Announcement

Collapse
No announcement yet.

Summary: EMG normalization during walking

Collapse
This topic is closed.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Summary: EMG normalization during walking

    Dear Biomch-L members,

    Following is a summary of replies to my question regarding amplitude
    normalization during walking. The initial question was:

    Dear Biomch-list members,

    I am interested in quantifying co-activation of antagonistic muscles
    during walking in 2 different adult population groups. Surface EMG
    will be used to measure the EMG signals from selected muscles during
    the activity, and the raw EMG will be processed to derive a
    time-series RMS signal for each muscle. The time-series RMS signals
    will be temporally normalized with regard to the cycle time, i.e. to a
    percentage of the gait cycle.

    My question is, should the amplitude of the EMG measured from a muscle
    during walking be normalized to a percentage of either: a) the peak
    EMG amplitude measured during a maximal voluntary isometric
    contraction, b) the peak EMG amplitude measured during a maximal
    voluntary isotonic contraction, or, c) the peak EMG amplitude measured
    during the activity?

    >From my review of the literature, it appears that methods a) & c) are
    the most common and both appear to have their strengths and
    weaknesses. Any comments on which of the above methods are:

    - the most valid,
    - show the greatest intra-subject reliability and
    reproducibility,
    and/or
    - show the least inter-subject variability,

    would be greatly appreciated. Also if anyone has used or knows of
    other methods to normalize the amplitude of dynamic EMG during
    submaximal activities, please let me know.

    I will post a summary of responses in a fortnight.

    Thank-you

    Peter Mills


    Following are the replies that were received. Thanks to all.

    ---------------------------------------------------------------------

    Peter:

    I have found the following references useful.

    Regarding reliability of normalization schemes:
    Yang, J.F., & Winter, D.A. (1984). Electromyographic amplitude
    normalization methods: Improving their sensitivity as diagnostic tools
    in gait analysis. Archives of Physical Medicine and Rehabilitation, 65
    (9), 517-521.

    (This reference was also supplied by Birgitte van Don)

    Birgitta J.C. van Don, Ph.D.
    Boston University
    NeuroMuscular Research Center
    Motor Control Laboratory
    44 Cummington Street>Boston, MA 02215
    Phone (617) 353-8984
    Fax: (617) 353-5737
    e-mail: bvandon@bu.edu


    Regarding a useful index of coactivation:
    Falconer, K. & Winter, D.A. (1985). Quantitative assessment of
    co-contraction at the ankle joint in walking. Electromyography and
    Clinical Neurophysiology, 25, 135-149.

    It sounds like you are doing something similar to something I've just
    done. I worked in collaboration with a colleague to estimate indices
    of coactivation during normal and pathologic gait at a variety of
    walking speeds. Here was my general procedure:

    1. Normalize all EMG data with respect to time (step time or stride
    time). This is straightforward. 2. Normalize the amplitude EMG data
    from each muscle with respect to the mean ensemble average (6 stride
    average) for condition 1 (lowest walking speed). Therefore, at
    subsequent walking speeds all EMG data are expressed as a percent of
    the baseline speed. I have found coefficients of variation to be
    lowest for this normalization scheme. It is also much less time
    consuming than any normalization scheme involving MVCs. 3. Compute
    coactivation indices using time and amplitude normalized EMG profiles
    using the method described in Falconer & Winter (1985). I favor this
    method because it is not affected by errors associated with the
    identification of muscle activity "onsets" and "offsets". However, you
    should be cognizant of the fact that many muscles of the lower
    extremity are biarticular in nature and therefore may confound your
    ability to make a straightforward interpretation of any coactivation
    index that uses EMG data from a biarticular muscle.

    Good luck,

    Peter
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Peter F. Vint, Ph.D.
    Department of Exercise and Sport Science
    256 HHP Building, PO Box 26169
    University of North Carolina at Greensboro
    Greensboro, NC 27402-6169
    Phone: (336) 334-3031
    Fax: (336) 334-3031
    E-mail: pfvint@uncg.edu
    URL: http://www.uncg.edu/~pfvint
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    ---------------------------------------------------------------------
    --------------------------------------------------------------------
    To my opinion, the c option is the most suitable for a main reason:
    Specificity of pattern.

