Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 6.19 Bipolar configurations of surface electrodes.
EMG amplifiers
have frequencies in the range 0–10 Hz. The use of high-pass filtering, high-quality
electrically shielded cables and careful taping to reduce cable movement can minimise
these. Cable artifacts can be reduced by using pre-amplifiers mounted on the skin near
the detection electrodes and by good experimental procedures (see next section). The
use of active surface electrodes can virtually eliminate cable artifacts.
The effect of high-pass filtering at 10 Hz is shown in Figure 6.20, where Figures
6.20(a) and (b) are the unfiltered and filtered signals, respectively. The SENIAM
recommendations for filtering the signal are to use a high-pass filter of 10 Hz if the
signal is to be analysed in the frequency domain (see below) and 10–20 Hz if the signal
is only to be used for movement analysis. To reduce high-frequency noise and aliasing,
they also recommend a low-pass filter with a cut-off frequency of about 500 or
1000 Hz, at sampling frequencies, respectively, of 1000–2000 and 4000 Hz, depending
on the application. Signal aliasing (see Chapter 4) will occur if the sampling rate is less
than twice the upper frequency limit in the power spectrum of the sampled signal.
These are the heart of an EMG recording system. They should provide linear amplification
over the whole frequency and voltage range of the EMG signal. Noise must
be minimised and interference from the electrical mains supply (mains hum) must be
removed as far as possible. The input signal will be around 5 mV with surface electrodes
(or 10 mV with indwelling electrodes). The most important amplifier characteristics are
as follows.
Gain
THE ANATOMY OF HUMAN MOVEMENT
This is the ratio of output voltage to input voltage and should be large and, ideally,
variable in the range 100–10 000.
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