Time Domain Analysis:
Time Domain Analysis uses computed functions. Learn more about how to create computed function by referring to the LabScribe Manual or view the video at Computed Functions in LabScribe.
To access the advanced EMG module analysis functions:
- Click on the channel’s “Add Function” button
- Choose “EMG” and then the “EMG analysis” function
Available Time Domain functions are:
- Root Mean Square (RMS)
- Average Rectified Value
- Teager-Kaiser Energy
- Threshold (used for Gait analysis)
- Maximal Voluntary Contraction (MVC)
Frequency Domain analysis are also available from this menu and we will cover these later.
Root Mean Square (RMS)
Choosing the Root Mean Square function, will open the setup dialog for the RMS calculations.
- Window Length: The length of the time window in msec over which the RMS value is calculated
- Baseline: Set the time at the beginning of each block to be used for calculating the baseline
- Averaging: Set the number of points to use for a moving average after the RMS data is calculated
Click on the “Preview” button to see the RMS value for the selection
Click “OK”
This calculates the RMS value for a timed window. To calculate RMS value for each cycle of EMG, use the Periodic→RMS function.
Average Rectified Value
The average rectifed value function is similar to the Timed RMS function described above.
It calculated the Average absolute value of the signal in the timed window selected.
Teagar-Kaiser Energy
The Teager-Kaiser Energy function is similar to the Timed RMS function described above.
It calculated the Teager-Kaiser Energy value of the signal in the timed window selected.
Threshold
The Suppression Time will take out any active or deactive periods in the output that are less than the user specified time. As seen in the example above, Channel C2 is setup with no Suppression Time, whereas channel C3 is setup with a 100ms suppression time.
Maximal Voluntary Contraction (MVC)
For calculating the MVC the RMS value is calculated first. Refer to the RMS calculation section above for more details.
Set the Window Length, the baseline points and averaging. Click on the “preview” button to see the RMS value for the current data.
If the data segment being viewed contains the MVC data, click the “Calculate MVC form current data”. This will calculate the maximum of the RMS data from the current data and use that as the MVC value. Click “OK”. The channel data will show the MVC value as percentage.
Envelope EMG Data:
There are 2 options to calculate the Envelope of the EMG signal:
- Smoothing the Rectified Data
- Calculating the Periodic (cyclic) maximum for each cycle of the EMG
Smoothing the Rectified Data:
To Smooth the rectified EMG signal:
- Click on the “Add Function” button of the Rectified EMG channel
- Choose “Smoothing”
- Choose the smoothing type: “Moving Average”
- Choose the number of points for the moving average
Cyclic Maximum of EMG:
To Smooth the Rectified EMG signal:
- Click on the EMG channel’s “Add Function” button
- Choose “Periodic” then “Cyclic Maximum”
- Set the Threshold and Tolerance as in the RMS setup
Filtering
A user defined filter can also be applied to the Rectified EMG signal.
To Filter the Rectified EMG signal:
- Click on the EMG channel’s “Add Function” button
- Choose “Filter”
- Choose “FIR Filter”
- Set the type of filter (from drop down menu) and filter cutoffs
- Choose Filter Order
Mean Derivative
Mean Derivative of the EMG signal decreases with time during the task that induces fatigue.
To calculate the Mean Derivative:
- Click on the EMG channel’s “Add Function” button
- Choose “Periodic” then “Mean dV/dt”
- The dialog setup is similar to the RMS setup
Pulsed Integrator
Use Absolute Integral and averaging to pulsed integration circuit which provides a voltage proportional to the area under the rectified EMG curve in discrete time windows.
- Click on the EMG channel’s “Add Function” button
Choose “Integral” then “Absolute”
The Integral dialog setup Choose “Average” and the time typically is 0.05 sec. We now have a channel that is the Pulsed Integral of the EMG signal.
Frequency Domain Analysis
Some Frequency Domain Analysis uses computed functions. Learn more about how to create computed function by referring to the LabScribe Manual or view the video at Computed Functions in LabScribe.
To access the advanced EMG module analysis functions:
- Click on the channel’s “Add Function” button
- Choose “EMG” and then the “EMG analysis” function
Available Frequency Domain functions are:
- Mean Frequency
- Median Frequency
- Peak Frequency
- Total Power
- Mean Power
- Frequency Ratio
The EMG Frequency Analysis Dialog will popup
Choose the window length and the number of points for a moving average
Mean Frequency (MNF)
Mean Frequency (MNF) is an average frequency which is calculated as the sum of the product of the EMG power spectrum and the frequency divided by the total sum of the power spectrum (e.g. Oskoei & Hu, 2008; Phinyomark et al., 2012a). MNF is also referred to as central frequency (fc), centroid, the spectral center of gravity, mean power frequency and mean spectral frequency.
Mean Frequency is defined as:
Where fj is the frequency value of EMG power spectrum at the frequency bin j, Pj is the EMG power spectrum at the frequency bin j, and M is the length of frequency bin.
MNF and MDF so far have been hailed as the gold standard for muscle fatigue assessment with surface EMG signals due to the fact that muscle fatigue results in a downward shift of frequency spectrum of the EMG signal.
Median Frequency (MDF)
Median Frequency (MDF) is a frequency at which the EMG power spectrum is divided into two regions with equal amplitude (e.g. Oskoei & Hu, 2008; Phinyomark et al., 2012a). MDF is also defined as a half of the total power, or TTP (dividing the total power area into two equal parts).
Peak Frequency (PKF)
Peak Frequency (PKF) is a frequency at which the maximum EMG power occurs.
Total Power (TTP)
TTP is an aggregate of EMG power spectrum (Phinyomark et al., 2012a). This is also defined as the energy and the zero spectral moment (SM0).
Mean Power
Mean Power (MNP) is the average power of the EMG power spectrum.
Frequency Ratio (FR)
FR provides information to differentiate between contraction and relaxation of muscle and it is defined as the ratio of power spectrum at low-frequency band and high-frequency band.
Where LLC and ULC are lower and upper frequency bounds for the low frequency band, while LHC and UHC are lower and upper frequency bounds for the high frequency band.
