Purpose: This study was designed to investigate whether the intensity modulation of a neuromuscular electrical stimulation (NMES) protocol delivered over the nerve trunk of the plantar flexors would lead to differential peripheral and central contributions of muscle fatigue.
Methods: Three fatiguing isometric protocols of the plantar flexors matched for the same amount of isometric torque-time integral (TTI) were randomly performed including a volitional protocol at 20 % of the maximal voluntary contraction (MVC) and two NMES protocols (one at constant intensity, CST; the other at intensity level progressively adjusted to maintain 20 % of MVC, PROG).
Results: No time x protocol interaction was found for any of the variables. The MVC decreased similarly (≈12 %, p < 0.001) after all protocols, so did the potentiated twitch responses (p = 0.001). Although voluntary activation of the plantar flexors did not change, maximal H-reflex to M-wave ratio of the soleus (SOL) and the gastrocnemius medialis (GM) muscles showed an overall increase (SOL: p = 0.037, GM: p = 0.041), while it remained stable for the gastrocnemius lateralis muscle (p = 0.221). A main time effect was observed only for the SOL maximal V-wave to the superimposed M-wave ratio (p = 0.024) and to the superimposed H-reflex (p = 0.008). While similar central and peripheral adaptations were observed after the three fatiguing protocols, the individual contribution of the three different triceps surae muscles was different.
Conclusion: Whether the current intensity was increased or not, the adaptations after a NMES protocol yield to similar muscle fatigue adaptations as voluntary contractions likely through similar pathways matching a similar TTI.