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Kieran Moran
10-31-2006, 04:31 AM
Hi

On the issue of sampling and filtering, Peter Konrad (The ABC of EMG
published by Noraxon) states (p13)

"In order to accurately “translate” the complete frequency spectrum of a
signal, the sampling rate at which the A/D board determines the voltage
of the input signal must be at least twice as high as the maximum
expected frequency of the signal. This relationship is described by the
sampling theorem of Nyquist: sampling a signal at a frequency which is
too low results in aliasing effects (Fig. 18). For EMG almost all of the
signal power is located between 10 and 250 Hz and scientific
recommendations (SENIAM, ISEK) require an amplifier band setting of 10
to 500 Hz. This would result in a sampling frequency of at least 1000 Hz
*(double band of EMG)* or even 1500 Hz to avoid signal loss".

This is also suggested by ISEK
(http://isek-online.org/pdf/ISEK_EMG-Standards.pdf) on page 2.

However, I am unsure why the sampling rate must be more than twice the
highest amplifier *band width* setting and not simply more than twice
the *"maximum frequency of interest in the signal"*. In other words, if
we use a band setting of 10 to 1000 Hz, and we are interested only in
signals up to 400 Hz, would it not be appropriate to sample at 1000Hz.
The recommendations from Peter Konrad would suggest the sampling rate
should be more than 2000Hz (twice the highest *band width*).

A number of published experimental studies would seem to agree that the
sampling rate does not have to be twice the highest band width, but
twice the highest maximum frequency of interest

J.R. Potvin & S. Brown (2004) Less is more: high pass filtering, to
remove up to 99% of the surface EMG signal power, improves EMG-based
biceps brachii muscle
force estimates. Journal of Electromyography and Kinesiology 14 (2004)
389–399:
(gain = 1000, input impedance = 10 GX, 10–1000 Hz, CMRR = 115 dB at 60
Hz, Bortec, Octopus AMT-8, Calgary, Canada) and sampling rate was 1024 Hz


M.G. Feltham et al. (2006) Changes in joint stability with muscle
contraction measured from
transmission of mechanical vibration
Journal of Biomechanics 39 (2006) 2850–2856
(gain = 1000, band-pass filtered between 10 Hz and 1000 Hz (NL820,
Neurolog System, Welwyn Garden City, UK), and sampled at 1000 Hz

Does any one have any thoughts on this issue?

Many thanks

Kieran

--
Dr Kieran Moran
Biomechanics Research Group
School of Health and Human Performance
Faculty of Science and Health
Dublin City University
Collins Avenue
Dublin 9

tel; 00 353 1 700 8011
fax: 00 353 1 700 8888

kieran.moran@dcu.ie