System Identification to Characterise Shoulder Joint Dynamics in Two Degrees of Freedom.

In this study, we present a new design of a shoulder perturbation robot that can characterise the dynamics of the shoulder in two degrees of freedom. It uses two linear electric motors to perturb the shoulder joint in internal/external rotation and abduction/adduction, and force and position sensors to measure the corresponding torque and angular displacement about the joint. System identification techniques are used to estimate the dynamics of the muscles around the joint. The advantage our apparatus offers over the existing ones is that it can efficiently transfer torque to the joint and measure its dynamics separately with minimal interference from soft tissues. We verified that the apparatus can accurately estimate joint dynamics by conducting tests on a phantom of known properties. In addition, experiments were conducted on a human participant. It has been demonstrated that the measured dynamics of participant's arm are repeatable. The potential impact of our apparatus is to be used in clinic as a diagnostic tool for rotator cuff injuries.