Recurrence quantification analysis of surface EMG detects changes in motor unit synchronization induced by recurrent inhibition

The systemic injection of L-Acetylcarnitine (L-Ac) induces a reversible increase in recurrent inhibition. In addition, L-Ac potentiation of recurrent inhibition has been found to increase the synchronous activity of single motor units, as detected by traditional linear analysis in the time domain. This result has been recently confirmed using a nonlinear method based on the analysis of embedded determinism (%DET) extracted from the surface EMG. The present study aimed at testing the general applicability of RQA methodology, as a viable tool for assessing motor unit synchronization, by extending the analysis of surface EMG, as revealed by changes in %DET induced by L-Ac, to many upper and lower limb muscles and to muscles that are not easily studied by needle electrodes, such as the orbicularis oculi. Subjects performed brief periods of tonic contractions, alternated to periods of rests to avoid muscle fatigue. Pharmacological enhancement of recurrent inhibition was obtained by a short-lasting intravenous injection of L-Ac. Control experiments were performed replacing L-Ac injection with saline injection. The average %DET showed a significant increase during L-Ac injection in the deltoid, biceps brachii, extensor carpi radialis, while no effect was observed in the opponens pollicis and abductor digiti minimi for the upper limb muscles. Similarly, the average %DET showed a significant increase during L-Ac injection in the quadriceps, soleus, and tibialis anterior, while no effect was observed in the abductor hallucis for the lower limb muscles. RQA of orbicularis oculi muscle activity showed no increase in %DET during L-Ac injection in analogy to what found in the intrinsic muscles of the hand and foot, known to be devoid of recurrent inhibition. The presence or absence of drug-induced increase in motor unit synchronization agrees with the known distribution of recurrent inhibition in the various motor nuclei. The overall significance of these findings is the potential application of RQA methodology as a reliable and independent tool for generally assessing motor unit synchronization from surface EMG under strictly controlled experimental condition.