A Parallel, 2-DOF Exoskeleton for the Human Shoulder: Device Characterization and Preliminary Results on Healthy Subjects

Abstract BackgroundHumans are highly reliant on the efficient function of their upper limb. Neuromuscular disorders that impair the function of the shoulder consequently reduce quality of life. Robotic rehabilitation serves as an attractive treatment choice due to its promising results and its ability to alleviate the demands on therapists and clinicians. Nevertheless, current robotic architectures are not optimized for the human shoulder but are more apt for industrial environments. Pneumatically powered soft robotic actuators present an attractive method to create shoulder exoskeletons due to their compliance and relatively low mass. However, current actuators lack the necessary functions to provide support to the entire shoulder’s range of motion.MethodsA modular, fabric pneumatic actuator was constructed. The actuator design allows it to perform three-dimensional (3-D) bends with minimal resistance. Four actuators were combined to create a soft shoulder exoskeleton. Each actuator drives one direction of motion: elevation and depression, rotation of the plane of elevation. The torque output of the actuator was measured using a customized two-axis torque measurement system. Exoskeleton functionality was tested through surface electromyography of relevant shoulder muscles. 10 healthy subjects were recruited and performed arm motions under the assistance of the exoskeleton.ResultsThe actuator can reach full bending (>360°) with low pressures (~10kPA). Its torque output is highly dependent on its geometry. Moreover, torque output is reduced as the bending angles increase. The actuators installed on the exoskeleton output 11.15N-m of torque at the neutral position, and 4.44 N-m at 90° shoulder elevation. The test on healthy subjects showed that use of the exoskeleton reduces muscle activation by up to 65% when performing shoulder elevation, and up to 34% when rotating the plane of elevation. Use of the exoskeleton also resulted in a change in arm trajectory when performing elevation and depression movements.ConclusionsThe reduction in muscle activation highlights the ability of a soft-robotic exoskeleton in supporting arm movements. Moreover, the presented exoskeleton design successfully demonstrated its ability to generate two degree-of-freedom support for the human shoulder.