Reproducibility of indirect persistent inward current indicators with neuromuscular electrical stimulation
Authors: Popesco Timothée , D'Amato Brunetta Davide , Pata Stefano , Place Nicolas 
Published: January 2025

Introduction Neuromuscular electrical stimulation (NMES) has been validated as an efficient tool to train and evaluate the neuromuscular system. Recently, it has been used to estimate persistent inward currents (PICs) in motoneurons (Donnelly et al., 2021). Notably, PICs play an important role in motor unit behavior (Heckman & Enoka, 2012) and are thought to be involved in the centrally mediated responses to NMES. This study aimed to examine the reproducibility of indirect PIC indicators, such as extra force production during wide-pulse high-frequency (WPHF) NMES, sustained electromyographic (EMG) activity following WPHF NMES and force production in response to Achille’s tendon vibration combined with NMES.
Methods Twenty-one healthy participants (8 women, 26±4 years) underwent two 10s-trains of WPHF NMES and two 33s-Achille’s tendon vibration combined with NMES bouts on the plantar flexors in two different sessions. Extra force was calculated as the relative percent increase in force between the last and the 2nd second of the stimulation. EMG activity was recorded from the gastrocnemius medialis (GM) and soleus (SOL) muscles with high density grids to evaluate sustained activity spatial distribution. The force increase observed during vibration + NMES, calculated as the difference between the reflexive force during vibration and the initial torque after the first NMES burst was measured and referred as the warm-up effect (Mesquita et al., 2021). Intrasession and intersession reproducibility were assessed with intraclass correlation coefficients (ICC).
Results Intrasession ICCs for extra force production were excellent, reaching 0.79 and 0.77 for sessions 1 and 2, respectively. Sustained EMG activity showed high reliability, with ICCs of 0.84 and 0.81 for GM, and 0.79 and 0.77 for SOL in sessions 1 and 2. The warm-up effect demonstrated excellent reliability in session 1 (ICC = 0.87) but moderate reliability in session 2 (ICC = 0.51). Intersession reliability was high for extra force (ICC = 0.79) and the warm-up effect (ICC = 0.75). However, sustained EMG activity showed moderate reliability between sessions, with ICCs of 0.51 for GM and 0.40 for SOL.
Discussion/Conclusion This study demonstrates excellent intrasession reliability for extra force production and sustained EMG activity as indirect PIC indicators but more variable reliability for the warm-up effect within sessions. Intersession reliability was high for extra force and the warm-up effect but moderate for sustained EMG activity. These findings have important implications for using these indicators in research to better understand motor unit behavior and in clinical settings to optimize NMES training.
References
Donnelly, C., Stegmüller, J., Blazevich, A. J., Crettaz von Roten, F., Kayser, B., Neyroud, D., & Place, N. (2021). Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training. Scientific Reports, 11(1), 6399. https://doi.org/10.1038/s41598-021-85645-0
Heckman, C. J., & Enoka, R. M. (2012). Motor unit. Comprehensive Physiology, 2(4), 2629–2682. https://doi.org/10.1002/cphy.c100087
Mesquita, R. N. O., Taylor, J. L., Kirk, B., & Blazevich, A. J. (2021). Involuntary sustained firing of plantar flexor motor neurones: Effect of electrical stimulation parameters during tendon vibration. European Journal of Applied Physiology, 121(3), 881–891. https://doi.org/10.1007/s00421-020-04563-7