Biomechanical modeling of subjective fatigue during high-frequency repetitive manual-handling tasks
Authors: Kuramoto Akisue , Noguchi Masaya , Nakashima Motomu 
Published: November 2024

Accumulation of muscle fatigue and subjective fatigue are significant causes of decline in individual performance. Those fatigues can also lead to work errors and the associated musculoskeletal disorders. Thus, a quantitative evaluation of fatigue accumulation during work is required to manage the risk of industrial accidents. Most of the current assessment methods of workload are based on observing and scoring the range of joint motion and work frequency at a point in time. In other words, these assessment methods do not fully consider the continuous accumulation of fatigue. However, even while repeating the same task, muscle fatigue-recovery states and work movements change over time. Therefore, risk management of industrial accidents is important to objectively evaluate the subjective sense of strain and muscle fatigue from work movement data. This study aims to biomechanically model muscle fatigue and subjective fatigue during high-frequency repetitive manual-handling tasks. In an experiment, participants were asked to repeatedly lift a bottle weighing approximately 1 kg, containing salt as ballast, from a chest-height shelf to an eye-level shelf every two seconds for ten minutes. Both start and end points were set at the point approximately 80% of the upper limb length from shoulders at those heights in the midsagittal plane. During the experiment, whole-body motion was measured using an inertial sensor-based motion capture system. In addition to the body motion, electromyograms and subjective evaluations based on the Borg-CR10 scale of the upper limb were recorded. The measured body motion data were applied to a human musculoskeletal model to simulate muscle activity at each sampling time in the experiment. The results were then applied to the Xia and Frey-Law muscle fatigue model to simulate each muscle's residual capacity and fatigue at each sampling time during the experiment. The ratio of the simulated muscle activity to the simulated residual capacity was defined as the substantial muscle activity rate (SMAR). Changes in the SMAR during the experiment were compared with the changes in subjective fatigue and EMG median frequency. Throughout the task, slight abduction and forward flexion were kept in the upper arm. Therefore, we focused our discussion on the deltoid muscle, which might be the most heavily loaded during the experiment. The frequency analysis result of electromyograms indicated that the frequency power spectrum in the medial deltoid shifted to a lower frequency band in the first few minutes and was generally constant in the rest. The residual capacity of the medial deltoid simulated by the muscle fatigue model declined nonlinearly in the first few minutes and was almost constant after that. These results indicate that the muscle fatigue model sufficiently represented the fatigue at the medial deltoid. The muscle activity rate simulated by the musculoskeletal model was almost the same throughout the experiment. On the other hand, the SMAR declined in the first few minutes and continued at a higher range than the muscle activity rate. This changing trend of the SMAR was similar to the time change of the subjective fatigue of the shoulder.