Motor Unit Identification in the M Waves Recorded by High-Density Electromyogram

Objective: We describe and test the methodology supporting the identification of individual motor unit (MU) firings in the motor response (M wave) to percutaneous nerve stimulation recorded by surface high-density electromyography (HD-EMG) on synthetic and experimental data. Methods: A set of simulated voluntary contractions followed by 100 simulated M waves with a normal distribution (MU mean firing latency: 10 ms, Standard Deviation - SDLAT 0.1-1.3 ms) constituted the synthetic signals. In experimental condition, at least 52 progressively increasing M waves were elicited in the soleus muscle of 12 males, at rest (REST), and at 10% (C10) and 20% (C20) of maximal voluntary contraction (MVC). The MU decomposition filters were identified from 15-20 s long isometric plantar flexions performed at 10-70% of MVC and, afterwards, applied to M waves. Results: Synthetic signal analysis demonstrated high accuracy of MU identification in M waves (precision >= 85%). In experimental conditions 42.6 +/- 11.2 MUs per participant were identified from voluntary contractions. When the MU filters were applied to the M wave recordings, 28.4 +/- 14.3, 23.7 +/- 14.9 and 20.2 +/- 13.5 MU firings were identified in the maximal M waves, with individual MU firing latencies of 10.0 +/- 2.8 (SDLAT: 1.2 +/- 1.2), 9.6 +/- 3.0 (SD (LAT): 1.5 +/- 1.3) and 10.1 +/- 3.7 (SD (LAT) : 1.7 +/- 1.6) ms in REST, C10 and C20 conditions, respectively. Conclusion and significance: We present evidence that supports the feasibility of identifying MU firings in M waves recorded by HD-EMG.