The effectiveness of the OMNI-RES (0-10) and the electromyographic signal for monitoring changes in the movement velocity was examined during a set to muscular failure using different percentages of one repetition maximum (1RM) in the parallel squat exercise (PSQ). Twelve males (26.3 ± 5.8 years) were evaluated on eight separate days with 48 hours of rest between sessions. After determining the 1RM value, participants underwent seven tests until achieving muscular failure with the following percentage ranges: 30 to <40%; 40 to <50%, 50 to <60%, 60% to <70%, 70 to <80%, 80 to <90% and >90%. An optical rotary encoder measured mean accelerative velocity (MAV) and the OMNI-RES (0-10) scale was used to express the Rating of Perceived Exertion (RPE) after every repetition of each set. Additionally, the normalized root mean square (RMS) signal of the surface electromyography (N-EMG) was calculated for the vastus medialis muscle. The RPE expressed after the first repetition and when the maximum value of MAV was achieved along the sets was lower (p <0.001, d >0.8) than the RPE that corresponded to a 10% drop in MAV and at failure. Additionally, the initial RPE was useful to distinguish different loading zones by anchoring the OMNI-RES value to the magnitude of the relative load (<60%, 60 to <70% or ≤ 70% 1RM). Similar patterns were observed using the N-EMG. In conclusion, apart from differentiating between relative loads during a set to failure in the PSQ, the RPE and the N-EMG can both reflect changes associated with the initial, maximal, 10% drop in movement velocity and the muscular failure.