Quantification of Postures for Low-Height Object Manipulation Conducted by Manual Material Handlers in a Retail Environment.

Technical Abstract Background Musculoskeletal disorders are relatively common among manual material handlers. This may be due in part to challenging postures used by workers. Purpose Studying the kinematics of manual material handlers in the workplace will provide quantitative data on how they move and what postures they adopt. With these data, some insights can be determined about why workers chose certain postures. Methods We conducted an on-site workplace study to capture the full-body kinematics of manual material handlers (stockers) using inertial measurement units. We organized the observed bends into six classes: stooping, fore-aft squatting, split-legged stooping with one-heel raised, split-legged stooping with no heels raised, one-legged lifting, and mixed lifting, which include multiple forms while remaining bent. These classes were based on a new general classification of bending and lifting postures that we developed, which enumerates all of the possible forms. We quantified how frequently and for what duration the workers bent and lifted, and determined how often they performed asymmetric motions while bending. We determined the range of motion of the hand positions during each bent posture, which provides a measure of the workspace afforded by the posture. Results Workers rarely used symmetric squats and infrequently used symmetric stoops typically studied in lab settings. Instead, they used a variety of different postures that have not been well-characterized. Of the 666 bending postures recorded during the experiment, 27.3% were stoops lifts, 22.1% were one-legged lifts, 20.3% were split-legged bends with both heels on the ground, and 12.3% were split-legged bends with a heel raised. Only 4.6% of the postures were squats and only one participant used this posture. Different bending postures were correlated with different ranges of hand position used in object manipulation. One-legged lifting corresponded to bends with the hands furthest away from the body along the sagittal axis. Conclusions While our study was exploratory, we observed many kinematic forms that have not been studied much in the past, such as split-legged stooping and one-legged lifting, suggesting that future work should be done to understand the biomechanics of these postures.