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4 Measurement of Biosignals and Analog Signal Processing

RE2

CE

RE1

RH

CH

Fig. 4.4: Electrical equivalent circuit of skin and electrode, composed of Figure 4.2 (right) and Fig-

ure 4.3 (right).

muscle up to the detection of the action potential of individual muscle fibers, which

is impossible with surface electrodes due to the significantly lower spatial resolution.

Surface electrodes in the form of electrode arrays, on the other hand, are better suited

for studies of the spatial and temporal propagation of the action potential along indi-

vidual motor units. For in vivo measurements, the needle electrode is usually used to

contact the extracellular area where the outer action potential is measured. Controlled

puncture of the inner cellular area of a muscle fiber and thus measurements of the in-

ner action potential are usually only possible in vitro. The finer the needle tip, the

higher the spatial resolution and more selective the measurement with respect to the

action potential of individual muscle fibers. With increasing distance of the electrode

to the muscle fiber, however, the EMG amplitude decreases.

In practice, both monopolar and bipolar needles are used (see Figure 4.5). The

monopolar version consists of a coated stainless steel needle with a diameter of 300 to

500 μm. The coating electrically insulates the needle body from the punctured tissue.

Only the tip contacting the area of interest is exposed. Teflon or medical grade silic-

one is used as the coating material. The counter electrode can be realized by means

of a surface electrode near the puncture site, but still outside the area of the motor

unit to be examined. Bipolar electrodes consist of a stainless steel cannula (counter

electrode) and a metal core (electrode) surrounded by an insulating layer. The outer

diameter is in the range of 300 to 700 μm, the diameter of the core is about 100 μm.

The material of the core is usually silver or platinum. In the monopolar electrode, the

active area of the needle tip is larger than in the bipolar electrode, which means that

a larger area of the motor unit is covered by the electrode, producing a stronger EMG

signal. In contrast, a higher spatial resolution is achievable with the bipolar electrode.

In addition, with this type the superimposed common mode signal² is much weaker,

since the electrode and counter electrode are very close to each other, which means

that the common-mode signal is present at the input of the measuring amplifier with

almost the same amplitude and phase and is largely eliminated by the high common-

mode rejection.

2 The term common mode signal is discussed in detail in the following sections.