Background: This study aimed to compare different smoothing window lengths used to compute the RMS and their impact on RMS slope during leg press exercise and to compare the RMS slope behavior with lactate threshold. Methods: Twelve subjects performed an incremental test on a leg press machine where blood lactate concentration was measured at each stage. The RMS of vastus medialis (VM), vastus lateralis (VL) and rectus femuralis (RF) were computed for 200, 1500 and 3000ms windows length periods, and the RMS linear slope was used to interpret the results in the amplitude domain. The EMG fatigue threshold (EMGth) was determined for the quadriceps muscle during the three smoothing window lengths. Results: There was no significant difference (p> 0.05) in RMS slope between the three different RMS window length analyses for VM and VL muscles. The RMS slope was significantly higher (p ≤ 0.05) for the window length period of 1500 and 3000ms compared to 200ms in some intensities of exercise. The ICCs between the RMS slopes were 0.94 for RF and 0.95 for VL and VM. There was no significant difference (p> 0.05) between the EMGth at different window length periods and the lactate threshold (28.0 ± 3.7% of 1RM). Conclusion: Different smoothing window lengths to computed RMS could be used during resistance exercise without differences in RMS slope. The smoothing window lengths don´t influence EMGth intensity and are related to the lactate threshold.