Few studies have assessed sex differences in neuromuscular fatigue after prolonged running. Compared to men, women exhibit less peripheral fatigue in plantar flexor muscles (PF) after running a 110-km-mountain-ultramathon [1]. The purpose of the present study was to further characterize sex differences by examining various running distances. Thirty-six runners completed the study: 18 men (36 ± 8 years, 72.5 ± 9.6 kg) and 18 women (36 ± 8 years, 59.1 ± 5.8 kg). Men and women were matched by relative performance (i.e. percent of winning time of their sex category). Participants completed various races of the Ultra-Trail du Mont-Blanc® ranging from 40 km with 2,300 m of positive elevation change to 170 km with 10,000 m of positive elevation change. One month before the race, participants visited the lab to be familiarized with electrical stimulation on both knee extensor (KE) and PF muscles. Neuromuscular function was tested before and after each race. The testing protocol consisted of a standardized warm-up followed by the assessment of maximal voluntary contractions (MVCs), maximal voluntary activation (superimposed 100 Hz doublet) and contractile properties (potentiated 100 Hz doublet, potentiated 10 Hz doublet and potentiated single twitch (Pt)). For analysis, participants were further subdivided into two groups of 18 runners by distance of race completed (SHORT < 100 km vs LONG ≥ 100 km). A factorial ANOVA with deficit (in percentage of PRE value) as a dependent factor and with sex and distance as between-subject factors was used for the analysis. MVC loss was greater after LONG than SHORT in both KE (p<0.05) and PF (p<0.01) and the decrease in Pt was significantly greater (p<0.05) in LONG compared to SHORT in KE, independent of sex. In PF, the decrease in MVC was greater (p<0.05) in men than women (-35 ± 11% vs -25 ± 13%, respectively), independent of the distance run. However, women exhibited a greater decrease in Pt compared to men in LONG (p<0.05) for PF. Neither sex nor distance effects were observed in maximal voluntary activation for either muscle group. The greater muscle strength loss in LONG compared to SHORT fits the previously reported relationship between strength loss and race duration (2). Surprisingly, the present study displayed greater peripheral fatigue in women compared to men after LONG races. However, women lost less muscle strength in PF than men, suggesting that they were globally more resistant to fatigue in plantar flexor muscles. [1] Temesi J et al. (2015). Med Sci Sports Exerc, 47: 1372-82. [2] Millet GY. (2011). Sports Med, 41(6): 489-506.