With the rapid development of China's express delivery industry, small parcel couriers generally report overtime, excessive intensity, and high work load, and are at a high risk of developing work-related musculoskeletal disorders (WMSDs). Using questionnaires to conduct research on WMSDs-associated postures may contain unavoidable subjectivity, and the simulation methods can quantitatively evaluate WMSDs, boasting advantages such as high efficiency and low cost, and become a research development trend.

To conduct ergonomic evaluation of typical postures of express couriers handling small parcels using a simulationmethod.

SolidWorks software was used to establish a geometric model of a parcel (530 mm depth × 290 mm width × 370 mm height), a geometric model of an express carriage (1.4 m depth × 0.9 m width × 1.17 m height), and a geometric model of a delivery truck (1.7 m height). These models were imported into Jack software, and the weight of parcel was set at 20 kg. The digital model of a male express courier was established in Jack software. A typical parcel handling task was defined as standing on one side of the express carriage and bimanually placing parcels at four different heights (top, upper half, middle, and bottom of the carriage) combined with three horizontal positions (outer side, center, and inner side of the carriage). The visual field and reach zones tools in Jack software were used to determine whether the parcels can be seen and touched by the express courier, and determine the operational posture of the express courier at their ends of motion ranges. The Ovako working posture analysis system (OWAS) and rapid upper limb assessment (RULA) tools in Jack software were used to evaluate postures applied at typical vertical and horizontal distances.

The maximum horizontal and vertical gaze fields effectively covered the area where the parcels were located, but some areas were not within the optimal field of view. When the horizontal distance between the parcel and the express courier exceeded 48 cm, the express courier was not able to directly reach it and had to adjust his standing position or posture to perform the task. The OWAS analysis found that when the express item was placed at the top, upper section, or middle of the carriage, the risk level of the courier's work posture was level 1, suggesting a normal posture. When the parcel was placed at the inner side bottom of the carriage, the courier needed to bend his knees and lower back, and the highest risk level for the posture was level 3, which had harmful effects on the musculoskeletal system, and correctiveactions were required as soon as possible. The other two work postures placing the parcel elsewhere in the bottom were graded level 2, which may cause harm to the musculoskeletal system , and corrective actions were required in the near future. The RULA analysis found that the risk of the posture was level 2 for handling the parcel in the upper or middle of the carriage, and level 3 for handling the parcel at the top or bottom of the carriage. When the parcel was placed at the bottom of the carriage, as the horizontal distance between the express item and the courier increased, the risk of posture increased to level 3 or above, and the risk reached level 4 for the parcel located at the innermost side of the carriage.

There is a significant risk of WMSDs in manually handling parcels at different heights and horizontal positions. The risk levels of associated postures are elevated from low to high following the express item pleased at the middle, upper, bottom, and top of the carriage and the extension of horizontal distances, if the express item is placed at the bottom of the carriage, between the courier and the parcel. By installing doors on both sides of the express delivery carriage, it is possible for the express courier to adjust his standing position and provide a vertical motion range, ensuring best visual field and reach zones.