Comparison between the degree of motor unit short-term synchronization and recurrence quantification analysis of the surface EMG in two human muscles

Abstract

The study provides a systematic evaluation of a technique previously proposed for the estimation of a clinically relevant characteristic of motor unit behavior.

Reliability of EMG determinism to detect changes in motor unit synchrony and coherence during submaximal contraction

“…As compared to the coefficient of determination of 70% reported in the ECR muscles by Del Santo et al (2006), our study, yielded variations in EMG DET that were only poorly explained by variation in motor unit synchronous impulse probability and frequency across the population of motor unit pairs tested (5 and 6%, respectively). In a recent study, no correlation at all was reported to occur between EMG DET and motor unit synchronous impulse frequency in two muscles showing a marked difference in synchronization strength (Dideriksen et al, 2009). It must be noted that in our study and that of Dideriksen et al (2009), EMG DET was related to different amounts of motor unit synchrony observed in natural conditions whereas in Del Santo et al (2006Santo et al ( , 2007, motor unit synchrony was pharmacologically enhanced by increasing recurrent inhibition activity with l-acetylcarnitine injection.…”

“…In a recent study, no correlation at all was reported to occur between EMG DET and motor unit synchronous impulse frequency in two muscles showing a marked difference in synchronization strength (Dideriksen et al, 2009). It must be noted that in our study and that of Dideriksen et al (2009), EMG DET was related to different amounts of motor unit synchrony observed in natural conditions whereas in Del Santo et al (2006Santo et al ( , 2007, motor unit synchrony was pharmacologically enhanced by increasing recurrent inhibition activity with l-acetylcarnitine injection. Another major methodological difference is that, in the two studies published by Del Santo et al (2006Santo et al ( , 2007, EMG DET values obtained across single 5-s periods were related to synchrony indices obtained across whole recordings lasting 2-3 min.…”

“…In the present study performed at lower contraction level using similar RQA parameters, DET was not found to co-vary consistently with the amount of synchronous impulses measured by cross-correlation analysis of single motor unit discharges, nor with the amount of oscillatory synchronous activity measured by coherence analysis. All in all, complementary to the report by Dideriksen et al (2009), the present data speak strongly against the use of RQA to detect changes in the synchronization of motor unit discharges in both the time and frequency domains at contraction levels lower than 20% of MVC.…”

“…Using 5-s steps did not average this variability out (unpublished data). Taking into account this variability, in our study and that of Dideriksen et al (2009), an average of DET values obtained in successive steps across whole recordings was used to investigate the relationship between EMG DET and synchrony indices obtained across the same period of time.…”

“…These results were taken as a major argument in favour of the use of RQA to evaluate changes in motor unit synchronization. The reliability of RQA as a tool for assessing single motor unit synchronization has been recently challenged, however, in a study in which similar EMG DET values were observed in two muscles showing a striking difference in motor unit synchrony (Dideriksen et al, 2009). The purpose of the present work was to further investigate this controversial issue.…”