¹Federal Institute for Occupational Safety and Health (BAuA), Berlin, Germany
²Berlin, Germany

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Introduction: Pushing and pulling loads is a common ergonomic solution to avoid the manual transport of loads. But under unfavorable working conditions, pushing and pulling loads can also cause high physical workloads and is associated with musculoskeletal stress and strains. The Key Indicator Method ‘Pulling and Pushing of Loads’ (KIM PP) can be used to assess the hazards of occupational activities requiring the pushing and pulling of loads. In particular for the pushing of hand trucks, the risk model included in the KIM PP is mainly based on expert judgements. There is a lack of data from high-quality laboratory studies on short-term physical workloads when using single-axle industrial hand trucks, such as sack trucks.

Objective: The aim of this study was to quantify the physical strain caused by pushing hand trucks in a laboratory study by varying the load to be moved, the inclination of the test track to be travelled and the path surface as performance conditions.

Methods: A total of 16 healthy male participants (age: mean 24 years, 20–38 years) repeatedly pushed a hand truck over a test track of 5 m with 6 steps. On two test days per subject, path surfaces (smooth, coarse stone, linoleum, road and plaster grit 2 to 4 mm), inclinations (0°, 3°, 7°, 12°) and load weights (60 kg to max. 225 kg, depending on inclination) were systematically varied (72 combinations of test conditions). Every test was performed twice. The muscle activity as the mean of the normalized integrated surface EMG signal of 22 muscles, subjective rating of exertion (Borg RPE scale) and cardiovascular load (heart rate) were recorded for each single test. Statistical analysis was performed using robust hierarchical linear regression models with repeated measures (GEE, SPSS procedure GENLIN) with respect to the three varied conditions, adjusted for height, weight and time of day. Ethical approval was obtained.

Results: The normalized muscular activity of almost all major muscle groups of the upper and lower extremities and the upper and lower back, the working heart rate and the subjective rating of exertion directly after the test showed a clear and linear increase of the parameters with increasing inclination and load weight and for the grit as surface in relation to the reference condition (0° inclination, smooth background, 60 kg load). Assuming an eight-hour working day the workload of the tested conditions can be rated as equivalent as ranging from light to high workload.

Discussion: The results confirm and quantify the significant and linear increase in muscular and cardiac load up to a high workload when pushing hand trucks with increasing loads, inclinations, and uneven/irregular ground conditions. Very high loads greater than 250 kg, as indicated by the KIM PP, are too heavy and therefore cannot be performed safely.

Conclusion: These results for pushing hand trucks provide an empirical basis for the derivation and discussion of adjustments to the KIM PP key indicator scores.