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Steve Vankoski
12-20-1996, 04:15 AM
Thank you for all your responses to my posting on EMG onset and cessation. I
was enlightened by the responses and some more of my own digging. It surely
seems to be a topic of interest as the reponses intimate. Sorry I took so
long to sum these up, but here they are and Happy Holidays to all:

Steve,
You have identified a major limitation in the emg literature. Authors
should provide their operational definitions for muscle onset and offset, or
should indicate that the determination was made qualitatively and should
indicate their reliability in making this qualitative determination.
One quantitative method that has been used in our labs at UNC is to first
collect resting emg levels and to report the conditions under which the
resting emg data were acquired. The resting emg can then be demeaned and
full-wave rectified. Any other signal processing that will be done to emg
data acquired during the experiment (eg, low pass filter) is also performed on
the data. The mean amplitude of the resting signal is then computed for a
specific time period, usually one second. Muscle onset is then defined as any
muscle activity whose amplitude excedes the mean plus three standard
deviations of the resting signal. Additional criteria can be specified, such
as: to qualify as an onset burst, the muscle must excede the minimum
threshold for a minimum number of msec, and the muscle burst duration does
not have an offset unless the amplitude falls below the threshold value for at
least a specified period of time.
I hope this helps.

Michael Gross
Division of Physical Therapy
University of North Carolina at Chapel Hill
email: mgross@css.unc.edu
phone: 919-966-4709

Dear Steve,

That's something I've been trying to find out also. I'd be interested
to see what other people have to say, but here's one criterion I
came across recently.

At the CSB conference in August, there was a seminar on EMG given by
Drs. Baratta and Solomonow. In their handout, they mention that
Harris et al. (ref below) took the on-off threshold as 5% of the
mean maximum voluntary muscle test value (after rectifying and
low-pass filtering the signal to give the mean absolute value).

The Harris reference is:
Harris, GF, and Wertsch, JJ. "Procedures for gait analysis."
Arch Phys Med Rehab, 75:216-225, 1994.

Hope this helps.
Yours,
Danielle.

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$$$$$$$$$$$$$$$$$$$$$
Danielle Toutoungi, D.Phil. Research Student,
Oxford Orthopaedic Engineering Centre, University of Oxford.
tel: ++ (0)1865 227684
fax: ++ (0)1865 742348 email: danielle.toutoungi@eng.ox.ac.uk

"Time flies like an arrow, Fruit flies like a banana" - Anon.
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Steve,

I do not know how to answer your question, but I would be very interested
in the responses that you get. I am a biomech grad student at the
University of Delaware, and will be doing an internship of sorts at a gait
analysis lab.

If you have the time, could you either post a summary of replies or
forward them my way?

Thank you in advance,
-Joe Seay (joeseay@udel.edu)
Dear Steve,

Here are references to two algorithms used by other researchers:

DiFabio RP (1987) Reliability of computerized surface electromyography for
determining the onset of muscle activity. Physical Therapy, 67:43-48

Bonato P, D'Alessio T, Knaflitz M (1994) A novel approach to the estimation
of muscle on-off timing durting gait. Proceedings of the 1994 ISEK conference.

We have implemented the DiFabio algorithm with limited success mainly
because of the problems imposed by a muscle that is on all the time. The
Bonato approach looks interesting in that it used more sophisticated signal
processing techniques but I haven't had any experience with it.

Richard
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+ Head, Biomechanics Division, +
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Hi Steve,

How are you and the planning of the NASGCMA conference ???

I just read your posting about EMG onset and cessation...

In March of this year a man named Leonard Caillouet posted a message asking
for volunteers to mark onset and cessation times on some EMG files after
they had been full wave rectified only or fwr and filtered at 6, 16, 39 Hz.
I participated in the study and sent my marked onset and cessations to him.

In August I wrote to him and asked for my results in comparison to others.
He said he was not finished recruiting volunteers, but from the results he
had observed thus far that increased filtering was increasing the
variability of people indicating onset and cessation.

He may still be looking for volunteers, or he may be ready to let you know
more than I know. His e-mail address is: lcaillo@unix1.sncc.lsu.edu

We had a really talented student in the lab last year who wrote some EMG
processing software, that we gave to Edi Cramp, that he now distributes.
The software has a level detector that will objectively indicate "on" or
"off" based on a user specified % of maximum contraction during a gait cycle
or some other maximum voluntary contraction.

You can see the output of the program in our clinical case that we sent out
to the interlab participants. If you want to know more about the software
please let me know.

I would really love to read what other people think about your posting ...


Suzanne Halliday
Texas Scottish Rite Hospital Gait Lab
ph (214)559-7580
TSRHGAIT@ix.netcom.com