EMG - Force Relationship Signal Processing.3 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 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 Half-wave Deletes the EMG signal below the baseline

Rectification - Types Raw EMG Full-wave Rectified EMG Half-wave 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 - msec 5.76 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 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.