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Dear Biomch-L subscribers,
May I thank everyone who responded to my post “Tilt Platform Ankle Perturbation”
Apologies for the delay in reply.
Please find a breif summary and the original posting below, followed by a selection of replies received.
Many Thanks,
Peter Thain
Postgraduate MSc by Research Student
University of Hertfordshire
England, UK
--------------------------Summary-------------------------
It appears that when calculating reaction time, MVC are not a required element. However, if one wishes to look at the muscle activity/magnitude, then normalisation is required as raw volts cannot be used to compare between individuals.
--------------------Original Message--------------------
Dear All,
I am currently conducting a study on reaction time and muscle activity during an ankle perturbation, using a tilt platform simulating a lateral ankle sprain mechanism.
When calculating EMG magnitude, are MVIC's required?
In numerous research papers conducting similar studies, MVIC's have not been used although they have not stated why. I would suggest a rationale that the reaction time occurs in a period of miiliseconds, far quicker than conscious control allows. I feel that performing static MVIC's does not reflect muscle activity during forced dynamic movements.
Any thoughts or opinions would be of great help
Thank you in advance.
Peter Thain
--------------------Reply Messages--------------------
If you are hoping to compare EMG amplitude between subjects, muscles and/or across trials performed on separate days then you will need to normalise your EMG signal to something. This is because there are so many factors that may affect the EMG signal (such as muscle size and composition, electrode placement, and electrical impedance of the fascia, subcutaneous fat and skin) that will change from day-to-day, muscle-to-muscle, and person-to-person. Assuming that there is a general linear relationship between EMG amplitude and force (at least during isometric conditions) then EMG amplitude during a MVIC should, in theory, provide maximal voluntary EMG to normalise all other EMG values to. Whilst there are problems with this theory, normalising the EMG signal to a MVIC is considered much more reliable than just reporting the absolute values, and it is the typical method used within the literature.
Neale Tillin (N.A.Tillin@lboro.ac.uk)
Loughborough University
--------------------Reply Messages--------------------
Normalizing to EMG MVC is needed when you wish to establish EMG-Force relations or between subject/muscles comparisons of activation levels. This process affects the amplitude computations, but timings are independent of activation level scales.
M.A. Sanjari (sanjarima@alum.sharif.edu)
Director of Biomechanics Lab
Rehabilitation Research Centre
Iran University of Medical Sciences
--------------------Reply Messages--------------------
Basically, you will only need to normalize if you're interested in the magnitude (vs. reaction time) and: 1) you are comparing between subjects, or 2) you are removing and replacing the electrodes in a within subjects design (eg - having them return on multiple days). Now, in saying that, whether you normalize to MVIC or to 'maximum dynamic EMG' (peak EMG during the dynamic tasks) is up to you - as long as you provide your rationale you should be OK.
My only note regarding 'maximum dynamic EMG' is that subject will habituate to the perturbation (see: Jackson NS, Gutierrez GM, Kaminski TW. The effect of fatigue and habituation on the stretch reflex of the ankle musculature. J Electromyography Kinesiol. Jan 2009; 19 (1): 75-84.), so you must give them a significant practice period (>10 trials) so they are already habituated (or come up with some other way around this).
Gregory M. Gutierrez, PhD (gmgutierrez@gmail.com)
Assistant Professor
Department of Physical Therapy
--------------------Reply Messages--------------------
In my opinion, standardizing EMG by MVIC provides a means to compare amplitudes across individuals. If you are looking at amplitude measures in any way, you can't compare volts across individuals due to variations in impedance. However, if you are simply looking at timing issues, then it shouldn't matter.
Laura Frey Law, PT, PhD (laura-freylaw@uiowa.edu)
Assistant Professor
Physical Therapy and Rehabilitation Science
The University of Iowa
--------------------Reply Messages--------------------
I would say only if you are examining EMG amplitude. To just measure EMG latency from the time of perturbation we've just had a timing marker relative to perturbation onset and measured the onset of activation based on a threshold of EMG that is greater than resting noise level. In the past we've used a certain number of standard deviations (anywhere from 5 to 10) beyond resting EMG amplitude to determine our onset of activation.
