Establishment of a robot-assisted ex vivo active shoulder simulator

Background Stable, repeatable, muscle-driven shoulder simulation remains difficult to achieve ex vivo , although it is essential for studying clinically relevant variables such as joint reaction forces, glenohumeral translations, and joint torques that cannot be measured directly during dynamic motion in vivo . Methods A robot-assisted active shoulder simulator (RAASS) was developed in which six cable-driven actuators applied forces to the rotator cuff and deltoid muscle group. An industrial robot maintained defined boundary conditions and recorded synchronized kinetic and kinematic data during active scapular-plane glenohumeral abduction in 19 fresh-frozen cadaveric shoulders. Closed-loop force control was used to regulate actuator loading. Internal rotation and horizontal adduction were constrained to 0°. Translational force targets maintained joint compression while muscle force ratios were updated across the range of motion. Results Active glenohumeral abduction to 60° was successfully reproduced in all specimens. The robot compensated for emerging torques and external loads, allowing the muscle actuators to remain the primary drivers of motion. Mean total muscle force increased significantly with abduction angle, from 81 ± 36 N/kg at 20° to 107 ± 28 N/kg at 60°. Superior translation increased significantly with abduction, whereas posterior and medial translations showed no significant angle-dependent changes. Horizontal adduction torque increased from 20° to 40° and remained elevated at 60°, and internal rotation torque reached its greatest magnitude at 40°. Conclusion This study demonstrates the feasibility of a robot-assisted ex vivo active shoulder simulator that combines muscle-driven motion with robotic load and constraint management. Beyond enabling controlled active abduction, the framework provides continuous, synchronized measurement of muscle forces, translations, torques, and kinematics. It allows the robot to serve as an active load-application and compensation interface. This represents a methodological advance for investigating shoulder stability, joint loading, and more complex dynamic shoulder motions.