.07-0.19 units). Conclusions – In this prospective cohort study, measures of internal spinal dose performed better than measures of daily vibration exposure (external dose) for the prediction of low back outcomes in professional drivers. The ISO boundary values of the risk factor R for low and high probabilities of adverse health effects on the lumbar spine tend to underestimate the health risk in professional drivers.