Optimal Motor Point Search Using Mm-Order Electrode Arrays

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.