Central Contribution to Electrically Induced Fatigue depends on Stimulation Frequency

This study analyzed the impact of several protocols of neuromuscular electrical stimulation (NMES), matched with a similar total torque-time integral, on muscle activation pathways and neuromuscular fatigue.Ten young healthy participants (age, 24.6 ± 4.2 yr) performed three randomized NMES sessions on the triceps surae muscles with 20-, 60-, or 100-Hz stimulation frequencies (pulse duration, 1 ms), with pulse amplitude (IES) set at 20% of isometric maximal voluntary contraction (MVC). Muscle activity during NMES was assessed by means of the twitch, the soleus H-reflex and M wave responses evoked by single muscle stimulation at IES. Neuromuscular fatigue was assessed as the changes in evoked and MVC torques and the underlying mechanisms by analyzing variations in superimposed maximal M-waves (Msup), normalized H-reflexes (Hsup/Msup) and V-waves (V/Msup) of the triceps surae muscles.Electromyographic responses at IES suggested that the relative contribution of the indirect muscle activation increases as the stimulation frequency was high and the pulse amplitude was low (P = 0.03). The decrease in MVC torque after NMES was significantly (P = 0.003) greater after 100-Hz protocol (20 Hz, -9.6% ± 3.3%; 60 Hz, -10.7% ± 3.2%; 100 Hz, -16.3% ± 2.7%). Hsup/Msup decreased significantly (P < 0.01) by 31% ± 4% after the 100-Hz protocol only and V/Msup decreased significantly (P <0.05) after both 60- and 100-Hz protocols.The combination of high-stimulation frequencies and low-pulse amplitude induced the greatest neuromuscular fatigue. Low frequencies (20 Hz) induced alterations mainly at the muscle level, whereas higher frequencies (60 to 100 Hz) rather induced modulations at both spinal and supraspinal levels.