Resistance training plays a key role in enhancing muscular performance; however, the effects of different combinations of loading intensity and velocity loss (VL) thresholds on muscle oxygen saturation (SmO²) dynamics during exercise remain insufficiently understood. This study aimed to investigate the influence of intensity (60% vs. 80% one-repetition maximum [1RM]) and VL (20% vs. 40%) on SmO² responses during the back squat exercise. Eighteen resistance-trained males (age: 20.06 1.63 years; height: 176.78 6.45 cm; body mass: 70.26 9.56 kg) performed four back squat protocols - 60%1RM-VL20% (60 - 20), 60%1RM-VL40% (60 - 40), 80%1RM-VL20% (80 - 20), and 80%1RM-VL40% (80 - 40) - using a repeated-measures, counterbalanced design. Each protocol comprised three sets with 5- minute inter-set rest periods. SmO² of the vastus lateralis was continuously monitored to determine changes in its magnitude and slope during exercise and recovery phases. Results revealed no significant differences were observed in the magnitude of SmO² decline across conditions, with values ranging from 47.28% to 57.67% across all sets (p > .05). The SmO² decline slope was significantly steeper (more negative) in the 80-20 condition (-1.71 to -2.04 %?s?¹) compared to both 60-20 (-0.80 to -1.13 %?s?¹) and 60-40 (-0.53 to -1.00 %?s?¹) across all sets (p < .001). No significant differences were observed in SmO² recovery slope during rest (range: 0.36 - 0.61 %?s?¹; p > .05). The present study demonstrated that combining 60% 1RM with a 40% VL% threshold elicited the slowest SmO² decline rate, potentially delaying fatigue onset and allowing greater repetition volume. Although both training intensity and velocity loss thresholds influenced muscle oxygenation dynamics, the rate of SmO² desaturation was particularly sensitive to changes in VL% thresholds under the 80% 1RM. These findings underscore the importance of integrating training intensity, VL% magnitude, and oxygenation dynamics when designing individualized resistance training protocols.