    If the aim is to express EMG normalized i think it can be misleading
    to normalize it to a measure which does not have anything to do with a
    dynamic performance such as gait. However it presents some problems.
    In literature most of the research has been conducted normalizing EMG
    to MVC or % of MVC, however it has been shown that MVC can be affected
    by a lot of parameters (motivation most of all and familiarity with
    the testing procedure). Second, MVC is performed at a specific limb
    angle....then how it can represent EMG baseline activity ? I think it
    is better to normalize to peak amplitude measured during activity.

    Marco Cardinale, M.Sc.

    ---------------------------------------------------------------------
    ---------------------------------------------------------------------

    > M a r c o

    >Thank-you for your reply. I tend to agree with you for
    the reason >that you stated, and also because by normalizing using
    the peak EMG >measured during the activity, any intersubject
    differences with >regard to the maximal amount of force developed by
    the individual >muscle (during an MVC) will not alter the
    intra-subject comparisons.

    This is another good point.

    > Using MVC to normalize muscle activity
    >during a dynamic task, 2 subjects with equal absolute dynamic force
    >patterns during an activity but differences in absolute maximal force
    >production (during MVC), of a muscle, would be identified as having
    >differences in muscle activation patterns, because the muscle with
    >the greater potential force production will have a lower activation
    >level at any point during the activity.

    I agree, this is why, to me, it is important to normalize EMG to some
    dynamic values when dynamic tasks have to be measured. However in
    literature most of the people used MVC. I was forced to use MVC
    normalization by my tutor in the M.S. Thesis....but I disagree
    completely with the results. I was measuring effects of dynamic
    exercise...and they obliged me to perform isometric MVC measurement...
    Anyway, I think from now it is better to use peak value to normalize
    activities such as gait.

    >Thank-you and I would welcome any further comments

    If something comes from my mind or come up from literature I will keep
    you updated. All the best for your research, Marco Cardinale

    ---------------------------------------------------------------------

    Peter,

    Did you check the biomechan-list archives? There are a few entries in
    the last couple of years summarizing viewpoints on normalizing EMG.
    Also, I suggest:

    Electroencephalogr Clin Neurophysiol 1987 Nov;67(5):402-11
    EMG profiles during normal human walking: stride-to-stride and
    inter-subject variability.
    Winter DA, Yack HJ


    ************************************************** ***********
    Gordon Chalmers, Ph.D.
    Dept. of Physical Education, Health and Recreation
    Western Washington University, MS-9067
    Bellingham WA
    U.S.A.
    98225-9067

    chalmers@cc.wwu.edu
    http://www.wwu.edu/~chalmers
    Phone: (360) 650-3113
    FAX: (360) 650-7447

    ---------------------------------------------------------------------
    --------------------------------------------------------------------

    Gordon,

    Thank-you for your reply. I have located summaries regarding
    amplitude normalization of dynamic EMG in the Biomch-L archives by
    Roger James (1996) and Kevin Sims (1997) that have been helpful.

    My concerns regarding the 'best' normalization method stems partly
    from an article: Knutson, L. M., Soderberg, G. L., Ballantyne, B. T.,
    & Clarke, W. R. (1994). A study of various normalization procedures
    for within day electromyographic data. Journal of Electromyography
    and Kinesiology, 4(1), 47-59.

    This article normalized EMG activity measured from the medial
    gastrocnemius during a balance board activity using 3 amplitude
    normalization procedures: 1) as a % of the peak ensemble averaged
    value measured measured during a MVIC, 2) as a % of the peak ensemble
    averaged value measured during the activity, and 3)as a % of the mean
    ensemble averaged value measured during the activity. They reported
    the lowest intra-subject CV's for the 2nd method, but also reported
    higher VR's and lower ICC's for the peak activity normalized data than
    for the MVIC normalized data. However, as only 2 trials for each
    subject were used, these results may not fully represent the
    reliability and reproducibility of these methods.

    I personally think it is more appropriate to represent muscle
    activation during a dynamic task as a percentage of the maximal muscle
    activation during that task, because using a percentage of the EMG
    measured during a MVIC can create differences in normalized activity
    between individuals that are due solely to differences in the maximal
    potential activation level of the muscle measured during an MVIC.

    I greatly appreciate your reply and would welcome any further thoughts
    ideas.

