Investigation of a tolerable time delay in SEMG-based elbow assistive device

Surface electromyography (SEMG) signal has been widely used to detect the intention of the user in assistive device, because the onset time of the SEMG signal is observed before the muscle force development. The time between the onset of SEMG activity and the development of force is defined as electromechanical delay (EMD). Owing to the existence of EMD, an SEMG-driven approach has advantage of early detection of the intention compared to other approaches, especially to a force sensor-based approach. However, few studies have been done on the investigation of the timing of the assistive force considering EMD. This study suggests the proper timing of the assistive force for better performance by analyzing the onset of muscle force development. Simultaneous measurements of four channels of SEMG, interaction torque, control input corresponding to the assistive torque, and joint angle allowed the investigation of the quantitative relation between the onset of SEMG and timing of the assistive torque. The timing of the assistive force was manipulated by varying the amount of artificial time delay. The experiments were carried out for 3 subjects, who were informed to complete 4 repetitive elbow flexion/extension motions as quickly as possible, with the help of the assistive device. Task completion time was examined as a measure of the performance of the assistance, and cross-correlation analysis was conducted. We observed that the reaching time and level of SEMG activation were reduced when the delay was consistent with the EMD of the user.