  Objective: Muscle contractions are commonly detected by performing EMG
measurements. The major disadvantage of this technique is that mechanical
disturbances to the electrodes are in the same frequency and magnitude range as
the desired signal. In this work we propose an approach and a realized
measurement system to combine EMG and bioimpedance measurements for higher
reliabilities of muscle contraction detections. Methods: We propose the
development of a modular four-channel measurement system, whereat each channel
is capable of acquiring EMG, the bioimpedance magnitude and phase,
simultaneously. The modules are synchronized by an additional interface board,
which communicates with a PC. A graphical user interface enables to control the
bioimpedance excitation current in a range from 100 {\mu}A to 1 mA in a
frequency range from 50 kHz to 333 kHz. Results: A system characterization
demonstrated that bioimpedance magnitude changes of less than 250 ppm and phase
changes below 0.05{\deg} can be detected reliably. Measurements from a subject
have shown the timing relationship between EMG and bioimpedance signals as well
as their robustness against mechanical disturbances. A measurement of five
exemplary hand gestures has demonstrated the increase of usable information for
detecting muscle contractions. Conclusion: Bioimpedance measurements of muscles
provide useful information about contractions. Furthermore, the usage of a
known high-frequency excitation current enables a reliable differentiation
between the actual information and disturbances. Significance: By combining EMG
and bioimpedance measurements, muscle contractions can be detected much more
reliably. This setup can be adopted to prostheses and many other human-computer
interfaces.
