Patient-specific upper-limb robotic replica integrating joint rigidity and physiological tremor

The implementation of an upper-limb rehabilitation simulator replicating patient-specific physical characteristics has garnered increasing interest as an effective alternative to human subjects for medical training and research. However, human upper-limb motion involves complex multi-joint coordination, whereas most existing patient simulators reproduce simplified mechanical properties of individual joints and often fail to mimic critical pathological characteristics, such as rigidity and physiological tremors. This study addresses these limitations by proposing a novel simulator that incorporates both compound joint coordination and patient-specific symptoms (e.g. lead-pipe rigidity and physiological tremors). A motion control system based on a linear quadratic regulator was designed, with patient-specific reference torques derived from adult male motion-capture data. The tremor parameters were selected within the range reported in neurophysiological literature, and rigidity was implemented based on spasticity-inspired torque models. The proposed controller achieved torque-tracking errors below 3%, while reproducing clinically relevant peak torque levels of 18–20 Nm at the shoulder and 3–7 Nm at the elbow under manual muscle testing–grade scaling. The experimental evaluations across rigidity levels highlighted the reliability and effectiveness of the proposed simulator in accurately mimicking patient characteristics, making it a practical alternative for patients in rehabilitation training. Additionally, a clinical similarity survey was conducted with eight rehabilitation specialists and 10 students following a standardized, blinded protocol, yielding a mean rating of 9.33 ± 0.72 and a significant group effect (analysis of variance F ( 2 , 57 ) = 12.83 , p < 0.001 ). To the best of our knowledge, this study is the first to combine multi-joint coordination with the simultaneous reproduction of rigidity and physiological tremors in a two-degree-of-freedom upper-limb robotic replica, demonstrating the novelty and practical value of the approach.