Introduction to Sports Biomechanics: Analysing Human Movement ...

Recorders
Frequency response
The ability of the amplifier to reproduce the range of frequencies in the signal is known
as its frequency response. The required frequency response depends upon the frequencies
contained in the EMG signal. This is comparable with the requirement for audio
amplifiers to reproduce the range of frequencies in the audible spectrum. Typical values
of EMG frequency bandwidth are 10–1000 Hz for surface electrodes and 20–2000 Hz
for indwelling electrodes. Most modern EMG amplifiers easily meet such bandwidth
requirements. About 95% of the EMG signal is normally in the range up to 400 Hz,
which explains why SENIAM recommend the use of a low-pass filter to remove higher
frequencies. This signal range unfortunately also contains mains hum at 50 or 60 Hz.
Also, SENIAM recommends that the input-referred voltage and current noise, respectively,
should not exceed 1 µV and 10 pA.
Common mode rejection
THE ANATOMY OF HUMAN MOVEMENT
Use of a single electrode would result in the generation of a two-phase depolarisation–
repolarisation wave (Figure 6.21). It would also contain common mode mains hum.
At approximately 100 mV, the hum is considerably larger than the EMG signal. This is
overcome by recording the difference in potential between two adjacent electrodes,
known as bipolar electrodes, using a differential amplifier. The hum is then largely
eliminated as it is picked up commonly at each electrode because the body acts as
an aerial – hence the name ‘common mode’. The differential wave becomes triphasic (it
has three phases, as in Figure 6.21). The smaller the electrode spacing, the more closely
does the triphasic wave approximate to a time derivative of the single electrode wave. In
practice, perfect elimination of mains hum is not possible and the success of its removal
is expressed by the common mode rejection ratio (CMRR). This should be 10 000 or
greater. The overall system CMRR can be reduced to a figure lower than that of the
amplifier by any substantial difference between the two skin plus cable resistances. The
effective system common mode rejection ratio in this case can be downgraded to a value
of R i/(R 1 − R 2); cables longer than 1 m often exacerbate this problem. For passive
electrodes, the attachment of the pre-amplifier to the skin near the electrode site reduces
noise pick-up and minimises any degradation of the CMRR arising from differences
between the cable resistances.
Various devices have been used historically to record the amplified EMG signal but at
present A–D conversion and computer processing are by far the most commonly used
recording methods. An A–D converter with 12-bit (1 in 4096) or 16-bit resolution is
recommended by SENIAM. High sampling rates are needed to reproduce successfully
the signal in digital form; telemetered systems do not always provide a sufficiently high
sampling rate.
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