the emg signal
TRANSCRIPT
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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 can’t 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 RateMotor 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)
EMG
Force
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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-waveRectified 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 - msec5.76 mV - msec
=Ratio: .37
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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 SourcesGottlieb, 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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