Introduction to Sports Biomechanics: Analysing Human Movement ...

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is non-stationary, when the statistical properties of the signal vary with time. A high cut-off frequency results in a noisy estimator for stationary activations and a linear envelope that follows closely the changes in the EMG. Typical values of the cut-off frequency are around 2 Hz for slow movements such as walking, and around 6 Hz for faster movements such as running and jumping. Also, SENIAM recommends that the cut-off frequency used should always be reported along with the type and order of filter. Most software packages for EMG analysis in sports biomechanics use a standard fourth-order Butterworth filter (as for filtering kinematic data in Appendix 4.1). A development of this approach is the use of an ensemble average calculated over several repetitions of the movement. This procedure theoretically – for physiologically identical movements – reduces the error in the amplitude estimation by a factor equal to the square root of the number of cycles. It is often used in clinical gait analysis but ignores the variability of movement patterns. It appears inappropriate for fast sports movements. The recommendations of SENIAM are that, if used, the number of cycles over which the ensemble average is calculated should be reported along with the standard error of the mean, to show up any movement variability. Frequency domain analysis (spectral estimation) THE ANATOMY OF HUMAN MOVEMENT Unless the data are explicitly time-limited, as in a single action potential, the EMG signal can be transformed into the frequency domain (as for kinematic data in Chapter 4). The EMG signal is then usually presented as a power spectrum (power equals amplitude squared) at a series of discrete frequencies, although it is often represented as a continuous curve, as in Figure 6.24. The epoch duration, over which the transformation of the time domain signal into the frequency domain occurs, which is recommended by SENIAM is 0.5–1.0 s. Again, these epoch durations considerably exceed the durations of muscle activity in many fast sports movements. The central tendency and spread of the power spectrum are best expressed by the use of statistical parameters, which depend upon the distribution of the signal power over its constituent frequencies. Two statistical parameters are used to express the central tendency of the spectrum. These are comparable to the familiar mean and median in statistics. For a spectrum of discrete frequencies, the mean frequency is obtained by dividing the sum of the products of the power at each frequency and the frequency, by the sum of all of the powers. The median frequency is the frequency that divides the spectrum into two parts of equal power – the areas under the power spectrum to the two sides of the median frequency are equal. The median is less sensitive to noise than is the mean. The spread of the power spectrum can be expressed by the statistical bandwidth, which is calculated in the same way as a standard deviation. The EMG power spectrum can be used, for example, to indicate the onset of muscle fatigue. This is accompanied by a noticeable shift in the power spectrum towards lower frequencies (Figure 6.25) and a reduction in the median and mean frequencies. This frequency shift is caused by an increase in the duration of the MUAP. 271
INTRODUCTION TO SPORTS BIOMECHANICS Figure 6.24 Idealised EMG power spectrum; f m is the median and f av the mean frequency. Figure 6.25 EMG power spectra at the start (a) and the end (b) of a sustained, constant force contraction. 272 This results either from a lowering of the conduction velocity of all the action potentials or through faster, higher frequency motor units switching off while slower, lower frequency motor units remain active.
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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 243 and 244: INTRODUCTION TO SPORTS BIOMECHANICS
Page 265 and 266: Figure 6.8 Main skeletal muscles: (
Page 293: 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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