1. Introduction
Surface electromyography (sEMG) is a technique for measuring electrical potentials generated by active motor units, comprising groups of muscle fibers [1]. sEMG can be detected by electrodes placed on the skin surface during muscle contractions, and have been used in various applications, such as diagnosis of neuromuscular disorders, evaluations of muscle fatigue, and control inputs for powered prosthetic/orthotic devices [2]–[5]. Two types of surface electrodes are commonly in use: wet and dry electrodes. Conventional silver/silver chloride (Ag/AgCI) wet electrode used electrolytic gel as a chemical interface between the skin and the conductive layer of electrode to measure good-quality sEMG signals. A dry electrode, on the other hand, measures sEMG signals via direct skin contact without gel. Commonly-used dry sEMG electrodes were made of noble metals (e.g. platinum, gold, or silver), carbon, or sintered Ag/AgCI [6].