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David Gabriel
06-14-2005, 11:39 PM
Greetings All:

In an earlier paper (Gabriel & Boucher, EJAP 1998, 79:37-40), we assumed
that EMD was related to excitation and contraction-coupling and generating
enough tension in the SEC so that force could be transmitted to the tendon
and move the limb. If so, it should be affected by dynamic training which
alters muscle activation . So, we divided the EMG burst into two
components EMD and END. The END portion was from the onset of movement
until the end of the burst. The portions did respond differently. The END
portion of the burst decreased in duration as subjects increased the speed
of limb movement. The duration EMD portion remained "relatively"
stable. We did note that 100 maximal effort contractions decreased its
duration "within" each training session, but were unable to find an across
sessions training effect.

How we filtered the data was an important issue. It affected the absolute
magnitude of the observed results, not the appearance of them. That is,
experimental affects were real, no matter how we filtered the data: i.e.,
band-passed versus linear envelope detection as per Winter (3rd Ed).

In my experience, EMG can be as long as 25-50 ms for isotonic contractions
of the elbow flexors (Gabriel & Boucher, 1998), or as low as 10-15 ms for
isometric contractions of the elbow flexors (Gabriel, Basford, & An, JEK
2001, 11: 123-129). There are figures in both of these papers which show
the differences in EMD for different contraction modalities. Both studies
used a zero phase Butterworth, band-pass filter. The earlier paper also
outline the algorithm for EMG burst onset and termination, which has been
requested many times in this forum.

One last point. I have noticed that EMD values for isotonic contractions
are shorter when using acceleration to determine the onset of movement
versus an event marker triggered by a microswitch; there are additional
'slight' delays associated with the electronics. I also read this is
another paper but can't remember the citation.

Best Wishes,

-d.g.


David A. Gabriel, Ph.D., FACSM
Department of Physical Education and Kinesiology
Brock University
St. Catharines, Ontario, CANADA
L2S 3A1

Phone: 905-688-5550 ext.4362
FAX: 905-688-8364
E-mail: dgabriel@brocku.ca

"I learn from my mistakes. I can repeat them perfectly"