Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 201

FIGURE 4.46: Plots of 8-s duration signal showing the output from the four post-
processing methods. (A) Hilbert. (B) ICA. (C) Hilbert + ICA. (D) Filtered. In (B),
the ICA component shows a momentary polarity inversion that results in missed
beats. In (C), the Hilbert method applied to ICA components removes the polarity
inversion. From [Wilson et al., 2008, Fig. 6].
4.3.1 Measurement techniques and physiological background
The EMG characteristics depend on the kind of applied electrodes. EMG may be
recorded by means of intramuscular (needle or fine wire) or surface electrodes. The
range of measured potential difference in case of intramuscular electrodes is 0.1-
20 mV, and for surface electrodes 0.05-1 mV. In clinical practice EMG is used for
routine diagnosis of neuromuscular disorders, based typically on single fiber (Figure
4.47 a) or concentric needle electrodes (Figure 4.47 b). Surface electrodes enable
non-invasive EMG recording of the global muscle electrical activity. Earlier, surface
electrodes were applied only in situations when the insertion of needle electrodes
was difficult or not possible, e.g., for examination of children, long-term record-
ings, ergonomics, sports, or space medicine. Nowadays surface EMG electrode ar-
rays ( Figure 4.48) are available, with which more precise information on muscle
activation can be obtained. This technique requires, however, advanced signal pro-
cessing methods, which will be described later.
The registered EMG signal is a superposition of activity of single motor units
(MUs). A single MU consists of a motoneuron and the group of muscle fibers in-
nervated by its axon ( Figure 4.49 a). Fibers belonging to a given MU are distributed
throughout a muscle (Figure 4.49 b). Every motoneuron discharge evokes contrac-