Validation of a Novel Parallel-Actuated Shoulder Exoskeleton Robot for the Characterization of Human Shoulder Impedance

This study validates the effectiveness of a recently developed parallel-actuated shoulder exoskeleton robot for the purpose of characterizing the neuromuscular properties of the human shoulder joint. In particular, shoulder mechanical impedance was quantified, which can be represented by a 2^nd order system consisting of spring, damper and inertia. The shoulder exoskeleton robot, which utilizes a new type of 4-bar spherical parallel manipulator (4B-SPM), has inherently low inertia and as a result can provide fast perturbations that are often essential for characterizing neuromuscular properties. The robot was first evaluated by using a physical shoulder mockup with adjustable and known spring and mass properties. The results of the mockup test confirmed the reliability of the robot for the characterization of the mockup properties. Stiffness of the tested springs was accurately quantified with an error of less than 1.6 Nm/rad in any of the tested conditions. A pilot study with 5 human subjects further confirmed that the robot could be successfully used to quantify multi-dimensional human shoulder impedance in both pitch and yaw directions with high reliability (R² > 0.97). The average human shoulder stiffness and damping at around the neutral arm posture under low muscle activation (< 5% maximum voluntary contraction) were 30.9 Nm/rad and 3.0 Nms/rad, respectively.