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 Frequency domain Examples… Amplitude

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