¹Vrije Universiteit Amsterdam, Department of Human Movement Sciences, Amsterdam, Netherlands
²Federal Institute for Occupational Safety and Health (BAuA), Berlin, Germany
³Berlin, Germany

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Introduction: Pushing and pulling loads is a common type of stress and is associated with musculoskeletal strain. The ‘Pushing and pulling’ key indicator method (KIM PP) is used for risk assessment. An assessment in the KIM PP is based on estimates. Reliable statements on individual stress (and strain) from laboratory studies for single-axle trucks (as hand trucks) are lacking. The aim of the study was to quantify the individual physical load when pushing hand trucks in a laboratory study under varying load, path inclination and surfaces as execution conditions.

Methods: A total of 16 healthy male participants pushed a hand truck over a short test track (approx. 4.5 m) on two days. In 72 conditions with repetition, the floor coverings (linoleum, smooth and coarse stone, grit), the inclinations (0°, 3°, 7°, 12°) and the load weights (60 kg to 225 kg) were varied. The duration of a pass in seconds [s] was measured from picking up to setting down the hand truck. The hand forces were measured in three dimensions using measuring grips (Kistler). Ground reaction forces were recorded using force plates (Kistler). The outcome was the averaged amount of the resulting force vector in Newtons [N] over one trial. The statistical analysis was performed using linear hierarchical regression with repeated measurements for the three conditions adjusted for body height and weight and the time of day of the measurement. A positive ethics vote was received.

Results: The manual forces averaged over the pushing process under reference conditions (60 kg, 0° inclination, smooth stone) were 37 N (95%CI 32 – 41 N) on the left and 32 N (95%CI 28 – 37 N) on the right. These forces increased linearly across the inclination categories by up to +24 N on the left and +36 N on the right at an inclination of 12°. Similarly, the averaged hand forces also increased across the load categories by up to +36 N on the left and +47 N on the right with a 225 kg load. The increment of the hand forces under the influence of the surfaces is negligible in comparison. For ground reaction forces an average amount of the resulting force of 884 N (95%CI 861 – 908 N) was determined for the reference test. This force amount reduced linearly by up to - 140 N across all load levels. In comparison, the ground reaction forces were only slightly decreased due to the influence of the inclination (by up to - 15 N at 12°) and the surface (by up to - 7 N on grit). Pushing the hand truck on the test track under reference conditions took an average of 6.2 s (95%CI 5.8 – 6.6 s). With increasing inclination, the duration increased exponentially by up to +2.3 s at an inclination of 12°. Across all weight categories, the duration increased by up to +2.6 s with a 225 kg load. In contrast, changes in duration due to the surfaces are small (less than +0.5 s).

Discussion: With regard to the individual load when pushing hand trucks, the results confirm and quantify the clear dependence on the load, inclination and ground conditions. High loads and steep inclines increase the individual load.

Conclusion: The results provide a basis for discussing adjustments to the point scores in the KIM PP for hand trucks and for better understanding the physical load when pushing hand trucks.