Quantification of Shoulder Stiffness at Various Arm Postures using a 4-Bar Parallel Exoskeleton Robot

Shoulder injuries are the leading cause of shoulder discomfort or disabilities. Assessment of the glenohumeral joint functions through system identification technique approach is beneficial considering glenohumeral joint has major contributing factors associated with shoulder movement and stability. This function is identified by estimating a mathematical model by perturbing the glenohumeral joint and measuring the input angle and output torque. In this study, a shoulder exoskeleton robot was utilized, which makes use of a 4-bar spherical parallel manipulator (4B-SPM). The 4B-SPM exoskeleton has the advantage of high acceleration, fast enough to satisfy the speed requirement for the characterization of distinct neuromuscular properties of shoulder. Thirty-four healthy subjects (17 female, 17 male) were appointed with no history of shoulder impairment to characterize shoulder joint stiffness by providing filtered gaussianrandom perturbations with RMS value, frequency of 2 degrees and 3 Hz respectively. These perturbations arecaptured by 3-D Motion capture system by placing markers on arm brace which allows arm to be locked at a particular pose. Participants were instructed to maintain a relaxed state to avoid the interference of the muscle activation on the mechanical properties of the shoulder. Torque was measured using Force-Torque (FT) sensor at 15 different postures. These postures were divided among 3 flexion angles of the shoulder with a set of 5 horizontal extension postural configuration quantified for each flexion angle. The stiffness characterization was performed by utilizing Short Data Segment (SDS) method of time-varying system identification. It was observed that shoulder joint stiffness varied significantly depending on the arm's posture. The shoulder joint stiffness was observed to increase as the flexion angle decreases. Notably, a convex pattern emerged, wherein stiffness values increased as the arm deviated further from the mid-range of the shoulder joint's range of motion (ROM) in horizontal extension directions. These findings suggest that maintaining the arm's posture near the mid-range of ROM decreases the stability of the shoulder joint. The shoulder joint stiffness was also observed to have significant difference on the basis of gender where in male subjects were observed to have higher joint stiffness than female subjects.