A Wearable Dual-Channel Bioimpedance Spectrometer for Real-Time Muscle Contraction Detection

The reliable detection of muscle contractions in real-time is important for many applications. Both for prosthesis control and in the field of human-computer interaction, the physiological commands of the user must be recognized. However, conventional methods such as electromyography (EMG) are susceptible to interferences. A particularly robust method is the electrical impedance myography (EIM). Especially the temporal changes of the bioimpedance phase response are of interest for muscle activity monitoring. However, available wearable measurement systems are not capable of detecting muscle contractions in real-time to control prostheses or human–computer interaction devices. This work presents the development and metrological characterization of a wearable real-time bioimpedance spectrometer for the detection of muscle contractions. It can record the frequency responses in the range of 20–230 kHz from two antagonistic muscles. The sampling rate of 25 impedance spectra per second and per channel provides sufficient temporal resolution for many applications. Phantom measurements show that the statistical errors are below 1% for the magnitude and below 0.4° for the phases, which is sufficient for EIM. This system is used to perform first subject measurements. For the first time, these measurements demonstrate the temporal impedance behavior and frequency responses of two antagonistic muscles during contraction. In addition, the directional dependence of the EIM during a muscle contraction is investigated for the first time. The presented measurement system and novel measurement approaches are promising for many EIM applications, especially for reliable muscle contraction detection e.g. in prosthetics or human–computer interaction applications.