Thomas W. Kernozek, Ph.D. (kernozek.thom@uwlax.edu)
University of Wisconsin-La Crosse
Department of Health Professions
Physical Therapy Program
--------------------Reply Messages--------------------
I would surmise that the reason that previous ankle perturbation reaction time studies have utilized MVICs is because they are interested in timing variables (ie, time to muscle activation) as opposed to amplitude variables (ie, %MVIC). In most of these studies, the dependent variable is the time (in milliseconds) between platform perturbation and onset of muscle activity. Onset of muscle activity is typically defined as a sustained period of EMG activity above a threshold. The thresholds vary across studies but are usually defined as 2 (or sometimes 3) standard deviations above the mean resting EMG amplitude for the given muscle. The SD comes from the same window of resting EMG amplitude as the mean. Thus, to calculate this dependent variable there is no need to record MVICs of the muscles you are testing
Jay Hertel, PhD, ATC (jhertel@virginia.edu)
Associate Professor of Kinesiology and
Physical Medicine & Rehabilitation
University of Virginia
--------------------Reply Messages--------------------
We did not use MVCs or MVICs because we were only interested in the latencies, not in the amplitude of the response.
Prof. Dr. Dieter Rosenbaum (diro@uni-muenster.de)
--------------------Reply Messages--------------------
We have been measuring EMG during cycling and we also understand that the MVIC's is not the most appropriate choice for EMG normalization. Some novel possibilities (i.e. maximal isokinetics) have been suggested to dynamical normalize the EMG data. I also believe that a reference condition, when the focus is comparing "non-conventional" situations, can be used for the normalization of the EMG data.
Rodrigo Rico Bini, PhD Student (bini.rodrigo@gmail.com)
Sport Performance Research Institute New Zealand
School of Sport and Recreation
--------------------Reply Messages--------------------
There are two aspects of EMG recordings that may be used in studies of reaction time. One is the timing of activation onsets, peaks, and offsets. The other is intensity or magnitude of muscle activation. If you are only concerned about the former, there is no real need to scale to MVIC. However if you are going to report muscle activation magnitude across multiple recording sessions, including across different people, it is advisable to scale each record to %MVIC. While it is certainly true that during dynamic situations the MVIC does not reliably represent the true maximum value
obtainable, it does serve to control for factors such as muscle size, skin impedance, and electrode placement which can have significant influences on recorded signal amplitude. This is particularly true when using signal amplitude as a measure of muscle activation intensity, but it also holds
when using frequency measures as indicators of the level of muscle activation.
Lawrence D. Abraham, Professor (l.abraham@mail.utexas.edu)
Kinesiology & Health Education
-------------------------------------------------------------
END
__________________________________________________ _______________
Send us your Hotmail stories and be featured in our newsletter
http://clk.atdmt.com/UKM/go/195013117/direct/01/
May I thank everyone who responded to my post “Tilt Platform Ankle Perturbation”
Apologies for the delay in reply.
Please find a breif summary and the original posting below, followed by a selection of replies received.
Many Thanks,
Peter Thain
Postgraduate MSc by Research Student
University of Hertfordshire
England, UK
--------------------------Summary-------------------------
It appears that when calculating reaction time, MVC are not a required element. However, if one wishes to look at the muscle activity/magnitude, then normalisation is required as raw volts cannot be used to compare between individuals.
--------------------Original Message--------------------
Dear All,
I am currently conducting a study on reaction time and muscle activity during an ankle perturbation, using a tilt platform simulating a lateral ankle sprain mechanism.
When calculating EMG magnitude, are MVIC's required?
In numerous research papers conducting similar studies, MVIC's have not been used although they have not stated why. I would suggest a rationale that the reaction time occurs in a period of miiliseconds, far quicker than conscious control allows. I feel that performing static MVIC's does not reflect muscle activity during forced dynamic movements.
Any thoughts or opinions would be of great help
Thank you in advance.
Peter Thain
--------------------Reply Messages--------------------
If you are hoping to compare EMG amplitude between subjects, muscles and/or across trials performed on separate days then you will need to normalise your EMG signal to something. This is because there are so many factors that may affect the EMG signal (such as muscle size and composition, electrode placement, and electrical impedance of the fascia, subcutaneous fat and skin) that will change from day-to-day, muscle-to-muscle, and person-to-person. Assuming that there is a general linear relationship between EMG amplitude and force (at least during isometric conditions) then EMG amplitude during a MVIC should, in theory, provide maximal voluntary EMG to normalise all other EMG values to. Whilst there are problems with this theory, normalising the EMG signal to a MVIC is considered much more reliable than just reporting the absolute values, and it is the typical method used within the literature.
Neale Tillin (N.A.Tillin@lboro.ac.uk)
Loughborough University
--------------------Reply Messages--------------------
Normalizing to EMG MVC is needed when you wish to establish EMG-Force relations or between subject/muscles comparisons of activation levels. This process affects the amplitude computations, but timings are independent of activation level scales.
