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Sean Keegan
01-27-2000, 03:57 AM
Thanks much to all those who replied to my message regarding running and EMG collection.

Posted below is the original question and the replies I received. A combination of techniques has resulted in a better signal.

Thanks again,

Sean Keegan

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Original Posting:

I am attempting to capture EMG data from the vastus medialis/ vastus lateralis/ tibialis anterior while running (2000Hz sampling). My problem is that there seems to be a movement artifact just prior to EMG activity of the leg - it looks like foot-contact is producing the artifact. Has anyone else found this type of signal response during a running movement and/or the best ways to avoid this problem?

I am collecting using typical surface EMG techniques (hair removal, skin abraision, etc) but due to the type of electrode (TSD150B) I am not using conductive gel.
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However, we have had the best
luck reducing movement artifact when using double-stick foam tape (approx 2
mm thick, with holes slightly smaller than the electrode surfaces), filling
those holes with conductive gel, and NOT using any other tape or wrapping
over the top of the electrode. I think the gel-filled "wells" in the foam
tape help to eliminate artifacts caused by movement of the electrode
relative to the skin. We do tape the wire to the subject proximal to the
electrode to avoid any stress on the electrode, but have found that any
tape or wrapping that actually goes over the electrode seems to contribute
to movement artifact.

If what you're seeing is mechanical artifact due to foot contact, you
should be able to reproduce it by having the leg propped up and relaxed and
then whacking on the bottom of the foot. If that doesn't reproduce it, you
could try tapping near the electrodes or leads to see if that reproduces
what you've been seeing. If you have been taping or wrapping over your
electrode, I think it's possible that changes in the pressure applied by
the wrap (e.g. due to hamstring muscle activity prior to heel contact)
could cause some artifacts.

Good luck, Greg

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As to your motion artifact. Those electrodes have a fairly large mass
especially compared to the size of the medialis so it's easy for
mechanical artifact to occur between the electrode and the skin. I don't
know what type of precautions you're taking with ahdering the electrode,
but some of your artifact may go away if you loop the leads near the
electrode and tape (or wrap) them down. I suspect that you'll not get
rid of all the artifact, and then your only choice is to filter out what
you can.

Motion artifact is typically low frequency, so you can either
high pass filter your data with a cutoff of 10 Hz, or even as much as 20 Hz,
as there is arguable NO real EMG down in that frequency range. Or
bandpass your data with cutoffs such as 20 and 250. I may be suggesting
things that you've already tried, but if not take a look at some raw data
after filtering and see if that didn't get rid of most of the problem.
A little filtering and a little caution with how the electrodes are
attached should take care of it.


good luck

Tim

================================================== =========
Timothy S. Hilliard, Ph.D. ofc: (617) 373-5994
Assistant Professor fax: (617) 373-3161
Physical Therapy Dept. e-mail: thilliar@lynx.neu.edu
206 Dockser Hall
Northeastern University
Boston, MA 02115

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I have would suggest trying a different electrode type just to
see if you get any differences (I assume the ones you are using
are the disposable type). I would not use the biopac standard
surface electrodes (at least those from a couple of years ago) --
we have found them to not stick well. We use electrodes from
sensormedics.

Alternate explanations could be some sort of static discharge
upon contact (unlikely perhaps), or when the leg is in a particular
position or on heel strike the artifact comes from cable movement
and not tissue movement.

Good luck,
Jeff
--
Jeff Ives, Ph.D.
Dept. of Exercise & Sport Sciences Phone: 607-274-1751
Ithaca College Fax: 607-274-1943
Ithaca, NY 14850 USA Email: jives@ithaca.edu
#####################################

If it is a movement artifact then it should be at a low frequency
relative to the EMG, so you should be able to filter it out using
matlab or a similar program.
Dr Jim Richards
Health Care Professions
University of Salford
Salford
M6 6PU
UK
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Sean Keegan
Biomechanics Graduate Program
Asst. Coach Men's Track and Field
Ball State University

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