Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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EMG electrodes
EMG cables
Extrinsic factors, by contrast, can be controlled. These include the location of the
electrodes with respect to the motor end-plates; the orientation of the electrodes with
respect to the muscle fibres and the electrical characteristics of the recording system
(see below). The use of equipment with appropriate characteristics is vitally important
and will form the subject of the rest of this section. The electrical signals that are to be
recorded are small – of the order of 10 µV to 5 mV. To provide a signal to drive any
recording device, we require signal amplification. The general requirement is to detect
the electrical signals (electrodes), modify the signal (amplifier) and store the resulting
waveform (recorder) (compare with the force platform system of Chapter 5). All of this
should be done linearly and without distortion. The following subsections summarise
the important characteristics of EMG instrumentation.
The electrodes are the first link in the EMG recording chain. Their selection and
placement are of considerable importance. Those used in sports biomechanics are
predominantly surface electrodes, which can be passive, having no electrical power
supply, and active, having a power supply. Indwelling electrodes are mainly used in
clinical research. Few students have the luxury of choosing which type of electrode they
will use and will normally use passive surface electrodes, which is the focus of the rest
of these three EMG subsections. It is worth noting that compared with indwelling
electrodes, surface electrodes are not only safer, easier to use and more acceptable to the
person to whom they are attached but also, for superficial muscles, provide quantitative
repeatability that compares favourably with indwelling ones. Many of the principles
and procedures covered will also apply to active surface electrodes; other equipment
and data processing are common to all types of electrode (see Burden, 2007; Further
Reading, page 280).
The European Recommendations for Surface Electromyography (Hermens et al.,
1999; see Further Reading, page 280), which are called the SENIAM (Surface Electromyography
for the Non-Invasive Assessment of Muscles) recommendations in the rest
of this chapter, have attempted to standardise EMG assessment procedures. They
recommend that only bipolar electrodes of pre-gelled silver–silver chloride material
should be used, which conforms to standard practice in sports biomechanics. They also
recommend a construction with a fixed distance of 20 mm between the centres of the
two pre-mounted electrodes (see Figure 6.19). They found no objective reason to
recommend any particular shape of electrode, instead advising that users should always
report the type, manufacturer and shape of the electrodes used.
Electrical cables are needed to connect the EMG electrodes to the amplifier or preamplifier.
These can cause problems, known as cable or movement artifacts, which