Obesity-related differences in shoulder and lumbar loading during manual material handling

Obesity is associated with increased risk of work-related musculoskeletal disorders; however, specific biomechanical effects during manual material handing (MMH) tasks remain unclear. This study examined the influence of body mass index (BMI) on shoulder and lumbar joint moments and postures during a lift/lower task and load transfer task. Sixty-three participants (29 male, 34 female), spanning normal weight to class III obesity, completed MMH tasks under varying load and distance conditions. Higher BMI was associated with greater lumbar and shoulder moments across tasks, with condition-dependent effects indicating loading did not increase uniformly with body size. During lifting, BMI-related increases in lumbar loading were most pronounced during floor-level lifts and under lighter load conditions, with effects amplified in males. During the load transfer, BMI increased shoulder moments, with stronger effects at lighter loads. Trunk and shoulder angles changed minimally, highlighting body composition as an important differential modifier of mechanical exposure during MMH.