Joowan Kim and Jaeheung Park are the corresponding authors. Abstract This paper presents a novel rehabilitation robot designed to address the challenges of passive range of motion (PROM) exercises for frozen shoulder patients by integrating advanced scapulohumeral rhythm stabilization. Frozen shoulder is characterized by limited glenohumeral motion and disrupted scapulohumeral rhythm, with therapist-assisted interventions being highly effective for restoring normal shoulder function. While existing robotic solutions replicate natural shoulder biomechanics, they lack the ability to stabilize compensatory movements, such as shoulder shrugging, which are critical for effective rehabilitation. Our proposed device features a 6 degrees of freedom (DoF) mechanism, including 5 DoF for shoulder motion and an innovative 1 DoF Joint press for scapular stabilization. The robot employs a personalized two-phase operation: recording normal shoulder movement patterns from the unaffected side and applying them to guide the affected side. Experimental results demonstrated the robot's ability to replicate recorded motion patterns with high precision, with root mean square error (RMSE) values consistently below 1 degree. These findings confirm the robot's potential as a rehabilitation tool capable of automating PROM exercises while correcting compensatory movements. The system provides a foundation for advanced, personalized rehabilitation for patients with frozen shoulders. Keywords: Rehabilitation robot, Shoulder exercise, Scapulohumeral rhythm, Compensatory movements 1 Introduction Frozen shoulder, also known as adhesive capsulitis, is a debilitating condition characterized by pain and progressive loss of shoulder movement [1]. The underlying cause of a frozen shoulder is not fully understood, and it is associated with inflammation and thickening of the capsule that surrounds the shoulder joint where the glenoid of the scapula and the proximal humerus meet [1, 2]. The condition affects approximately 2-5% of the general population, with a higher incidence in people aged in the mid-50s [3]. Frozen shoulder problems extend beyond limited range of motion (ROM), involving compensatory movements that can lead to secondary issues. These compensations mainly include excessive scapular upward rotation and trunk adjustments during arm elevation [4]. Especially, excessive scapular upward rotation results as shoulder shrugging, which directly affects the scapulohumeral rhythm [5, 6]. Scapulohumeral rhythm, traditionally described as a 2:1 ratio between glenohumeral elevation and scapular upward rotation, but in reality it is a nonlinear motion, is crucial for shoulder stability [7-9].

This paper critically analyzes conventional and biomimetic robotic arms, underscoring the trade-offs between size, motion range, and load capacity in current biomimetic models. By delving into the human shoulder's mechanical intelligence, particularly the glenohumeral joint's intricate features such as its unique ball-and-socket structure and self-locking mechanism, we pinpoint innovations that bolster both stability and mobility while maintaining compactness. To substantiate these insights, we present a groundbreaking biomimetic robotic glenohumeral joint that authentically mirrors human musculoskeletal elements, from ligaments to tendons, integrating the biological joint's mechanical intelligence. Our exhaustive simulations and tests reveal enhanced flexibility and load capacity for the robotic joint. The advanced robotic arm demonstrates notable capabilities, including a significant range of motions and a 4 kg payload capacity, even exerting over 1.5 Nm torque. This study not only confirms the human shoulder joint's mechanical innovations but also introduces a pioneering design for a next-generation biomimetic robotic arm, setting a new benchmark in robotic technology.