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

OCCUPATIONAL APPLICATIONS Manual material handlers performing stocking tasks spent substantial amounts of time in bent postures but used traditional stoops and squats infrequently. Instead, they often used split-legged stoops and squats, where one foot is further forward than the other, and one-legged ("golfer's") lifts. During object manipulation, the distance workers reached away from their body, and the height at which they manipulated objects, were correlated with the posture used by the worker. Workers also stayed in different postures for different lengths of time. It is likely that certain postures are more comfortable for the workers to remain in, provide additional mobility or operational radius, or require less energy to use. Understanding these factors in more detail could lead to improved worker training programs, where the postures taught not only have low injury risk but are comfortable so are actually adopted and used by the workers. 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.