The EMG Signal

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The EMG Signal

• EMG - Force Relationship
• Signal Processing.3

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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 cant pick-up all
  motor units

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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

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EMG - Force Relationship Amplitude

• Motor unit firing rate will increase as force
  demand increases
• Initially force rises rapidly due to increased
  firing rate
• EMG amplitude will increase less rapidly

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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

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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
• Total EMG amplitude increases more than force
  output (i.e., non-linear)

EMG
Force
Motor Unit Firing Rate
Motor Unit Firing Rate
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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

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EMG - Force Relationship Isometric vs.
Isotonic Contractions
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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
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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)

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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

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Rectification

• Translates the raw EMG signal to a single
  polarity (usually positive)
• Facilitates signal processing
• Calculation of mean
• Integration
• Fast Fourier Transform (FFT)

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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

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Rectification - Types

• Full-wave
• Half-wave

• Deletes the EMG signal below the baseline

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Rectification - Types
Raw EMG
Full-wave Rectified EMG
Half-wave Rectified EMG
Delete
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Rectification

• Full-wave rectification takes the absolute value
  of the signal (array of data points)

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Rectification

• To rectify the signal turn the toggle switch to
  the On position

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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

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Integration
Area Under a Curve
Units mV - msec
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Integration - Procedure

• EMG signal is

• Full-wave rectified
• (Usually) lowpass filtered
• 5 - 8 (10) Hz
• Segment selected
• Integral read (mV- msec or secs)

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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

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Answer

• Probably not!

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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

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Isometric or Isotonic Effort?

• Intuitively, it seems to make sense that the
  normalizing maximal effort should be the same as
  the nature of the effort
• Isometric - Isometric
• Isotonic/Isokinetic - Isotonic/Isokinetic

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Isometric or Isotonic Effort?

• Intuitively, it seems to make sense that the
  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)

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Therefore...

• Isometric contraction normalized with an
  isometric MVC
• and
• Isotonic/isokinetic contractions normalized with
  an isometric MVC

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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

2.13 mV - msec 5.76 mV - msec
Ratio

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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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