Commercial truck drivers experience approximately 8% of all work-related musculoskeletal disorders (MSDs) in the U.S. and report an above-average number of lost workdays per injury (BLS, 2007). Prevalence rates for low back pain among truck drivers range between 50% and 81% (Miyamo-to, Shirai, Gembun, et al., 2000; Robb & Mansfield, 2007), and truck drivers report experiencing low back pain intensity that is higher than any other occupational driving group (Okunribido, Magnusson & Pope, 2008).

Contributing factors to MSDs and low back pain among drivers include exposures to whole body vibration, prolonged sit-ting and material handling tasks. Material handling exposures are highest among short-haul truck drivers [also known as less-than-truckload (LTL) drivers] who typically deliver freight to customers within a single metropolitan area (Burks, Belzer, Kwan, et al., 2010). These drivers make multiple stops each day, and use their hands, hand trucks, pallet jacks and forklifts to load and unload freight. The physical demands on a short-haul route are affected by factors such as the freight being hauled, trailer technologies and customer environments. Little is known about how often drivers should be observed to obtain reliable measures of ergonomic exposures. The current study was designed to ad-dress this knowledge gap.

Observational ergonomic assessments help evaluate workers' exposures to injury hazards such as the frequency or duration of severe or awkward body postures. A fundamental issue in conducting such assessments is deciding how many observations are required to obtain reliable exposure data. To provide guidance on this topic, some researchers have conducted secondary analyses of relatively large observational data sets and estimated confidence intervals (CI) for exposure means for a range of different observational sampling strategies.

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