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Andrew Chapman
09-11-2002, 04:05 PM
Message to: Biomech-L subscribers

I am attempting to collect telemetered EMG data during running, but the artefact content of the data is overwhelming.

I have searched the Biomech-L archives for previous postings relating to EMG noise content, but all suggestions mentioned in these postings have failed to resolve my current problems.

The data is being captured with a Noraxon TeleMyo 900 8 channel telemetered EMG system. This system has a fixed output voltage range (+/- 2.5 volts when in bipolar mode) and a fixed gain of 2000.

To date, all measures taken to reduce the noise content of the data have failed. Any running, even at the slowest of speeds, causes clipping and thus a high frequency artefact which cannot be filtered - rending the data useless. Even a comparatively gentle foot 'stomp' causes an artefact of amplitude beyond the range of the voltage band and thus clipping.

When the signal does not contain clipped data (for example: from isolated limb movements), filtering is effective, and the quality of the data, while not extraordinary, is reasonable. With a fixed output range and fixed gain, I can not see any adjustments that can be made to expand the voltage band and thus prevent the clipping without actually reducing the noise. However, if such adjustments are possible, I would welcome advice as to how these adjustments can be made.

Interestingly, the artefact content of the data is enhanced when fine wire electrodes are used, but regardless, the artefact is at levels which render the data useless even when surface electrodes are used as per the intended use the Noraxon system (thus the fine wire electrodes themselves do not appear to be the main/only source of the problem).

The fine wire electrodes used consist of 75 micrometer teflon coated stainless steel with ~1mm bared tips for recording. Custom made connectors were used to connect the wire to the Noraxon EMG cables, via an alligator clip to allow connection to the electrode wires, and a 'male' press stud to fit the 'female' press stud connection of the Noraxon cable.

All cables were secured to the subject as best as possible to minimize movement, and all cables were wound around one another. Channels not in use were shorted out with back to back surface electrodes. The cable was secured to prevent tension on the input point to the transmitter (which was strapped as tightly as possible around the subject's waist) and the input point to the electrodes.

Varying the ground electrode position does not alter the artefact content of the recorded signals.

I would appreciate any suggestions as to how I can reduce the noise content of my EMG recordings? I can provide sample data if this will assist others in determining the cause of this noise.

Regards,
Andrew Chapman

BPhty(Hons.)(UQ)
PhD Candidate
Department of Physiotherapy
The University of Queensland
St. Lucia, 4072, QLD, Australia.

and

Department of Physical Therapies
Australian Institute of Sport
P.O.Box 176
Belconnen, 2616, ACT, Australia.

email: andrewchapman@ozemail.com.au
phone: + 61 2 6214 7943
fax: + 61 2 6214 7953
mobile: + 61 438 115 605


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