Dear Dr Cho,
The diffference between tthe above are not worth bothering too much
about.
#1 and #3 are identical. 'Integration' is in the literature
often misused for 'low-pass
filtering'.
RMS, #2) is slightly different, but in practice it gives the
same curves after low-pass
filtering as #1 & #3.
In fact, the choice of a sensible low-pass cutoff frequency after
rectification is much more
relevant.
A good overview is in DA Winter, Biomechanics of human movement.
At Hof
Department of Medical Physiology
University of Groningen
Bloemsingel 10
NL-9712 KZ GRONINGEN
THE NETHERLANDS
Tel: (31) 50 3632645
Fax: (31) 50 3632751
e-mail: a.l.hof@med.rug.nl
Hi,
The main problem is not the method used but the length of
EMG epochs that is analyzed.
Shortest EMG epochs reveals muscle excitability, longer epochs- muscle
power and longest epochs
muscle work.
Sincerely Yours,
Plamen Gatev MD, PhD
Dear Dr. Sang-hyun Cho
basically, the linear envelope
(LE) or Smoothed Rectified EMG (SRE) and the RMS of a
noisy signal with zero mean (e.g. the EMG), are representing the same
property of the signal:
an estimation of the actual "intensity"of the signal. The low-pass
characteristics of the filter after squaring or
rectifying the signal, determines the time period over which this "intensity"of
the filter is
estimated.
For a gaussian distribution
(eg. the EMG), the RMS and the LE of a
signal x are related:
E{|x|}=sqrt( 2/pi * E{x^2} )
The reason that in signal-theory
RMS is preferred is that it is nicely proportional to
the power of the signal (expressed in decibel: dB). The
reason that LE is used so much is that
it is easily implemented in hardware (bi-pasic rectifying).
There also might be minor
differences, though (ask a signal-processing expert).
So use what you prefer, the
low-pass filter time constant is the most critical
parameter, with regard to interpretation.
Jaap Harlaar
University hospital VU
Amsterdam
Dear Dr. Sang-hyun Cho
Your e-mail was forwarded to me today. Although I have been doing
EMG research for several
years now, I do not have much an answer to your question. (It
seems that the nervous system is
a rather complicated universe, with room for a great many questions!)
Anyway, Hylander and Johnson have been quite
successful in associating contractile force and EMG scored by
RMS. Their goal was to develop
statistical correlations, because EMG is easier to measure than force.
Hylander,WL; Johnson,KR (1993): Modelling relative masseter force from
surface electromyograms
during mastication in non-human primates. Arch. Oral Biol. 38(3), 233-240.
Best of luck to you,
David J. Eliot
Assistant Professor
University of Bridgeport College of Chiropractic
Bridgeport, CT
Hello.
These two signal processing techniques perform two very different functions.
The RMS is an amplitude normalisation technique. It is
one of many available and it can be
useful for comparing the relative amplitude of the signal between subjects.
A good reference
for you to look at is Yang and Winter (1984) Electromyography amplitude
normalisation methods:
improving their sensitivity as diagnostic tools in gait analysis. Arch
Phys med rehab Vol 65,
Sept 1984.pg517-521.
A linear envelope is a signal processing technique which is used
to gain an 'average trend of
EMG activity'. it is used because the RAW EMG signal
is relatively useless because it
fluctuates in amplitude too quickly and too often. The linear envelop
can take many forms. The
most common of which is where bins of raw EMG data are taken and averaged.
This averaged figure
is then used as a representation of the activity in that bin. A bin
usually is 25-50ms. The
resultant signal is significantly smoother and hence more meaningful.
This signal is usually then lowpass filtered to remove the
peaks which occur due to this processing technique to leave a smooth
EMG signal. This process
of 'binning' the data and then lowpass filtering is known
as a linear envlope. Winter (1990)
Biomechanics and motor control of human movement.
The linear enveloped signal in it's own right is relatively useless
for determining muscle
activity. Determining periods of muscle activity is a function of many
criteria. usually a
threshold is set based on some measure of muscle resting activity.
The EMG signal must be able
this threshold by some arbitary measure usually 2-3 SD and some
authors also specify a time
which the signal must remain above the threshold. The filter used to
process the signal also
has an effect on muscle on off determination.
Hodges and Bui (1996) A comparison of computer based methods for the
determination of onset of muscle contraction using EMG. Electroencephalography
and clinical
neurophysiology 101. 511-519
It would be useful for you do so some literature reviewing and look
through the Biomech-l
archives as this topic is well documented even though there remains
great controversy and
little consensus.
kindest regards.
Michael Dillon
B P&O Hons. PhD Student
Centre for Rehabilitation Science and Engineering
Queensland University of Technology
School of Mechanical, Manufacturing and Medical Engineering
GPO Box 2432
Brisbane. 4001.
Ph. +61 07 3864 2751 E-mail: m.dillon@qut.edu.au
Fax. +61 07 3864 1469
http://www.bee.qut.edu.au/mech/staff/mdillon.html