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The EMG signal

With EMG, we measure a voltage curve over time. We pay attention to the amplitude (the magnitude) of the signal and the frequency. Depending on the musculature, considerable differences become plain. An EMG signal from a small muscle; e.g. from a finger on the hand, is significantly lower in amplitude and frequency range than the signal from the large thigh muscle.

The eSense Muscle can measure the EMG raw signal and transmit it via Bluetooth, as well as already three preset bandpass signals (RMS) in the frequency ranges:

20 Hz to 950 Hz, 20 Hz to 300 Hz and 100 Hz to 200 Hz

You can select and use these in the eSense web app and the mobile app. In biofeedback, we work exclusively with these bandpass RMS signals, as these are related to amplitude and applied muscle force. The amplitude ranges from a few µV (millionths of a volt) to several hundred µV in particularly strong muscles.

The frequency range of the EMG signal does not play a significant role in EMG biofeedback. It only emphasizes the signal part you want to look at. EMG activity has a broad frequency spectrum, from 10 Hz to 500 Hz, focusing between 25 and 300 Hz. Choose a wide filter (20-950 Hz) if you want to capture and feedback all components in the EMG and choose a narrow filter (100-200 Hz) if you’re going to capture the majority and filter out possible sources of interference such as pulse artifacts or mains hum.

It is advisable to always choose the narrowest filter (100-200 Hz) first and to check whether the measured signal is represented sufficiently well with it. The filter should only be widened and set wider if the amplitude appears too small and not very reactive.

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