The EMG Signal

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					The EMG Signal
     EMG - Force Relationship
       Signal Processing.3
EMG - Force Relationship
   An EMG signal will not necessarily reflect
   the total amount of force (or torque) a
   muscle can generate
    – The number of motor units recorded by
      electrodes will be less than the total number of
      motor units that are firing - electrodes can’t
      pick-up all motor units
EMG - Force Relationship:
Amplitude
   If a newly recruited motor unit is close to
    the electrode the relative increase in the
    EMG signal amplitude will be greater than
    the corresponding increase in force
   If a motor unit is too far from the electrode
    the amplitude will not change but the force
    will increase
EMG - Force Relationship:
Amplitude
   Motorunit firing rate will increase as force
    demand increases
    – Initially force rises rapidly due to increased
      firing rate
       » EMG amplitude will increase less rapidly
EMG - Force Relationship:
Firing Rate
   As force output increases beyond the rate of
    newly recruited motor units
         » Firing rate will increase
         » Force produced by the motor unit will saturate
EMG - Force Relationship:
Firing Rate
   Asforce output increases beyond the rate of
   newly recruited motor units
         » Firing rate will increase
         » Force produced by the motor unit will saturate
   TotalEMG amplitude increases more than
   force output (i.e., non-linear)

 EMG                              Force

         Motor Unit Firing Rate           Motor Unit Firing Rate
EMG - Force Relationship:
Isometric vs. Isotonic Contractions
    Lippold (1952), Close    (1972) & Bigland-
     Ritchie (1981) often cited in suggesting
     there is a linear relationship between IEMG
     and tension.
    Zuniga and Simmon (1969) &
     Vrendenbregt and Rau (1973) suggested a
     non-linear relationship exists
EMG - Force Relationship:
Isometric vs. Isotonic Contractions
EMG - Force Relationship:
Isometric vs. Isotonic Contractions
    During isotonic contractions force
     production lags EMG
      – Motor unit twitch (contraction) reaches peak 40
        - 100 msec after motor unit activates
      – Summation of twitch contractions summates
        the delay (Inman et al., 1952; Gottlieb and
        Agarwal (1971)


   Force

   EMG
EMG - Force Relationship:
Isometric vs. Isotonic Contractions
    Working Model:  Probably a consensus of
    opinion that EMG and force are “linear”
    under isometric condition and non-linear
    under isotonic conditions (Weir et al., 1992)
EMG - Force Relationship:
Concentric vs. Eccentric Contractions
   EMG amplitudes    are generally less during
    negative (eccentric) work vs. positive
    (concentric) work (Komi, 1973; Komi et al.,
    1987)
     – Preloaded tension in tendons (non-contractile
       elements) requires less contribution from
       muscle (contractile elements)
        » Less metabolic work required
     – EMG ~ muscle metabolism
Rectification
   Translates  the raw EMG signal to a single
    polarity (usually positive)
   Facilitates signal processing
    – Calculation of mean
    – Integration
    – Fast Fourier Transform (FFT)
Rectification - Types
   Full-wave          Adds the EMG signal
                        below the baseline
                        (usually negative
                        polarity) to the signal
                        above the baseline
                        – Conditioned signal is
                          all positive polarity
                       Preferred method
                        – Conserves all signal
                          energy for analysis
Rectification - Types
   Full-wave          Deletes the EMG
   Half-wave           signal below the
                        baseline
 Rectification - Types

   Raw EMG



Full-wave
Rectified EMG




     Half-wave
     Rectified EMG       Delete
Rectification
   Full-wave  rectification takes the absolute
    value of the signal (array of data points)
Rectification
   To rectifythe signal turn the toggle switch
    to the “On” position
Integration
   A method      of quantifying the EMG signal
    – Assigns the signal a numerical value
    – Permits manipulation
       » Calculation
              Example: Normalization
       » Statistical analysis
   A form     of linear envelope procedure
    – Measures the area under a curve
Integration
 Area Under a Curve