M.A. Sanjari (sanjarima@alum.sharif.edu)
Director of Biomechanics Lab
Rehabilitation Research Centre
Iran University of Medical Sciences
--------------------Reply Messages--------------------
Basically, you will only need to normalize if you're interested in the magnitude (vs. reaction time) and: 1) you are comparing between subjects, or 2) you are removing and replacing the electrodes in a within subjects design (eg - having them return on multiple days). Now, in saying that, whether you normalize to MVIC or to 'maximum dynamic EMG' (peak EMG during the dynamic tasks) is up to you - as long as you provide your rationale you should be OK.
My only note regarding 'maximum dynamic EMG' is that subject will habituate to the perturbation (see: Jackson NS, Gutierrez GM, Kaminski TW. The effect of fatigue and habituation on the stretch reflex of the ankle musculature. J Electromyography Kinesiol. Jan 2009; 19 (1): 75-84.), so you must give them a significant practice period (>10 trials) so they are already habituated (or come up with some other way around this).
Gregory M. Gutierrez, PhD (gmgutierrez@gmail.com)
Assistant Professor
Department of Physical Therapy
--------------------Reply Messages--------------------
In my opinion, standardizing EMG by MVIC provides a means to compare amplitudes across individuals. If you are looking at amplitude measures in any way, you can't compare volts across individuals due to variations in impedance. However, if you are simply looking at timing issues, then it shouldn't matter.
Laura Frey Law, PT, PhD (laura-freylaw@uiowa.edu)
Assistant Professor
Physical Therapy and Rehabilitation Science
The University of Iowa
--------------------Reply Messages--------------------
I would say only if you are examining EMG amplitude. To just measure EMG latency from the time of perturbation we've just had a timing marker relative to perturbation onset and measured the onset of activation based on a threshold of EMG that is greater than resting noise level. In the past we've used a certain number of standard deviations (anywhere from 5 to 10) beyond resting EMG amplitude to determine our onset of activation.
Thomas W. Kernozek, Ph.D. (kernozek.thom@uwlax.edu)
University of Wisconsin-La Crosse
Department of Health Professions
Physical Therapy Program
--------------------Reply Messages--------------------
I would surmise that the reason that previous ankle perturbation reaction time studies have utilized MVICs is because they are interested in timing variables (ie, time to muscle activation) as opposed to amplitude variables (ie, %MVIC). In most of these studies, the dependent variable is the time (in milliseconds) between platform perturbation and onset of muscle activity. Onset of muscle activity is typically defined as a sustained period of EMG activity above a threshold. The thresholds vary across studies but are usually defined as 2 (or sometimes 3) standard deviations above the mean resting EMG amplitude for the given muscle. The SD comes from the same window of resting EMG amplitude as the mean. Thus, to calculate this dependent variable there is no need to record MVICs of the muscles you are testing
Jay Hertel, PhD, ATC (jhertel@virginia.edu)
Associate Professor of Kinesiology and
Physical Medicine & Rehabilitation
University of Virginia
--------------------Reply Messages--------------------
We did not use MVCs or MVICs because we were only interested in the latencies, not in the amplitude of the response.
Prof. Dr. Dieter Rosenbaum (diro@uni-muenster.de)
--------------------Reply Messages--------------------
We have been measuring EMG during cycling and we also understand that the MVIC's is not the most appropriate choice for EMG normalization. Some novel possibilities (i.e. maximal isokinetics) have been suggested to dynamical normalize the EMG data. I also believe that a reference condition, when the focus is comparing "non-conventional" situations, can be used for the normalization of the EMG data.
Rodrigo Rico Bini, PhD Student (bini.rodrigo@gmail.com)
Sport Performance Research Institute New Zealand
School of Sport and Recreation
--------------------Reply Messages--------------------
There are two aspects of EMG recordings that may be used in studies of reaction time. One is the timing of activation onsets, peaks, and offsets. The other is intensity or magnitude of muscle activation. If you are only concerned about the former, there is no real need to scale to MVIC. However if you are going to report muscle activation magnitude across multiple recording sessions, including across different people, it is advisable to scale each record to %MVIC. While it is certainly true that during dynamic situations the MVIC does not reliably represent the true maximum value
obtainable, it does serve to control for factors such as muscle size, skin impedance, and electrode placement which can have significant influences on recorded signal amplitude. This is particularly true when using signal amplitude as a measure of muscle activation intensity, but it also holds
when using frequency measures as indicators of the level of muscle activation.
Lawrence D. Abraham, Professor (l.abraham@mail.utexas.edu)
Kinesiology & Health Education
-------------------------------------------------------------
END
__________________________________________________ _______________
Send us your Hotmail stories and be featured in our newsletter
http://clk.atdmt.com/UKM/go/195013117/direct/01/