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vnaing65
08-14-2006, 04:05 AM
Hi,

Here is a summary of responses regarding my query on methods of reducing
noise and signal loss from EMG measurements underneath a knee brace. It
seems that others with similar problems caused by shear and axial pressure
on electrodes have tried low profile electrodes or ones with connectors not
directly over the recording area. Alternatively, pressure can be relieved
from the electrodes by high density foam placed around the electrodes so the
brace sits on the foam instead of the electrode. Based on some suggestions,
we have ordered Ambu neuroline 700 electrodes which have a lead instead of a
snap connector. We have also tried a Delsys system and did not have any
problems.
Much thanks to all that responded,

Veronica Naing
Simon Fraser University

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I ve been using the biometrics EMG system with elite downhill skiiers, i
have been fixing the elecrtodes to VL and VM, and Peroneus Longus, the
skiiers then put a thermals and an extremly tight catsuit over the top -
so far the results have been good and i dont get too much nosie.
sometimes the cause of noise is the electrodes being placed too close
together and "crosstalking"?
also are you sure the electrodes arnt moving around under the brace - as
electrode movement creates a massive amount of noise.
Have a look at the biometrics system - i would recommend it.
charlotte elsworth
celsworth@brookes.ac.uk
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Maybe you can try to fix
the electrode with some kind of elastic tapping material. We did some sports
projects, used elastic tape for athletics, Johnson & Johnson Co. But that
costs a lot.

On the other hand, the earlier discussed issue on [BIOMCH-L], EMG noise on
treadmill, for your reference to decrease the noise during data collecting.

Regards,
Pan Hui-Fen, M.S., P.T.
a066038@apple.cmu.edu.tw
.................................................. ...........

I used the Noraxon system for 18 months during walking experiments and noise
from electrode lead movement was a problem. I minimized the noise with
careful taping of the lead. The trick was to prevent leads from hitting
other structures but not taped so tightly that mechanical stretch caused
another source of noise.

Recently I have been using a Bortec system and it is much less sensitive to
lead noise. Taping the lead is a cursory exercise now.

I donít have the added problem of braces fixed over the electrodes.

I doubt you can fully solve the latter problem unless you position the
electrodes away from the braces.

If that is not possible, and the braces are large enough in diameter, you
could try a layer of 10mm thick high density foam on the inside of the brace
and leave a gap where the electrodes are positioned.

Jim
j-stinear@northwestern.edu
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Hi Veronica,

I have experienced similar problems when recording EMG when subjects
walking while wearing different types of knee and ankle braces. There
were a couple of things we did that help with the problem. First we
used low-profile disposable sticker electrodes. I am not sure what type
of electrode you are using but the thicker plastic electrodes did not
handle shear forces well. Second we bought a sheet of thick foam that
was sticky on one side. We cut strips of this foam and placed it around
the electrodes so that the brace would sit on the foam rather then
directly on the electrode. We also tried making more ridgid plastic
coverings for the eletrodes but this was not more effective then the
thick foam strips. Finally when possible we modified our braces so that
they would have openings where we were going to place the electrodes.
These things helped but getting good EMG signals from under a tight
fitting brace was always a lot of work.

Keith
kegordon@umich.edu
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Hi Veronica,

When recording surface EMG from
superficial muscles underneath a brace or AFO, to minimize movement
artifact we routinely use conventional disposable non-preamplified
electrodes in conjunction with the usual preamplifier-equippped
electrode supplied with our EMG system. We secure the non-preamplified
electrodes (e.g., VIASYS Neurocare) over the site of interest with BSN
Medical (formerly Biersdorf) Cover-Roll Stretch Tape. (This product,
while expensive, was the hands-down winner after an epic search to find
the optimal skin prep tape.) We attach the preamplified electrode (in
our case, Motion Lab Systems) nearby, mounted upside down with the
transducer surfaces exposed. The brace itself often provides a
convenient rigid, stable site for mounting this electrode. Trim the
output leads from the plain electrodes to an appropriate length, strip
the insulation from the last centimeter, and tape the bare ends against
the respective + and - transducer faces on the preamp'd electrode.
Secure everything with the same stretchy pre-wrap that athletic trainers
use when taping ankles.

