Power assistance using surface electromyography and its influence on human movement stability

Abstract

Robotic power assistive devices that employ SEMG signals can provide mechanical power to supplement human physical strength according to the estimated torque of users. While the device is expected to be useful solution for individuals who have deficits in motor functions, the use of the devices may be limited depending on assist conditions. The noisy nature of SEMG signals can cause fluctuation of assistive torque, which can result in mechanical perturbations. With respect to stability represents the reproducibility of movements and robustness to perturbations, this characteristic of SEMG-based power assistance can deteriorate the stability of assisted movements. How to affect the movement stability by power assistance is one of the main concerns in the fields of robotic assistance. However, there has been a paucity of studies that have quantified the influence of this type of assistance and the stability of the assisted movements while SEMG-based assistive power is transferred. This thesis mainly investigates the influence of SEMG-based power assistance on human movement stability. Firstly the upper limit of the amount of torque assistance was investigated using a computational model. Elbow flexion movements were simulated under twelve sets of conditions, each having different proportion of torque assistance. The results imply that the model could describe the expected movements of flexion torque assistance using SEMG and the movements could become unstable while the amount of assist ratio was out of proportion to SEMG noise level. Secondly, the stability of the assisted elbow flexion was quantitatively assessed by employing the kinematic variability and the averaged divergence rate of the movement trajectory referred to as a maximum finite time Lyapunov exponent. Twelve subjects performed self-paced elbow flexion tests under five sets of conditions, each having different proportion of torque assistance. Experimental results indicated that the user was able t...