Determining the maximum feasible box dimensions and box masses for one-handed box transfers from varying heights

Abstract

Background: To address continued prevalence of musculoskeletal disorders (MSD) related to manual materials handling (MMH) in logistics and warehousing industries, more effort is being devoted to proactive ergonomics when (re)designing the workspace through the use of digital human models (DHM). However, effectively using a DHM requires the user to estimate how a worker will interact with an object. One example includes guessing when a worker might use one hand versus two when lifting and transferring a box. To avoid guessing we need a better understanding of the determinants for performing one-handed lifts and transfers to inform DHM users on when human-object interactions should be modeled with one-hand verses two. Purpose: The aim of this work was to determine the maximum feasible box dimensions and masses that would permit a one-handed box lift and transfer from three shelf heights. Methods: Two sex-balanced samples of 20 participants completed a series of box transfer tasks from varying shelf heights (ankle, greater trochanter (GT), shoulder height) to a table with their dominant hand. For Study One, participants were instructed to grab a minimally weighed retractable box from either front or top and expand/collapse its dimension until they perceived that they could no longer transfer it with only one hand. For Study Two, participants were instructed to grab the box from either front or top (using the maximum acceptable dimensions from Study One) and to adjust the mass of the box until they perceived that they could no longer transfer it with only one hand. Within-subject repeated measure ANOVA with an alpha value of 0.05 were used for both studies to detect for shelf height or grip orientation differences. Results: The perceived maximum acceptable box width ranged from 70-73% of participant’s hand length. Perceived maximum width decreased with higher shelf height when grasping the box from the top, while staying consistent when grabbing the box from the front regardless of shelf height. Perceived maximum mass was lower when grasping the box from the top regardless of shelf height and was lower when grasping the box from shoulder height compared to ankle height regardless of grip orientation. Discussion: When the box width exceeded 124mm of the person’s hand length and required more than 36.7% wrist strength, people may be more likely to use two hands. The discrepancy in perceived maximum acceptable box width and box mass when grasping from the top at shoulder height was potentially due to awkward hand/wrists posture or line of sight issues. The effects between grip orientation and shelf height on the perceived maximum lifting capacity for one-handed transfers should be considered when proactively designing stock-picking tasks to optimize safety. Additionally, an understanding of the maximum determinants helped set manual material handling limits for one-handed box transfers. Findings from the current investigations provided psychophysical insights into the one-handed lifting capacity of standard cardboard boxes with no handles.