Circuit elements: Electromyography and passive filters

Electromyography, or EMG, is a means of measuring electric potentials produced by a person’s muscles in order to determine if there are any abnormalities within the muscle, such as neuromuscular disease. Various electric properties can be measured via electrodes placed in a muscle, including the amplitude and the duration of an action potential. Additionally, an EMG will not record any electrical activity when a muscle is at rest, so the presence of electrical activity can be a sign of pathology as well.

To perform the test, two electrodes can be inserted into muscle tissue which measure the difference in voltage between the two points. The signals generated during muscle contraction are then passed through an amplifier to boost the signal, a filter to remove noise from the amplified signal, and then an analog-to-digital converter.

Though the filtering process is now digital itself, the original design of an EMG involved an analog system of fundamental circuit components, the simplest being a resistor and capacitor, known as a passive low pass filter (Figure 1). As long as the current passes through the circuit below the cut-off frequency, it behaves essentially like a normal circuit. However, frequencies above the cut-off frequency of the filter will cross the capacitor and go to the ground. The cut-off frequency is determined by both the resistance and capacitance of the circuit; the higher the capacitance or resistance, the lower the cut-off frequency will be.

Figure 1. Passive, first order low-pass RC filter