Introduction to Sports Biomechanics: Analysing Human Movement ...

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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. 261
INTRODUCTION TO SPORTS BIOMECHANICS Figure 6.20 Effect of high-pass (10 Hz) filter on cable artifacts: (a) before filtering; (b) after filtering. 262 Input impedance The importance of obtaining a low skin resistance can be minimised by using an amplifier with a high input impedance – by which we mean a high resistance to the EMG signal. This impedance should be at least 100 times the skin resistance to avoid attenuation of the input signal. This can be seen from the relationship between the ratio (e i/e emg) of the detected signal (e emg) to the amplifier input signal (e i), and the amplifier input impedance (R i) and the two skin plus cable resistances (R 1 and R 2): e i/e emg = R i/(R i + R 1 + R 2). The SENIAM recommendation for the minimum input impedance of an amplifier for use with passive surface electrodes is 100 MΩ (10 8 Ω). Input impedances for high-performance amplifiers – used with active electrodes – can be as high as 10 GΩ (10 10 Ω).
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Introduction to Sports Biomechanics
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First edition published 1997 This e
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Contents List of figures x List of
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Study tasks 216 Glossary of importa
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FIGURES 1.26 Underarm throw - femal
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FIGURES 5.9 Typical path of a swimm
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Tables 2.1 Examples of slowest sati
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Preface Why have I changed the cove
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Introduction MISSING TEXT The first
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INTRODUCTION principal movements in
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INTRODUCTION TO SPORTS BIOMECHANICS
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Figure 2.2 ‘Principles’ approac
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Figure 2.13 Identifying critical fe
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Page 265 and 266: Figure 6.8 Main skeletal muscles: (
Page 283: INTRODUCTION TO SPORTS BIOMECHANICS
Page 305 and 306: INDEX 282 balls air flow past 173 b
Page 307 and 308: INDEX 284 energy 219 kinetic 219 mi
Page 309 and 310: INDEX 286 isokinetic contraction 24
Page 311 and 312: INDEX 288 muscle length-tension rel
Page 313 and 314: INDEX 290 three-dimensional 146-7,
Page 315: INDEX 292 validity 220 vantage poin
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