A novel biomechanical model for predicting ankle moments and assessing static balance in users of shoulder-support exoskeletons

Abstract

Shoulder work-related musculoskeletal disorders (WMSDs) pose significant challenges, leading to substantial economic burdens and negatively impacting worker well-being. Shoulder-support exoskeletons have emerged as a promising solution to prevent shoulder WMSDs by providing ergonomic support to workers. However, the exoskeletons may restrict joint motion and affect the user balance, necessitating a thorough understanding of balance effects during the exoskeleton design and evaluation process. Currently, the absence of a balance prediction model for the shoulder-support exoskeleton users poses challenges in incorporating balance considerations into the early design stages. To fill this gap, this study proposes a novel biomechanical model to assess static balance by predicting ankle moment changes in shoulder-support exoskeleton users. Our model integrates principles from the Stiff Rod Inverted Pendulum model and introduces a novel function to compute the ankle moment resulting from exoskeleton reaction forces. The model's performance was validated by comparing it with experimental data and a widely used commercial benchmark software, the 3D Static Strength Prediction Program (3DSSPP). Follow-up experimental validation demonstrated that the model predicted ankle moments closely aligned with the measured body sway and subjective instability ratings. Strong correlations were observed between the predicted ankle moment and subjective instability ratings. The mean difference between predicted ankle moments of the proposed model and those of the 3DSSPP in the no-exoskeleton condition was less than 1.15%. These findings indicate that the proposed model offers valuable insights to facilitate the design and evaluation of exoskeletons for improved static balance.