The influence of input excitation on the inter- and intra-day reliability of the motor unit firing rate versus recruitment threshold relationship

Abstract

Decomposition of the surface electromyographic (sEMG) signal is commonly used to examine motor unit (MU) firing behavior. However, the intra- and inter-day reliability of these measurements has yet to be quantified or reported. This investigation 1) examined the effect of input excitation on the mean firing rate (MFR) vs. recruitment threshold (RT) relationship and 2) determined the inter- and intra-day reliability of the MFR vs. RT relationship at 30%, 50%, and 70% of maximum voluntary isometric strength (MVIC). Twenty-eight healthy males (23 ± 3 yr) completed two experimental visits, during which they performed MVIC testing and isometric ramp contractions at 30%, 50%, and 70% MVIC. sEMG signals were recorded from the vastus lateralis during the ramp contractions and decomposed to establish the MFR vs. RT relationship for the detected MUs. Intra- and inter-day reliability was then established for the slopes and y-intercepts of the MFR vs. RT relationship at each contraction intensity. All participants displayed significant MFR vs. RT relationships ( r range: −0.662 to −0.999; P ≤ 0.001–0.006). Intra- and inter-day intraclass correlation coefficients (ICCs) ranged from 0.766–0.824 and 0.867–0.919 for the slopes and from 0.780–0.915 and 0.804–0.927 for the y-intercepts, respectively. Furthermore, the slope coefficient was significantly greater at 70% than at 30% MVIC, and the y-intercepts increased with increasing contraction intensities. Changes in input excitation to the MU pool alter the magnitude, but not the reliability, of the slopes and y-intercepts of the MFR vs. RT relationship. NEW & NOTEWORTHY The firing behavior of the motor unit (MU) pool is often characterized using the mean firing rate vs. recruitment threshold relationship of the active MUs. Although this relationship has been widely used, this is the first study to report the effects of input excitation (contraction intensity) on the intra- and inter-day reliability of this relationship. The criteria used for MU analysis and the model utilized in this study allow for generalization to outside investigators and laboratories.

Sex-related differences in motor unit firing rates and action potential amplitudes of the first dorsal interosseous during high-, but not low-intensity contractions

“…The slope of the regression from EMG variables and joint torque was then correlated with the slope of the regression lines between MU variables and RTs. Furthermore, all subjects and contraction intensities were collapsed and analyzed, rather than assessing by individual subjects and contraction intensities Decomposition is a common technique to analyze EMG from the surface of the skin to determine MU behavior (Beck et al 2011;Colquhoun et al 2018b;Dimmick et al 2018;Enoka 2019; . Recently, Colquhoun et al (2018b) quantified intra and inter-day reliability of the slopes and y-intercepts of the Delsys PDIII decomposition algorithm (PDIII) and found ICC's between 0.766-0.919 for the slopes and 0.780-0.927 for the y-intercepts.…”

“…Furthermore, all subjects and contraction intensities were collapsed and analyzed, rather than assessing by individual subjects and contraction intensities Decomposition is a common technique to analyze EMG from the surface of the skin to determine MU behavior (Beck et al 2011;Colquhoun et al 2018b;Dimmick et al 2018;Enoka 2019; . Recently, Colquhoun et al (2018b) quantified intra and inter-day reliability of the slopes and y-intercepts of the Delsys PDIII decomposition algorithm (PDIII) and found ICC's between 0.766-0.919 for the slopes and 0.780-0.927 for the y-intercepts. These findings provide information for the PDIII system as a reliable method for assessing changes in firing rate behavior and excitation to the motorneuron pool during an isometric contraction at differing intensities (Colquhoun et al 2018b).…”

“…Recently, Colquhoun et al (2018b) quantified intra and inter-day reliability of the slopes and y-intercepts of the Delsys PDIII decomposition algorithm (PDIII) and found ICC's between 0.766-0.919 for the slopes and 0.780-0.927 for the y-intercepts. These findings provide information for the PDIII system as a reliable method for assessing changes in firing rate behavior and excitation to the motorneuron pool during an isometric contraction at differing intensities (Colquhoun et al 2018b).…”

Reduced blood oxygen levels induce changes in low-threshold motor unit firing that align with the individual’s tolerance to hypoxia

“…The four channels of EMG recorded during each ramped contraction were decomposed into their constituent MUAPTs using the Precision Decomposition III (PD III) method (Delsys). This method has been previously described in detail (De Luca et al 2006;Kline and De Luca 2014;Nawab et al 2010), independently two-source validated against intramuscular EMG assessment during low levels of isometric force contractions (Hu et al 2014), and shown to have good intra-and interday reliability (Colquhoun et al 2018). In the current study, validation of MUAPTs was performed in accordance with De Luca and Contessa (2012), where each identified MUAPT was compared with a synthesized sum of MUAPTs with additional Gaussian noise and a root mean square (RMS) value equal to the original signal.…”

Characteristics of the Electrophysiological Properties of Neuromuscular Motor Units and Its Adaptive Strategy Response in Lower Extremity Muscles for Seniors with Pre-Sarcopenia: A Preliminary Study

“…Additionally, there are two kinds of motor unit physiology: a smaller motor unit, which is slow-twitching, a type I muscle fiber, and fatigue-resisted, and a larger motor unit, which is fast-twitching and has type II muscle fiber fatigability [41]. In addition, during an isometric contraction, there is a negative correlation between the recruitment threshold and the mean firing rate; a higher RT has been found to respond to a lower MFR [28,42,43]. One study found that a type I motor unit's MFR was higher than that of type II.…”