Impact of Force Scaling on Physician Fatigue in a Bilateral Tele-Ultrasound System

Tele-ultrasound systems enable remote diagnostic imaging by transmitting both motion commands and haptic feedback between a sonographer and a robotic probe. While these systems aim to replicate conventional ultrasound procedures, they rarely address the physical strain typically experienced by sonographers. In this study, the effect of applying a force scaling strategy to haptic feedback on reducing muscular fatigue and task-induced stress during a realistic tele-ultrasound task is studied. First, a thorough operational and biomechanical analysis of the abdominal US procedure was performed to reconstruct a representative task in the laboratory. Then, a bilateral position-force tele-ultrasound architecture was implemented, and a total of 11 subjects performed the reconstructed remote ultrasound task under two randomized conditions: with and without force scaling. Surface electromyography (sEMG) signals were acquired from seven upper-limb muscles (posterior deltoid, trapezius, anterior deltoid, biceps, triceps, wrist flexors, and wrist extensors). Teleoperation-related stress was also assessed using a seven-item Likert-scale self-report questionnaire administered after each trial. Statistical significance was tested using Repeated Measures ANOVA for EMG data and the Wilcoxon signed-rank test for stress scores. The results showed a statistically significant reduction in muscle activation in 5 out of 7 muscles, and a clear mitigation of fatigue progression over time in the scaled condition. Additionally, perceived stress levels were significantly lower in the presence of force scaling in overall stress scores. These findings support the effectiveness of force scaling as a tool to enhance ergonomics in tele-ultrasound procedures without compromising the operator's ability to perform the task. The proposed methodology proved robust and generalizable, offering valuable insight into the integration of human-centered design in tele-operated diagnostic systems.