Decreased excitability of motor axons contributes substantially to contraction fatigability during neuromuscular electrical stimulation

The present study was designed to: 1) determine the time course of changes in motor axon excitability during and after neuromuscular electrical stimulation (NMES), and 2) characterise the relationship between contraction fatigability, NMES frequency, and changes at the axon, neuromuscular junction, and muscle. Eight neurologically-intact participants attended three sessions. NMES was delivered over the common peroneal nerve at 20, 40, or 60 Hz for 8 min (0.3 s “on”, 0.7 s “off”). Threshold tracking was used to measure changes in axonal excitability. Supramaximal stimuli were used to assess neuromuscular transmission and force-generating capacity of the tibialis anterior muscle. Torque decreased 49 and 62% during 8 min of 40 and 60 Hz NMES, respectively. Maximal twitch torque decreased only during 60 Hz NMES. Motor axon excitability decreased by 14, 27, and 35% during 20, 40, and 60 Hz NMES, respectively. Excitability recovered to baseline immediately (20 Hz), 2 min (40 Hz), and 4 min (60 Hz) following NMES. Overall, decreases in axonal excitability best predicted how torque declined over 8 min of NMES. During NMES, motor axons become less excitable and motor units “drop out” of the contraction, contributing substantially to contraction fatigability and its dependence on NMES frequency. NOVELTY BULLETS • The excitability of motor axons decreased during neuromuscular electrical stimulation (NMES) in a frequency-dependent manner. • As excitability decreased, axons failed to reach threshold and motor units dropped out of the contraction. • Overall, decreased excitability best predicted how torque declined and thus is a key contributor to fatigability during NMES.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author