    Peter

    ---------------------------------------------------------------------
    ---------------------------------------------------------------------


    I am not an expert in this area, but, I notice that Winter DA, Yack HJ
    (reference given before) (and Winter is an expert) agree with you.
    See>page 405 - normalizing to MVC increased variability observed, so
    they normalized to the average value seen during the step cycle. I
    know (but don't have references at hand) that others have nomalized
    to max seen in step cycle as you suggest. So I suggest you pick one
    such method that is established by a prominent publication(s) and/or
    researcher and go with it. In fact, normalizing to mean or max level
    seen in step cycle is something you can compare in your preliminary
    analysis, because both can be done from the same data after the data
    is collected.

    Good luck GC

    Gordon Chalmers, Ph.D.
    Dept. of Physical Education, Health and Recreation
    Western Washington University, MS-9067
    Bellingham WA >U.S.A.
    >98225-9067 > >chalmers@cc.wwu.edu >http://www.wwu.edu/~chalmers
    >Phone: (360) 650-3113 >FAX: (360)
    ---------------------------------------------------------------------
    ---------------------------------------------------------------------


    From: "a.l.hof"
    Organization: faculty of medical sciences (RuG)
    To: p.mills@MAILBOX.GU.EDU.AU
    Date sent: Wed, 16 Jun 1999 09:31:07 GMT+0100
    Subject: Re: EMG normalization during walking
    Priority: normal

    Dear peter, I am late in answering, but this is because your question
    requires quite a long answer. > > I am interested in quantifying
    co-activation of antagonistic muscles during > walking in 2 different
    adult population groups. Surface EMG will be used to > measure the
    EMG signals from selected muscles during the activity, and the > raw
    EMG will be processed to derive a time-series RMS signal for each >
    muscle. The time-series RMS signals will be temporally normalized
    with > regard to the cycle time, i.e. to a percentage of the gait
    cycle. > > My question is, should the amplitude of the EMG measured
    from a muscle > during walking be normalized to a percentage of
    either: a) the peak EMG > amplitude measured during a maximal
    voluntary isometric contraction, b) the > peak EMG amplitude measured
    during a maximal voluntary isotonic contraction, > or, c) the peak EMG
    amplitude measured during the activity? > > From my review of the
    literature, it appears that methods a) & c) are the > most common and
    both appear to have their strengths and weaknesses. Any > comments on
    which of the above methods are: > > - the most valid, >
    - show the greatest intra-subject reliability and reproducibility, >
    and/or > - show the least inter-subject variability, > > would
    be greatly appreciated. Also if anyone has used or knows of other >
    methods to normalize the amplitude of dynamic EMG during submaximal >
    activities, please let me know.

    What method is the best, depends very much on the subject of your
    research. 1) In general, the best EMG calibration is by using a
    submaximal isometric contraction with slowly increasing force. This
    can give an rmsEMG/force or - moment ratio. (J Biomech 10:529-539,
    1977). 2) Scaling with EMG at MVC is much more convenient, no
    dynamometer needed, but not accurate. Maximum EMGs are very variable.
    3) For walking I use in many cases a regression of the individual
    averaged profile Ei with the 'normal' profile E , obtained from a
    group of 'normal' subjects.
    gain = sum( Ei*E / E^2)
    Then (Ei/gain) gives a profile of the same magnitude as the normal
    one. Disadvantage: cocontracting muscles are not quantified in moment.
    4) For your work, if you cannot do 1), you may record a situation in
    which two antagonist muscles are working against each other " keep the
    foot / knee as stiff as possible". The muscles are working against
    each other, but the net moment is zero. What is the ratio of the EMGs.

    In the past several studies have come with a measure of cocontraction
    in which just raw ┬ÁVs of EMG were used. This is of course incorrect.

    As you will see, there is not one best answer.
    Good luck anyway,

    Best wishes,

    At Hof
    Department of Medical Physiology &
    Laboratory of Human Movement Analysis AZG
    University of Groningen
    Bloemsingel 10
    NL-9712 KZ GRONINGEN
    THE NETHERLANDS
    Tel: (31) 50 3632645
    Fax: (31) 50 3632751
    e-mail: a.l.hof@med.rug.nl

    ---------------------------------------------------------------------
    ---------------------------------------------------------------------

    The end. Thanks again!


    Peter Mills PES GUGC
    p.mills@mailbox.gu.edu.au

    ---------------------------------------------------------------
    To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
    For information and archives: http://isb.ri.ccf.org/biomch-l
    ---------------------------------------------------------------
Working...
X