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Re: Electromechanical Delay

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  • Re: Electromechanical Delay

    Just a quick reply to Ton's comments. The paper he is talking about is
    "Olney SJ and Winter DA (1985) Predictions of knee and ankle moments
    of force in walking from EMG and kinematic data Journal of
    Biomechanics 18(1):9-20" (abstract below) but I actually wasn't really
    thinking of that one in particular. There's also "White SC and Winter
    (1992) Predicting muscle forces in gait from EMG signals and
    musculotendon kinematics. Journal of Electromyography and Kinesiology

    I haven't got Winter's book with me and these papers aren't on Science
    Direct, but I think the original work he quotes was this one:
    "Milner-Brown HS, Stein RB (1975)The relation between the surface
    electromyogram and muscular force. J Physiol ; 246:549-569". Frank
    Borg also mentions Soechting as having something to do with it.

    But anyway, my point is that there is no time-shifting (such as Ton
    suggests) involved - you just simply smooth the rectified EMG (i.e.
    envelope it) with a bog-standard Butterworth zero-lag low-pass filter.

    This is not just an academic question, because in analysing raw EMG
    from patients it is assumed that the timing of electrical activity
    indicates the timing of force production - I have never met any
    clinician who takes into account the electromechanical delay, which is
    surely present in the raw EMG. To my mind it's yet another reason why
    raw EMG should not be used clinically.

    Look forward to your further comments!


    A deterministic model was developed and validated to calculate
    instantaneous ankle and knee moments during walking using processed
    EMG from representative muscles, instantaneous joint angle as a
    correlate of muscle length and angular velocity as a correlate of
    muscle velocity, and having available total instantaneous joint
    moments for derivation of certain model parameters. A linear
    regression of the moment on specifically processed EMG, recorded while
    each subject performed cycled isometric calibration contractions,
    yielded the constants for a basic moment-EMG relationship. Using the
    resultant moment for optimization, the predicted moment was
    proportionally augmented for longer muscle lengths and reduced for
    shorter lengths. Similarly, the predicted moment was reduced for
    shortening velocities and increased if the muscle was lengthening. The
    plots of moments predicted using the full model and those calculated
    from link segment mechanics followed each other quite closely. The
    range of root mean square errors were: 3.2–9.5 Nm for the ankle and
    4.7–13.0 Nm for the knee.

    Dr. Chris Kirtley MD PhD
    Associate Professor
    Dept. of Biomedical Engineering
    Catholic University of America
    Washington DC 20064