Effect of damped shoulder tracking mechanism for shoulder rehabilitation

Abstract

As the number of patients with shoulder disease is increasing, the demand for shoulder rehabilitation devices such as continuous passive motion device (CPM) is also increasing. The devices mainly facilitate repetitive exercises that the arm of the patient to improve the ROM and muscle strength. Due to the complex anatomic structure of the shoulder, the shoulder joint has three-dimensional translational movements when raising the arm. However, most of the commercial rehabilitation devices do not compensate such movements. If the movement of the shoulder joint is not considered, there will be a difference between the position of the shoulder joint and the axis of rotation of the device, which may induce incorrect motion to patients and reduce the effect of the rehabilitation. To solve this problem without actuators, a device that passively follows the translational movement of the shoulder joint has been developed [1]. In addition, a variable damping mechanism was designed to improve the tracking performance of the device when conduct resistive exercises [2]. However, the damping intensity was not optimized for patients and exercises. In this study, the experimentation of the shoulder joint tracker with various damping values was carried out and the tracking performance was evaluated. To evaluate the tracking performance, the movements of the upper limb and the tracking device were analyzed for one healthy subject using a motion capture system. In the experiment, we performed flexion/extension and abduction/adduction under four damping conditions. As a result, the angle difference between the arm and the device, and the translational movement difference between the shoulder and the tracker decreased for damping conditions. The abrupt translational movements of the device compared to the shoulder were also reduced. It indicates that adding damping can improve the tracking performance of the tracker. However, excessive damping can decrease the ROM. Therefore, by optimizing the damping value, the passive tracking device can be used for various rehabilitation devices to induce natural motions.