Biomechanical modeling of subjective workload considering muscle fatigue in a repetitive task

Accurate, objective estimation of subjective workload is critical issue in occupational health and safety, yet existing methods largely rely on observational assessments rather than quantitative metrics. To address this gap, this paper presents a novel approach for objectively estimating subjective workload by combining the substantial muscle activity ratio, which accounts for fatigue-induced reductions in muscle strength, with a logistic function approximating individual sensitivity curves. The method was applied to three shoulder muscles during a repetitive upper-limb task performed by healthy participants. The results indicated that, for each participant, the estimated subjective workload closely approximated the perceived workload in at least one of the three muscles. This finding demonstrates that the proposed method can simulate temporal changes in perceived workload despite inter-individual variability. Incorporating the logistic function allowed the model to account for nonlinear perception of workload. Further analysis suggested that the dominant muscle contributing to perceived workload differed across participants, depending on individual movement strategies during the task. Motion analysis confirmed that differences in shoulder joint kinematics were associated with the muscles most closely tracking perceived workload. These findings highlight the necessity of accounting for both biomechanical and kinematical factors when estimating subjective workload. Overall, the study demonstrates a physiologically grounded framework for predicting perceived workload and provides a foundation for developing generalized models applicable to real-world tasks, supporting more informed strategies for worker load management.