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EMG Analysis and Interpretation

EMG Analysis and Interpretation. Extracting information from the signals. Interference EMG. Superimposed muscle action potentials Positive deflections Negative deflections Isoelectric line Algebraic sum of muscle action potentials Amplitude cancellation I.e., interference.

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EMG Analysis and Interpretation

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  1. EMG Analysis and Interpretation Extracting information from the signals

  2. Interference EMG • Superimposed muscle action potentials • Positive deflections • Negative deflections • Isoelectric line • Algebraic sum of muscle action potentials • Amplitude cancellation • I.e., interference

  3. Two Signal Domains • Time domain • Amplitude • Frequency domain • Frequency • Examples…

  4. Time Domain • Changes in EMG amplitude over time • Root-mean-square (rms) value • Same as standard deviation • Time-averaged integrated EMG values • Integrated = AUC • AUC ÷ time

  5. Normalization • Drawbacks of surface (interference) EMG is comparing raw values • Between subjects • Between muscles • Repeated over time • Solution = normalize EMG amplitude as a percentage of the maximal voluntary contraction (MVC). • Result = comparing the patterns of response under different conditions or tasks, not the absolute value. • Example…HRmax

  6. Frequency Domain • Determining the frequency content of an EMG signal epoch • Epoch = any given signal portion/duration • Measured in Hertz (Hz) • Cycles (turns) per second • Every signal can be represented by a set of sine and cosine terms = Fourier series. • The fundamental, its harmonics, and the amplitude of each harmonic • Fourier Transformation • Determine the Power Density Spectrum (PDS) • Mean Power Frequency (MPF) • Mean (Hz) of the PDS • Median Power Frequency (MDF) • Frequency (Hz) that bisects the PDS in half

  7. EMG Signal Conditioning • Units • Volts (V), millivolts (mV), and microvolts (μV) • Zero Mean • Subtracting the mean value • Digital filtering • Reducing unwanted signal components • Low-frequency noise (movement artifact, etc) • High-frequency noise (random white noise, etc) • More about this after Fourier analysis…

  8. Quantified Values • EMG Amplitude • RMS • Represents: • Algebraic sum of muscle action potentials traveling within the recording area • Number of motor units recruited • Firing rate of activated motor units • Linear or curvalinear relation with force production

  9. Quantified Values • EMG Median Power Frequency (MDF) • Derived from Fourier Transformation • Indication of what frequencies dominate the EMG signal (power of the signal) • Related to: • The shape of the action potentials • Most significant contributor… • The relative timing of the action potentials discharged by different motor units • The discharge rate of motor units (firing rate) • Least significant contributor… • Mostly used to track fatigue

  10. Displacement Sensor Accelerometer Laser Beam Bipolar EMG Electrodes Force Transducer Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003. Isometric muscle action at 30% MVC

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