Movements stability analysis of SEMG-based elbow power assistance by Maximum finite time Lyapunov exponent

A human who uses a SEMG-based power assist robot suffers from unstable assistance due to noisy nature of SEMG signals. This study aimed at analyzing the quantitative stability of the human elbow movements during the power assistance. During self-paced elbow flexion in the sagittal plane, an exoskeleton robot provided the assistive torque that was proportional to the estimated human elbow torque using SEMG. Maximum finite time Lyapunov exponent (MFTLE), the average logarithmic rate of the divergence of neighboring trajectories, during elbow flexion assisted by the robot was computed as an index of movement stability. The results showed a trade-off between decrease of physical effort for the movements and the stability of the movements. The stability of the SEMG-based assistance by MFTLE deteriorated as the amount of the assistive torque was increased to the amount of the human torque, although the physical effort required by the user his forearm was decreased. This study can be used as a guide to determine the amount of SEMG-based assistive torque for maintaining the stability of the assisted movements.