Quantification of Shoulder Joint Impedance during Dynamic Motion: A Pilot Study Using a Parallel-Actuated Shoulder Exoskeleton Robot

Previous studies characterizing shoulder joint impedance were either strictly limited to 2D planar motion or static postures in 3D space. It is still not clear how shoulder joint impedance is regulated during dynamic motion in 3D space. To address this knowledge gap, this study presents our initial efforts to quantify shoulder joint impedance during dynamic shoulder motion using a parallel-actuated shoulder exoskeleton robot. The robot's 4-bar spherical parallel manipulation mechanism, characterized by low inertia, allows for transparent and natural arm motion in 3D space and applies rapid perturbations to the upper arm during dynamic shoulder motion. Two unimpaired individuals participated in an experiment involving repeated shoulder flexion and extension motions at a fixed horizontal shoulder extension angle of 45.. Shoulder impedance was quantified by estimating the relationship between the kinematics of input position perturbations, which were applied in the orthogonal direction of the arm motion, and the output torque responses resulting from these perturbations. This relationship was approximated by a second-order model consisting of inertia, damping, and stiffness. Both subjects showed high reliability in impedance quantification during shoulder flexion and extension movements, evidenced by a high percentage Variance Accounted For that exceeds 96%. The experimental results showed the following notable trends. First, the contribution of stiffness to the shoulder torque was greater than that of the other two impedance parameters. Next, damping was larger during shoulder extension (downward motion) as opposed to flexion (upward motion). Lastly, inertia remained relatively constant regardless of shoulder motions. This pilot study validated the reliability of the presented robotic approach, paving the way for future shoulder impedance studies involving various dynamic motions in 3D space.