Introduction to Sports Biomechanics: Analysing Human Movement ...

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THE ANATOMY OF HUMAN MOVEMENT Figure 6.23 Time domain processing of EMG: (a) raw signal; (b) full wave rectified (FWR) EMG; (c) smoothed, rectified EMG using a 6 Hz low-pass filter (linear envelope). and are, used to express EMG amplitude, usually over a specified time interval, known as the epoch duration, such as the duration of the contraction or one running stride. The SENIAM recommendations specify epoch durations of 0.25–2 s for isometric contractions, 1–2 s for contractions less than 50% of an MVC and 0.25–0.5 s for contractions greater than 50% MVC. Such epoch durations considerably exceed the durations of muscle activity in many fast sports movements. 269
INTRODUCTION TO SPORTS BIOMECHANICS 270 Average rectified EMG The average rectified EMG is the average value of the full-wave rectified EMG, and is easily computed from a digital signal by adding the individual EMG values for each sample and dividing by the sample time. This is recommended by SENIAM for amplitude estimation of the EMG in non-dynamic contractions. The average rectified EMG is closely related to the integrated EMG (see below). As for other amplitude estimators of the EMG, the average rectified EMG is affected by the number of active motor units; the firing rates of motor units; the amount of signal cancellation by superposition; and the waveform of the MUAP. The last of these depends upon electrode position, muscle fibre conduction velocity, the geometry of the detecting electrode surfaces and the detection volume. Root mean square EMG The RMS EMG is the square root of the average power (voltage squared) of the signal in a given time. It is easily computed from a digital signal by adding the squares of the individual EMG values for each sample, then taking the square root of the sum before dividing by the sample time. It is considered to provide a measure of the number of recruited motor units during voluntary contractions where there is little correlation among motor units. This is also recommended by SENIAM for amplitude estimation of the EMG in non-dynamic contractions. Integrated EMG This is simply the area under the rectified EMG signal. It is not recommended at all by SENIAM and is mentioned here only because of its wide use – and abuse – in earlier research. Smoothed, rectified EMG The standard way of obtained the smooth, rectified EMG is by the use of a linear envelope detector (hence another name for this estimator, the ‘linear envelope’), comprising a full-wave rectifier plus a low-pass filter. This is a simple method of quantifying signal intensity and gives an output (Figure 6.23(c)) that, it is claimed, follows the trend of the muscle tension curve with no high-frequency components. It is the only intensity detector recommended by SENIAM for dynamic contractions – which predominate in sport – because of the statistical properties of such contractions. The main issue with this estimator is the choice of the epoch duration, which, for this estimator, is related to the filter cut-off frequency (see also Chapter 4). A low cut-off frequency gives a reliable estimate of the signal intensity for ‘stationary’ activation of the muscle – this is a statistical term meaning, roughly, that the statistical properties of the activation signal do not change with time, not that the activation is constant. However, the amplitude estimates with a low cut-off frequency are inaccurate if the activation
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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 241 and 242: INTRODUCTION TO SPORTS BIOMECHANICS
Page 265 and 266: Figure 6.8 Main skeletal muscles: (
Page 291: 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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