                      Units = mV - msec
Integration - Procedure
   EMG signal   is    Full-wave rectified
                       (Usually) lowpass
                        filtered
                          – 5 - 8 (10) Hz
                       Segment selected
                       Integral read (mV-
                        msec [or secs])
Normalization
   Question: Is it valid to directly compare the
    EMG output (e.g., integral) of a muscle
    across subjects?
   Subjects will have muscles with
    –   different   physiological cross-sections
    –   different   lengths - geometry
    –   different   ratios of slow- to fast-twitch fibers
    –   different   recruitment patterns
    –   different   firing frequencies
Answer
  Probably   not!
Solution
   Normalize  the measurement value against a
    maximal effort value
   Divide the sub-maximal effort value (e.g.,
    50%, 75%, etc.) by the maximal effort value
   The resultant ratio (no units) is the
    normalized signal making direct comparison
    possible
Isometric or Isotonic Effort?
               it seems to make sense that the
   Intuitively,
    normalizing maximal effort should be the
    same as the nature of the effort
     – Isometric - Isometric
     – Isotonic/Isokinetic - Isotonic/Isokinetic
Isometric or Isotonic Effort?
               it seems to make sense that the
   Intuitively,
    normalizing maximal effort should be the
    same as the nature of the effort
     – Isometric - Isometric
     – Isotonic/Isokinetic - Isotonic/Isokinetic
   Because  the relationship between the EMG
    signal and isotonic/isokinetic contractions
    is probably not linear, most sources
    recommend normalizing with the isometric
    maximal effort value (i.e., during MVC)
Therefore...
   Isometric  contraction normalized with an
    isometric MVC
                         and
   Isotonic/isokinetic contractions normalized
    with an isometric MVC
Example
   Integral during MVC of VM of
    quadriceps = 5.76 mV - msec
   Integral of VM at 50% of a sub-maximal
    effort = 2.13 mV - msec

    Ratio:     2.13 mV - msec   =   .37
               5.76 mV - msec
Reference Sources
  Bigland-Richie, B. (1981). EMG/force relations and
    fatigue of human volunatry contractions. In D.I.
    Miller (Ed.), Exercise and sport sciences reviews
    (Vol.9, pp.75-117), Philadelphia: Franklin
    Institute.

  Close, R.I. (1972). Dynamic properties of
    mammalian skeletal muscles. Physiological
    Review,52, 129-197.
Reference Sources
  Gottlieb, G.L., & G.C. Agarwal, G.C. (1971).
    Dynamic relatiosnhip between isometric muscle
    tension and the electromyogram in man. Journal of
    Applied Physiology, 30, 345-351.

  Inman, V.T., Ralston, J.B. Saunders, J.B., Fienstein,
    B, & Wright, E.W. (1952). Relation of human
    electromyogram to muscular tension. Medicine,
    Biology and Engineering, 8, 187-194.
Reference Sources
  Komi, P.V. (1973). Relationship between muscle
    tension, EMG, and velocity of contraction under
    concentric and eccentric work. In J.E. Desmedt,
    New developments in electromyography and
    clinical neurophysiology (pp. 596-606), Basel,
    Switzerland: Karger.
Reference Sources
  Komi, P.V., Kaneko, M., & Aura, O. (1987). EMG
    activity of the leg extensor muscles with special
    reference to mechanical efficiency in concentric
    and eccentric exercise. International Journal of
    Sports Medicine, 8 (suppl), 22-29.

  Lippold, O.C.J. (1952). The relationship between
    integrated action potentials in a human muscle and
    its isometric tension. Journal of Physiology, 177,
    492-499.
Reference Sources
  Vrendenbregt, J., & Rau, G. (1973). Surface
    electromyography in relation to force, muscle
    length and endurance. In J.E. Desmedt (Ed.) New
    developments in electromyography and clinical
    neurophysiology (pp. 607-622), Basel,
    Switzerland: Karger.
Reference Sources
  Zuniga, E.N., & Simons, D.G. (1969). Non-linear
    relationship between averaged electromyogram
    potential and muscle tension in normal subjects.
    Archives of Physical Medicine and Rehabilitation,
    50, 613-620.
Reference Sources
  Weir, J.P., McDonough, A.L., & Hill, V. (1996).
   The effects of joint angle on electromyographic
   indices of fatigue. European Journal of Applied
   Physiology and Occupational Physiology, 73, 387-
   392.
Reference Sources
  Weir, J.P, Wagner, L.L., & Housh, T.J. (1992).
   Linearity and reliability of the IEMG v. torque
   relationship for the forearm flexors and leg
   extensors. American Journal of Physical Medicine
   and Rehabilitation, 71, 283-287.

				
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posted:11/29/2011
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