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

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

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

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

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)

EMG

Force

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 Half-wave

Deletes the EMG signal below the baseline

Rectification - Types

Raw EMG

Full-waveRectified EMG

Half-wave Rectified EMG Delete

Rectification

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

Rectification To rectify the 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? 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

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)

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

2.13 mV - msec5.76 mV - msec

=Ratio: .37

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 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.

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.