Introduction to Sports Biomechanics: Analysing Human Movement ...

EXPERIMENTAL PROCEDURES IN ELECTROMYOGRAPHY
THE ANATOMY OF HUMAN MOVEMENT
The results of an electromyographic investigation can only be as good as the preparation
of the electrode attachment sites. Although skin resistance ceases to be a problem if
active electrodes are used, many EMG studies in sports biomechanics currently use
passive surface electrodes, and good surface preparation is, therefore, still beneficial.
The location of the electrodes is the first consideration. For the muscles covered by
their recommendations, the advice on electrode placements in the SENIAM report are
well illustrated and simple (see Box 6.5 for a few examples). The separation of the
electrodes was discussed above. For muscles not covered by those recommendations,
simple advice is that:
The two detector electrodes – or the detector pair – should be placed over the
mid-point of the muscle belly.
The orientation of the electrode pair should be on a line parallel to the direction of
the muscle fibres (for example Box 6.5).
If the muscle fibres are neither linear nor have a parallel arrangement, the line
between the two electrodes should point to the origin and insertion of the muscle
for consistency.
The following are good experimental practice for skin preparation to improve the
electrode-to-skin contact, thereby reducing noise and artefacts:
The area of the skin on which the electrodes are to be placed is shaved if
necessary.
The attachment area is then cleaned and degreased using an alcohol wipe, the
alcohol being allowed to evaporate before the electrodes are attached to the skin.
None of the procedures used historically, requiring abrasion with sandpaper or a
lancet to scratch the skin surface, is recommended by SENIAM; such procedures
raise ethical, or health and safety, issues at many institutions and are not necessary
if the amplifier input impedance conforms to that recommended in the previous
section.
Preferentially, the electrodes should be pre-gelled; if they are not then, after the
electrodes have been attached, electrode gel should be injected into the electrodes;
any excess gel should be removed.
It should not normally be necessary to check the skin resistance; however, if using
older equipment, a simple DC ohmmeter reading of skin resistance should be
taken. The resistance should be less than 10 kΩ and, preferably, less than 5 kΩ. If
the resistance exceeds the former value, the electrodes should be removed and the
preparation repeated.
The reference electrode, also known as the earth or ground electrode, should be
placed on electrically inactive tissue near the site of the recording electrodes or
electrode pair. These sites are specific to each electrode (see examples in Box 6.5)
and are usually on a bony prominence.
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