Selecting the best sEMG channel based on amplitude parameters is straightforward in principle; the channel/pair with the highest amplitude are due to electrodes being placed over the most active muscle area. Since amplitude is lower over the IZ, choosing the best pair based on maximum amplitude parameters would reduce the possibility of choosing an electrode pair over the IZ. Also, by increasing amplitude, the signal-to-noise ratio is effectively increased.
For sEMG data collection, repeatability of the measured signal given common measurement conditions is essential. Based on repeatability between five trials, across all subjects and lower extremity muscles, six parameters had repeatability-4 values greater than 75%. From the highest repeatability value to lowest, these six parameters were RMS, WL, MAV, MNF, MDF, and MAX. Only RMS and MAV had a repeatability-5 value greater than 70%, suggesting that these parameters are more robust.
Good inter-parameter agreement was found between RMS and MAV, which is expected since they are both proportional to the average EMG amplitude (RMS corresponds to square root of the average square EMG magnitude and MAV corresponds to the average absolute magnitude). There was also good inter-parameter agreement between these parameters and the MAX parameter; however, the agreement was weaker than RMS-MAV. Since the MAX parameter corresponds to a single value over the EMG signal, rather than an average, its value is susceptible to higher variability and would be expected to be less reliable. The lower repeatability value associated with MAX reinforces this statement. Also, the WL parameter has only a modest inter-parameter agreement with RMS, MAV, and MAX, despite these parameters all having good repeatability values. WL is associated with the EMG signal derivative, computed as the sum of the absolute difference between adjacent samples. This single parameter provides a measure of waveform amplitude, frequency, and duration . Its sensitivity to frequency content may explain why WL only had a modest inter-parameter agreement with the amplitude measures.
The spectral parameters associated with EMG signal quality (i.e., SMR, DPR, SNR, and Ω) exhibited low repeatability. The EMG data used in this study were from isometric contractions in a laboratory setup; therefore, EMG signal quality was expected to be quite high in all cases. Indeed, the mean ± standard deviation of these parameters across all cases were: SMR 26.25 ± 12.46 dB, DPR 42.25 ± 7.19 dB, SNR 30.99 ± 5.57 dB, and Ω 0.73 ± 0.17. While these parameters do not appear to be beneficial for identifying an optimal electrode pair, they could be useful for excluding EMG channels with a low signal quality.
There is a correlation between amplitude and frequency content as a function of electrode position, with EMG from the IZ corresponding to a minimum in amplitude and maximum in MNF and MDF ; however, our results showed a low inter-parameter agreement between amplitude parameters (RMS, MAV, and MAX) and the MNF and MDF spectral parameters. These spectral parameters may have been more sensitive to electrode position along the muscle fiber axis than the orthogonal direction. Repeatability values of the spectral parameters decreased much more than the time domain parameters from repeatability-4 to repeatability-5. This suggests that time domain parameters would provide a more consistent method for electrode selection. Larger differences were observed with amplitude parameters than spectral parameters as a function of distance from the IZ , which also suggests that amplitude parameters can be expected to be more repeatable.
It is important to note that muscle geometry is an important factor in EMG [19–21]. For example, muscle fibers may also have inhomogeneities due to various fiber orientations . In the context of this work, the influence of the IZ on surface EMG may not be as prevalent in pinnate muscles (e.g., GM, GL, VL, and RF), where muscle fibers insert at an oblique angle to the tendon, compared to fusiform muscles (e.g., TA, BF, and ST); however, IZ effects are still observable in surface EMG measurements .
Agreement was examined to determine whether the electrode pair placed over the guideline-recommended location was the same as the top ranked pair for any of the twelve parameters. For all parameters, agreement was very low. This suggests that an array approach has a better chance of successfully locating an electrode pair based on maximal output as compared to the standard anatomical approach.
The electrode array approach could be considered for cases where the clinician has difficulty locating anatomical landmarks, such as for people with thick adipose tissue. The array could also be beneficial for test-retest situations where consistent electrode placement over time is difficult to reproduce. Variations in the EMG due to electrode positioning can be a confounding factor in analysis. In , an electrode array was also used for measurement of EMG from the lower extremities to assess this variability in the context of gait analysis. While the objective of this work is different, the results are consistent in that certain electrode pairs exhibited less variability than others. In addition, appreciable differences in robustness were observed between electrode pairs within the array, despite the array being placed near those adopted in clinical practice. An automatic electrode array approach would also provide results that are independent of a person’s experience with electrode placement; such as in home care or isolated healthcare environments.