When human movement is assisted or controlled with a muscle actuator, such as electrical muscle stimulation, a critical issue is the integration of such induced movement with the person's motion intention and how this movement then affects their motor control. Towards achieving optimal integration and reducing feelings of artificiality and enforcement, we explored perceptual simultaneity through electrical muscle stimulation, which involved changing the interval between intentional and induced movements. We report on two experiments in which we evaluated the ranges between detection and stimulus for perceptual simultaneity achievable with an electromyography-triggered electrical muscle stimulation system. We found that the peak range was approximately 80-160 ms, with the timing of perceptual simultaneity shifting according to different adaptation states. Our results indicate that perceptual simultaneity is controllable using this adaptation strategy.

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Human-machine integration has been widely implemented in various fields, such as entertainment, human movement assistance, and rehabilitation. This study focuses on electrically-induced muscle contractions. In this technology, it is important to stimulate the optimal position, called the motor point (MP), on the muscle belly to induce the contraction with the lowest current injection. Because the positional relationship between the skin surface and muscles varies according to muscle contraction and body posture, it is difficult to achieve constant and accurate stimulation. Therefore, targeted motor intervention is not possible in several instances. We propose an approach that automatically identifies and stimulates a single precise MP in response to individual differences and postural changes using mm-order electrode arrays in two dimensions (2D). In this study, we develop a system to search for MPs by arranging 2-mm diameter electrodes in a 2D array. Our experiments successfully visualized the MPs in a 2D skin surface, which shifts according to the elbow joint angle change and twisting movement. The results demonstrated the feasibility of 2D electrode arrays. These findings contribute to the design of future devices and an algorithm for electrical muscle stimulation that enables effective muscle contraction.