Motor unit recruitment and bursts of activity in the surface electromyogram during a sustained contraction

Abstract

Bursts of activity in the surface electromyogram (EMG) during a sustained contraction have been interpreted as corresponding to the transient recruitment of motor units, but this association has never been confirmed. The current study compared the timing of trains of action potentials discharged by single motor units during a sustained contraction with the bursts of activity detected in the surface EMG signal. The 20 motor units from 6 subjects [recruitment threshold, 35.3 +/- 11.3% maximal voluntary contraction (MVC) force] that were detected with fine wire electrodes discharged 2-9 trains of action potentials (7.2 +/- 5.6 s in duration) when recruited during a contraction that was sustained at a force below its recruitment threshold (target force, 25.4 +/- 10.6% MVC force). High-pass filtering the bipolar surface EMG signal improved its correlation with the single motor unit signal. An algorithm applied to the surface EMG was able to detect 75% of the trains of motor unit action potentials. The results indicate that bursts of activity in the surface EMG during a constant-force contraction correspond to the transient recruitment of higher-threshold motor units in healthy individuals, and these results could assist in the diagnosis and design of treatment in individuals who demonstrate deficits in motor unit activation.

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

“…These results are consistent with indirect evidence supporting a tendency toward repetitive motor unit discharge in older adults as suggested by the decreased rate of bursts of activity in the surface EMG recordings (Riley et al 2008b) when the elbow flexor muscles supported an inertial load and pulled up against a rigid restraint during fatiguing contractions that were sustained at 20% MVC force (Hunter et al 2005). In contrast to the present findings, young adults varied the CV for ISI but not mean discharge rate across the two target forces with the rigid restraint, but again the old adults modulated neither discharge property across the two target forces (Table 2).…”

Relation between motor unit / muscle activity and fine motor performance

“…Potvin and Brown 44) speculated that this pre-processing would improve the signal quality because the low-frequency component of the original surface EMG signals contains the distorted components due to unwanted low-pass filtering when passing from muscle fibers to electrodes, resulting in signal components unrelated to motor unit action potentials. In line with this notion, Riley and colleagues 45) indicated that high-pass filtering of interference EMG resulted in stronger associations between surface EMG and trains of motor unit action potentials as compared with those obtained from …”

Increase in joint stability at the expense of energy efficiency correlates with force variability during a fatiguing task

“…Compared to traditional filtering (e.g., 10-20 Hz highpass cutoff), whitening and extreme highpass filtering improve muscle sEMG-to-force estimates during rested (Potvin and Brown, 2004;Staudenmann et al, 2007;Riley et al, 2008) and fatiguing contractions (Cort et al, 2006;Potvin et al, 2010;La delfa et al, 2014). Importantly, Cort et al (2006) showed that the ratio between whitened sEMG amplitude and elbow moment was relatively constant during fatiguing isometric elbow flexor contractions.…”