Skeletal muscle is a vital organ for many aspects of health and the age-related loss of mass and function, termed sarcopenia, contributes to not only decreased functional capacity but also increased mortality (1). Neuromuscular electrical stimulation (NMES) has been shown to attenuate sarcopenia (2), and has also proven useful in situations of critical illness to recover muscle function (3). However NMES methodologies are inconsistent and exertional effects are not always apparent (3). The m-wave, a collation of motor unit potentials recorded from an activated muscle, has been studied previously using both muscle and nerve NMES (4) but with little data available in relation to fatigue. Our aim was to investigate electrophysiological factors associated with vastus lateralis (VL) muscle fatigue induced by NMES, applied via the femoral nerve or directly to the muscle. Sixteen young, healthy individuals (27(5) years, 8 male, BMI 23.3(3.8) kg/m2) participated in two separate modalities of NMES; muscle stimulation (mStim), applied over the quadriceps and nerve stimulation (nStim), applied over the femoral nerve. Maximal voluntary contraction (MVC) was measured before and after each test. Stimulation intensity was determined by a single 30 Hz pulse eliciting 30% MVC, and each protocol consisted of sixty 30 Hz pulses, 1-second on/1-second off, over 2 minutes. Involuntary force was recorded throughout using a force transducer. Surface EMG was recorded throughout from the medial motor point of the VL. Repeated-measures ANOVA with Šidak’s multiple comparisons were used for analysis unless otherwise stated. Significance was accepted as p<0.05. Where relevant data are displayed as mean (SD). Stimulation intensity required to reach 30% MVC was greater for mStim versus nStim (132(55) mA vs 90(25) mA, p<0.001). Voluntary force decreased following both modalities by a similar extent (-12(9) % and -10(8) % respectively, p<0.001). Involuntary force (mean of first and final three stimulated contractions) also decreased following both modalities to a similar extent (-45(12) % and -27(27) % respectively, p<0.001). Relaxation delay was measured from the peak of the final m-wave (representing the final muscle excitation) to the onset of force decline, and was seen to progressively increase throughout mStim (4.3(1.1) mS to 8.3(2.5) mS; p<0.001), but did not differ throughout nStim (4.6(1.3) mS to 5.5(1.8) mS, p=0.75). Similarly, no other electrophysiological characteristics of the m-wave (area, amplitude, stimulus conduction time, m-wave duration) changed throughout nStim. With mStim, m-wave duration increased progressively (0.9(1.7) mS to 1.1(2.2) mS; p<0.001). These data demonstrate that the m-wave characteristics of area and amplitude are not affected by fatigue in the VL of healthy young people, regardless of stimulation modality or fatigue type (voluntary or involuntary). M-wave duration was not altered in response to fatigue via nStim, however it did progressively increase with mStim. This, combined with the differences observed in relaxation delay, suggests mStim acts only to stimulate and fatigue a localized area of muscle located superficially, potentially limiting its beneficial effects on muscle hypertrophy and function. As such, nStim may be preferable to mStim for the purposes of rehabilitation.