2.2.1. EAA Ratio and EAA Sum for Joint Angle Control

We have proposed control signals that focus on the coordination between the agonist and antagonist muscles: the EAA ratio and the EAA sum. These signals were inspired by the agonist-–antagonist muscle ratio (AA ratio) and the agonist-–antagonist muscle sum (AA sum) and contributed to the joint equilibrium point and joint stiffness [47,48]. The AA ratio and AA sum are given by Equations (1) and (2). The flexor and extensor muscle activities of the AA muscles obtained from electromyography (EMG) are defined as me and mf, respectively. Notably, r and s were only used to indicate that rE and sE refer to them in this study.

The proposed signals, the EAA ratio and EAA sum, are given by Equations (3) and (4). The normalized FES intensity to the flexor and extensor are defined as Ve and Vf, respectively. To minimize the differences in the characteristics of the flexor and extensor, the stimulus voltage values are normalized. The subjects sat on a chair with their elbow joint kept at 135° and their shoulder joint kept at 45°, and the finger end was connected to a fixed force gauge ZP-50 N (IMADA Co., Ltd., Aichi, Japan) at an MP joint angle of 180°. The maximum stimulus voltages Vemax′ and voltage Vfmax′ were determined as the voltages just before the subjects felt pain. The minimum stimulus voltages Vemin′ and voltage Vfmin′ were determined at the point where muscle contraction commenced, and the threshold was 0.05 kgf. These were used for normalization (Equations (5) and (6)), where Ve′ and Vf′ are the voltages prior to normalization. For details, refer to previous studies [16,17,19].