PAS-INDUCED CHANGES IN EMG ACTIVITY OF THE LOWER LEG MUSCLES DURING VOLUNTARY EFFORT

Abstract

Aim: this study aims to identify changes in EMG activity of the lower leg muscles during maximal plantar flexion under various modes of paired associative stimulation (PAS) targeting the motor cortex and the afferent fibers of the dorsal roots of the spinal cord at the L2–L3 level. Materials and me­thods. The study involved sixteen apparently healthy sprinters and middle-distance runners. Two PAS application protocols were used: one with a 10 ms advance of cortical stimulus arrival to spinal motoneurons (PAS₁₀) and one with a 20 ms advance of spinal stimulus arrival (PAS_–20). Results. A PAS₁₀ session significantly increased EMG amplitude and turn frequency of the working muscles immediately after stimulation and for up to 10 minutes post-stimulation. In the GM muscle, activity increased by 42% during the tension phase and by 39% during the relaxation phase immediately after stimulation. By the 10^th minute, the increases reached 30% and 34% compared to baseline, respectively. A PAS₂₀ session reduced EMG amplitude and turn frequency during maximal voluntary plantar flexion. Mean EMG turn frequency decreased by 5–20% immediately after stimulation, with recovery to baseline values within 10 minutes. Conclusion. The direction of EMG changes in the lower leg muscles during maximal plantar flexion is determined by the order of arrival and the interval between the arrival of potentials from cortical and spinal stimuli to target motoneurons during PAS. The observed changes in EMG parameters during maximal voluntary effort are likely explained by the modification of corticospinal synaptic connections at the level of spinal motoneurons induced by different PAS application modes and, consequently, by the reorganization of motor unit recruitment in the working muscles.