When working with tall thermoplastic AFO's, alternatively we'll merely
cut out access windows of the appropriate size and location so that we
can use the usual preamplifier-equipped electrode; however, this
solution probably won't apply to your knee brace mounted device. Good luck!

Terry Horn, Ph.D
Director, MRC Motion Analysis & Human Performance Laboratory
gaitdr@MMRCREHAB.ORG
.................................................. ...........
Dear Veronica,

is the brace device passive or active? In case it is active, I am
wondering if the device is interfering with EMG electrodes because they
are not shielded. I am not sure if you would gain an increase signal to
noise ratio by some sort of shielding of the electrodes. However, this
might not be possible.

One other possible source of interference is also some capacitive
effects between the electrodes and the brace. This might affect the
recording by picking up some external electromagnetic field in the
room. Some times a preamplifier before the acquisition board might
reduce the effect of this.

Massimo.
mac50@pitt.edu

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Veronica, insulate the electrodes...go to an electrical dept or store and
ask for some insulating tape. It's proper name is friction tape. It
doesn't stick very well, but unless the noise is caused because the
electrode is moving, it should help isolate the 'trodes and stop the noise
that might be caused because the brace itself is linking the 'trodes.
K
jkmiller37067@yahoo.com
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Veronica,

IS there any metal in the knee brace? If so, this
may be attracting EMF. As well, do you get the same effect if you press the
sensors to the skin hard and jiggle them a bit? IF so, then what your seeing
is called biomechanical noise OR motion artifact. I know it seems weird
cause the brace should hold the sensor in place but that's precicley the
problem. The skin stretches, bends and contorts as someone moves around.
Typically, a sensor attached with double sided tape is left to move with the
skin the relative tension/shear between the two is close to zero. However,
with the brace in place the skin still moves but the sensor is held by the
brace. Hence, relative tension shear between the skin and the sensor creates
the artifact.

Geoff Desmoulin
gdesmoulin@hotmail.com
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I haven't worked with your system but it seems as if you have an isolation
problem. Either one of the components is generating noise and the noise is
making it into the 1200 when you tighten the brace or the tightening is
bridging two of the linear amp input circuits in the 1200.

I just had a few ideas or questions that you might think about:

Could the brace be making electrical contact with one of the electrodes or
one of the lead wires and causing a spurious crossover between two of the
pre-amp circuits in the 1200?

Could the brace be pressing a worn wire up against the skin of the subject
and letting ground or treadmill noise enter the 1200?

Does the noise continue if the treadmill is turned off and the subject walks
in place?

Is the subject holding on to a non-grounded support rod?

Is the 1200 grounded to the same ground as the treadmill?

Does the noise continue if the treadmill is unplugged?

Does the noise continue if the fluorescent lights are turned off?

Is the spark suppressor on the treadmill's DC motor not working?

Wrapping the entire leg with an insulating blanket of some type, even thick
newspaper, would help you determine which part of securing the brace is
causing the problem. If you use newspaper and there is no noise, then you
can tear away sections of the insulating paper and localize the problem.

Regards,

Ted Andresen
Tjacmc@aol.com
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1) What is the typical impedance condition? In difficult measurement
conditions it is very important to make a good skin preparation. The
impedance should be below 10 KOhm - see ABC of EMG.


2) Which electrode type is used?
Pressure artifact can not be solved by changing the EMG system. Most
important is the selection of a proper electrode. One that may work fine
is the AMBU QR electrode, which is flat and has no snap connection near
the electrode but a thin cable:

AmbuR Blue Sensor

Description

The medium-size, radio translucent Ambu Blue Sensor QR features highly
conductive wet gel, superior adhesion, and a large measuring area to
ensure optimal signal quality during medium-term applications. Thanks to
the special occlusive backing material, the electrode will not absorb
liquids. The QR is only available with pre-attached lead wires.
Features

http://www.ambu.com/ambu/Final.nsf/(pubid)/i91BCC5F1C7C9AD69C1256ED6003C
E800i645AF5538F35DF4AC1256ED600490F9Ei5FEFAB67CACB F17BC1256ED6004CF769?O
pen&SubName=&ParentName=MRI%20and%20X-ray
Hope that helps - best regards Peter
Todd.shewman@noraxon.com