Effects of external hand force modeling on validity of inverse analysis of lifting

Manual lifting tasks pose the risk of musculoskeletal injury, including damage to lumbar intervertebral discs. To assess the risk, spinal joint reaction forces are often estimated using musculoskeletal simulations. However, such analyses require information of external hand force and moment (EHF&M), which has mostly been estimated using simplified models without sufficient validation. This study evaluates the validity of two common EHF&M models by comparing their analysis outcomes to those resulting from directly measured EHF&M. Simplified models yielded negligible errors during the middle phase of symmetric lifting and lowering but caused substantial errors at the beginning and end of each task, leading to inaccurate estimation of peak spinal joint reaction forces. These findings highlight the limitations of simplified models in evaluating mechanical factors of injury risk. The measurement platform and analysis method developed in this study can also contribute to validating EHF&M modeling for other tasks and improving assessment accuracy.