Estimates of persistent inward currents in lower limb muscles are not different between inactive, resistance-trained and endurance-trained young individuals

ABSTRACT Persistent inward currents (PICs) increase the intrinsic excitability of α-motoneurons. The main objective of this study was to determine whether estimates of α-motoneuronal PIC magnitude is influenced by chronic endurance and resistance training. We also aimed to investigate whether there is a relationship in the estimates of α-motoneuronal PIC magnitude between muscles. Estimates of PIC magnitude were obtained in three groups of young individuals: resistance-trained (n=12), endurance-trained (n=12), and inactive (n=13). We recorded high-density surface electromyography (HDsEMG) signals from tibialis anterior, gastrocnemius medialis, soleus, vastus medialis, and vastus lateralis. Then, signals were decomposed with convolutive blind source separation to identify motor units spike trains. Participants performed triangular isometric contractions to a peak of 20% of their maximum voluntary contraction. A paired-motor-unit analysis was used to calculate ΔF, which is assumed to be proportional to PIC magnitude. Despite the substantial differences in physical training experience between groups, we found no differences in ΔF, regardless of the muscle. Significant correlations of estimates of PICs magnitude were found between muscles of the same group (VL-VM, SOL-GM). Only one correlation (out of 8) between muscles of different groups was found (GM and TA). Overall, our findings suggest that estimates of PIC magnitude in the lower limb muscles are not influenced by physical training experience in healthy young individuals. They also suggest muscle-specific and muscle group-specific regulations of the diffuse monoamine inputs.