Identification of Motor Unit Firings in H-Reflex of Soleus Muscle Recorded by High-Density Surface Electromyography

Abstract

We developed and tested the methodology that supports the identification of individual motor unit (MU) firings from the Hoffman (or H) reflex recorded by surface high-density EMG (HD-EMG). Synthetic HD-EMG signals were constructed from simulated 10% to 90% of maximum voluntary contraction -MVC, followed by 100 simulated H-reflexes. In each H-reflex the MU firings were normally distributed with mean latency of 20 ms and standard deviations (SDLAT) ranging from 0.1 to 1.3 ms. Experimental H-reflexes were recorded from the soleus muscle of 12 men (33.6 ± 5.8 years) using HD-EMG array of 5×13 surface electrodes. Participants performed 15 to 20 s long voluntary plantarflexions with contraction levels ranging from 10% to 70% MVC. Afterwards, at least 60 H-reflexes were electrically elicited at three levels of background muscle activity: rest, 10% and 20% MVC. HD-EMGs of voluntary contractions were decomposed using the Convolution Kernel Compensation method to estimate the MU filters. When applied to HD-EMG signals with synthetic H reflexes, MU filters demonstrated high MU identification accuracy, especially for SDLAT > 0.3 ms. When applied to experimental H-reflex recordings, the MU filters identified 14.1 ± 12.1, 18.2 ± 12.1 and 20.8 ± 8.7 firings per H-reflex, with individual MU firing latencies of 35.9 ± 3.3, 35.1 ± 3.0 and 34.6 ± 3.3 ms for rest, 10% and 20% MVC background muscle activity, respectively. Standard deviation of MU latencies across experimental H-reflexes were 1.0 ± 0.8, 1.3 ± 1.1 and 1.5 ± 1.2 ms, in agreement with intramuscular EMG studies.

Decoding firings of a large population of human motor units from high‐density surface electromyogram in response to transcranial magnetic stimulation

“…The nature of elicited contractions represents a computing challenge due to highly synchronised and superimposed MU firings. Promising results have been shown previously whereby MU firings could be identified in induced reflexes (Kalc et al, 2022a;Yavuz et al, 2015), and a limited number of MU firings were identified in response to maximal percutaneous nerve stimulation (i.e. M wave; Kalc et al, 2022b).…”

“…To assess the feasibility of the methodology we first simulated synthetic HDsEMG signals for which the ground truth about MU firings was known. We previously showed in simulations that the approach of estimating MU filters from the decomposition of voluntary submaximal contractions and applying them to evoked responses allows accurate identification of MU firings at a fixed proportion of motor pool recruitment, even in cases of supraphysiological synchronisation of firings (Kalc et al., 2022a , 2022b ). Here, we simulated MEPs corresponding to a wide range of the proportion of motor pool activation (10–100%) and demonstrated high precision and accuracy of MU identification (>90%).…”

“…Furthermore, because of the geometry and relatively low levels of adipose tissue, HDsEMG signals recorded from these muscles typically yield higher number of identified MUs compared to some proximal muscles (Del Vecchio et al., 2020 ). We have previously identified MU firings during evoked reflexes in the soleus, another distal muscle (Kalc et al., 2022a ). Thus, the methodological approach presented herein should be evaluated further on other muscle groups, particularly proximal and/or fusiform muscles.…”

“…In this study, we aimed to ascertain the validity and accuracy of identifying firings of a large population of individual MUs that constitute an evoked motor response to single pulse TMS. We utilised the principles of the convolution kernel composition (CKC) method of HDsEMG decomposition that we recently developed for identification of MU firings underpinning the H‐reflex (Kalc et al., 2022a ) and M‐wave (Kalc et al., 2022b ); here we adapted and applied this methodology to more complex evoked responses to TMS. We relied on MU filters, a unique property of HDsEMG decomposition (Francic & Holobar, 2021 ), obtained from a range of voluntary contractions to estimate firing instances of many MUs during MEPs.…”