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Van Den Bogert, Ton
06-14-2005, 06:07 AM
Chris Kirtley always brings up great discussion topics.

Chris:

> EMG, but I have always followed David Winter's reasoning that EMG
> should always be smoothed at a frequency related to the twitch time of
> the muscle (usually about 3 Hz for lower-limb muscles) so that its
> envelope most closely matches the force generated by the muscle.

Chris is probably referring to Olney & Winter (J Biomech 1985), who showed that a 3 Hz low-pass filtered EMG envelope can predict joint moments during gait. If I remember correctly, they used a recursive filter that caused the EMG envelope to be not only smoothed but also delayed to become synchronized with muscle force. I think it would be be better to filter with a zero-lag filter and then do a time shift as a separate EMG processing step. The smoothness and time shift can then be varied independently. Otherwise you may not even be aware that you are time shifting the EMG signal, and by how much.

As Chris says, most people do not smooth the EMG envelope that much. By doing the delay and smoothing as separate processing steps, you can avoid oversmoothing and still have the right amount of delay.

But perhaps Olney and Winter were on to something. If joint moments do not have much signal above 3 Hz, this is because muscles can't change their force output very quickly. And that very same property of muscles is also responsible for the electromechanical delay. So there may be a fundamental reason why a low pass filter with cutoff frequency F0 and a time delay C/F0 (where C is some dimensionless constant) is exactly the right way to process EMG into a signal related to muscle force. I do not have Olney & Winter's paper in front of me, but from a description of their filter you could figure out what the constant C was.

> 1. Is there, in fact, such a thing as electro-mechanical delay?

I think yes. It may seem to disappear if you process the EMG with a filter that has time lag, but this does not mean it does not exist.

> 2. If so, how should it be defined?

I am not familiar with that literature, but it makes sense to do an experiment where you ask a subject to perform rapid variations in muscle force, superimposed on some baseline. The cross-correlation function between force and EMG envelope would have a peak at the electromechanical delay time. This is how it was done by Vint et al. (Med Sci Sports Exerc 2001).

> 3. What is it for common muscles?

I have seen so many different values, it would be good to have a critical review with a discussion of how different methods can give different results. Muraoka et al (J Appl Physiol 2004) showed that EMD depends on tendon strain, due to nonlinear series elasticity. Vint et al (2001) found that EMD decreases with increasing baseline force, which is consistent with that mechanism. Therefore, EMD may not even be a well-defined concept at the whole muscle level.

There must be a Biomch-L subscriber who is more familiar with this literature. Please post your contribution!

--

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
Phone: (216) 444-5566
http://www.lerner.ccf.org/bme/bogert/
http://www.isb2005.org

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