1. EMG - Force Relationship Signal Processing.3
The EMG Signal
EMG - Force Relationship
Signal Processing.3

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. EMG - Force Relationship: Isometric vs. Isotonic Contractions

9. EMG - Force Relationship: Isometric vs. Isotonic Contractions
During isotonic contractions force production lags EMG
Motor unit twitch (contraction) reaches peak msec after motor unit activates
Summation of twitch contractions summates the delay (Inman et al., 1952; Gottlieb and Agarwal (1971)
Force
EMG

10. 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)

11. 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

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

13. 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

14. Rectification - Types Full-wave Half-wave
Deletes the EMG signal below the baseline

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

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

17. Rectification
To rectify the signal turn the toggle switch to the “On” position

18. 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

19. Integration
Area Under a Curve
Units = mV - msec

20. Integration - Procedure
EMG signal is
Full-wave rectified
(Usually) lowpass filtered
5 - 8 (10) Hz
Segment selected
Integral read (mV- msec [or secs])

22. 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

23. Answer
Probably not!

24. 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

25. 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

26. 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)

27. Therefore... Isometric contraction normalized with an isometric MVC
and
Isotonic/isokinetic contractions normalized with an isometric MVC

28. 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: =
.37

29. 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 ), Philadelphia: Franklin Institute.
Close, R.I. (1972). Dynamic properties of mammalian skeletal muscles. Physiological Review,52,

30. 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,
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,

31. 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 ), Basel, Switzerland: Karger.

32. 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),
Lippold, O.C.J. (1952). The relationship between integrated action potentials in a human muscle and its isometric tension. Journal of Physiology, 177,

33. 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 ), Basel, Switzerland: Karger.

34. 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,

35. 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,

36. 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,