Onset and Offset voltage values for EMG

[Malindu Fernando]

Hi everyone,

I am after the onset and offset values people are using for EMG of the lower limb in healthy people during normal gait and also any deviations of this in neuropathic gait?

I am having trouble finding these values and need to know what values you are using to indicate when a muscle is 'active' and when it ceases being 'active' during a movement i.e the voltage threshold you are using and any references to these values.

Thanks,

Mal

[Torrence Welch]

Re: Onset and Offset voltage values for EMG

I have always recorded a background EMG level during either quiet stance or at rest, and then determined onset as whenever the activity exceeds the mean EMG during this background period + 2 standard deviations. You may also need to incorporate rules for how long the EMG signal has to remain above that threshhold level before you consider it 'on' and how long it has to remain below that threshhold level before you consider the muscle 'off'. Usually somewhere in the 30 ms range has worked for me in that regard. I should also mention that I perform this analysis after the raw EMG signal has been transformed into an EMG envelope through filtering.

Best of luck!
Torrence

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

I have always recorded a background EMG level during either quiet stance or at rest, and then determined onset as whenever the activity exceeds the mean EMG during this background period + 2 standard deviations. You may also need to incorporate rules for how long the EMG signal has to remain above that threshhold level before you consider it 'on' and how long it has to remain below that threshhold level before you consider the muscle 'off'. Usually somewhere in the 30 ms range has worked for me in that regard. I should also mention that I perform this analysis after the raw EMG signal has been transformed into an EMG envelope through filtering.

Best of luck!
Torrence

Thanks kindly for that Torrence, that was useful. Have you got any papers that you use to quote this method? Or any of your previous papers?

Kind regards,


Mal

[Douglas Powell]

Re: Onset and Offset voltage values for EMG

Mal,
You may also consider using the Teager-Kaiser Energy Operator to improve the clarity of the onset and offset. There are several papers that have reported improvement in the determination of EMG onset and offset using this method. Solnik and Hortobagyi published these data in 2009 or 2010 I believe.
DP

[Giulia Mantovani]

Re: Onset and Offset voltage values for EMG

Hi everyone,

I am after the onset and offset values people are using for EMG of the lower limb in healthy people during normal gait and also any deviations of this in neuropathic gait?

I am having trouble finding these values and need to know what values you are using to indicate when a muscle is 'active' and when it ceases being 'active' during a movement i.e the voltage threshold you are using and any references to these values.

Thanks,

Mal

Hello
I would also suggest this paper by Paolo Bonato "A Statistical Method for the Measurement of Muscle Activation Intervals from Surface Myoelectric Signal During Gait", IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 45, NO. 3, MARCH 1998.

Giulia

[Gannon White]

Re: Onset and Offset voltage values for EMG

Mal,

Your voltage is relative to placement and will vary between individuals due to exact placement, adipose layer, how well you clean/shave the surface etc... The 2-3 SD method should also be on an individual/per visit/per electrode placement basis. So to be clear, getting a baseline and choosing a 2-3 SD threshold should be done every time you remove/replace the electrode and for each muscle/channel. Sorry if I pointed out "the obvious" but it is better to be clear vs not!

Gannon

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Mal,

Your voltage is relative to placement and will vary between individuals due to exact placement, adipose layer, how well you clean/shave the surface etc... The 2-3 SD method should also be on an individual/per visit/per electrode placement basis. So to be clear, getting a baseline and choosing a 2-3 SD threshold should be done every time you remove/replace the electrode and for each muscle/channel. Sorry if I pointed out "the obvious" but it is better to be clear vs not!

Gannon

Thanks for this Gannon, much appreciated. So this is the method you routinely use? And what time period do you use to determine when a muscle is active and not active?

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Mal,
You may also consider using the Teager-Kaiser Energy Operator to improve the clarity of the onset and offset. There are several papers that have reported improvement in the determination of EMG onset and offset using this method. Solnik and Hortobagyi published these data in 2009 or 2010 I believe.
DP

Cheers Doug, will have a look into this. You use the Teager-Kaiser operator?

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Mal,
You may also consider using the Teager-Kaiser Energy Operator to improve the clarity of the onset and offset. There are several papers that have reported improvement in the determination of EMG onset and offset using this method. Solnik and Hortobagyi published these data in 2009 or 2010 I believe.
DP

Cheers Doug, will have a look into this. You use the Teager-Kaiser operator?

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Thanks Giulia.

[Gannon White]

Re: Onset and Offset voltage values for EMG

In our lab I'm usually looking at something like a clean and jerk or a gait cycle so the time periods are expressed as %s of the cycle or movement. I measure a few pretrials to look at how quickly the "burst" happens or rather how defined the muscle bursts are. I forgot to mention earlier you need to use a window size (50 ms or so) and once that crosses the SD threshold it's on until it crosses it again. I've seen some that specify after so many consecutive samples (10 strait samples over the threshold) are above the threshold then the muscle is on. One issue you will see are muscles that "pre-activate", turn off, then back on. Also, for motions like a sit to stand we've seen very slow increases in the signal (generally not a problem for explosive movements)...then it becomes more subjective.

Cheers,
Gannon

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Thanks Gannon. What do you suggest in terms of window time? And do you use the RMS (Root mean square) method to obtain your linear envelops?

[Malindu Fernando]

Re: Onset and Offset voltage values for EMG

Also I note that the 2-3 SD method does not work in some instances and a multiplication factor of 7-8 is needed..Is this also the case? Or have you been able to get values with using 2-3 SD to identify onset and offset?