Text

Introduction: Traditionally, occupational health and safety (OHS) practitioners and epidemiological studies assess ergonomic risk factors as single, independent exposures—such as duration of arm elevation above shoulder height. For years, many OHS practitioners targeted ergonomic risk factors individually. However, this “single-exposure approach” overlooks that many ergonomic exposures co-occur in time throughout the workday. For example, during window cleaning, a worker may simultaneously be standing and raising their arms above shoulder height. While each exposure is associated with adverse health outcomes, co-occurring ergonomic exposures may put the worker at even higher risk. In other words, when two ergonomic work factors co-occur in time, their risk of ill health can be higher than when considered independently (so, 1 + 1 > 2). Thus, focusing only on single exposures may fail to capture the complete health effects from ergonomic work factors. This study compares how well single vs. co-occurring ergonomic exposures explain perceived physical exertion, using objective measurements of physical behavior, arm elevation, and trunk inclination in a large occupational sample.

Methods: Between 2011 and 2014, 1299 Danish workers wore accelerometers on the thigh, right upper arm, and upper back for 1–5 workdays. Objective data were derived on physical behavior (sitting, standing, and active), upper arm elevation (below/above 90°), and trunk forward bending (below/above 60°). These exposures were analyzed both individually and as co-occurring combinations based on time-series alignment. Co-occurrence variables included behaviors combined with arm elevation (6 categories), behaviors with trunk inclination (6 categories), and behaviors with both arm elevation and trunk inclination (12 categories). Perceived physical exertion was assessed using a 0–10 Borg Scale. Linear regression models were run using either single or co-occurring exposures as predictors, using adjusted R-squared (R2) to assess how exposures explained the perceived exertion.

Results: Regression models using single exposures showed that standing explained 5.6% of the variance in physical exertion (R2 = 0.056), arm elevation above 90° explained 0.2% (0.002), and trunk forward bending above 60° explained 5.0% (0.050). In contrast, models using co-occurring exposures explained more variance: behaviors co-occurring in time with arm elevation explained 10.8% (0.108), behaviors co-occurring in time with trunk inclination 12.5% (0.125), and behaviors co-occurring in time with both arm and trunk exposures explained 12.9% (0.129).

Discussion: These findings suggest that the co-occurring approach of ergonomic exposures better captures workers’ perceived physical exertion, compared to treating them as single factors. Thus, the increase in explained variance, while modest, indicates that workers’ perceptions of physical exertion are shaped not just by isolated ergonomic exposures, but by how multiple ergonomic exposures interact in time.

Conclusions: The results support a shift from single-exposure approaches to models that account for co-occurrence of exposures in time. Such approaches may lead to better ergonomic risk assessments and preventive workplace interventions.