## 4 Discussion
In the multiple regression analysis (Figure 3), increased muscle oxygen consumption was associated with elevated sympathetic activity, as evidenced by decreased R-R interval and increased SNS index and mean HR. HRV variables influenced desaturation (r\({}^{2}\) = 0.69) and resaturation (r\({}^{2}\) = 0.60). While no studies have combined VOT-NIRS with HRV, it is known that localized muscle fatigue after specific exercises increases blood flow oscillations and muscle oxygen consumption [55]. [PERSON] et al. (2021) measured SmO\({}_{2}\) and tHb using spectral analysis, showing that muscle oxygen increases depending on the duration and intensity of previous exercise [56]. Sustained sympathetic nervous system activation in response to muscle metabolic demands can elevate blood pressure, heart rate, andsympathetic vasoconstrstrictor activity [57], especially when muscle blood flow demand exceeds resting levels [58]. Increased metabolic demand and the need to maintain adequate oxygen and nutrient supply to muscles activate the sympathetic nervous system, leading to increased blood flow and decreased R-R interval, supporting recovery, effective circulation, and metabolite clearance post-exercise [59]. VOT-NIRS data is considered valid after ischemia with desaturation and resaturation values between \(\geq\)30% and \(\leq\)70% SmO\({}_{2}\), similar to other studies showing 40-58% in desaturation and resaturation slopes [8]. MRT responses between \(\leq\)10 s and \(\geq\)60 s are also consistent with [PERSON]'s findings [16], which reported values from 20 s (fastest) to 50 s (slowest), suggesting comparable time constant measurements despite methodological differences in VOT-NIRS. Increased resting muscle oxygen consumption was associated with VAS pain (Figure 4) during desaturation (r = 0.81), resaturation (r = 0.69), and MRT (r = \(-\)0.58). This aligns with [PERSON] et al. (2020), who found a higher VAS pain correlating with increased resting SmO\({}_{2}\) (r = 0.65), with similar VAS pain values under acute fatigue conditions (6.1 and 6.8, respectively). While higher resting SmO\({}_{2}\) is debated, physiologically it might prevent increased muscle activation during exercise due to reduced vasoconstriction capacity, as more blood flow suggests enhanced metabolite clearance, which is known to cause pain [60]. Additionally, rapid oxygen consumption in acute measures could indicate pathology and potential lower limb injuries [14], all related to perceived muscle pain. Regarding the reliability data found in this study between different days and without fatigue, moderate ICC was found for resaturation, hyperemia, and MRT. These results contrast with previous studies that reported baseline SmO\({}_{2}\) measurements with ICCs between 0.90 and 0.99 [13; 14; 19] and SmO\({}_{2}\) slopes ICCs from 0.77 to 0.97 [13; 14; 61]. However, these measures were taken in the gastrocnemius muscle, which has less error than the vastus lateralis. The vastus lateralis showed ICCs ranging from 0.35 to 0.91 [62]. NIRS-derived SmO\({}_{2}\) slope of resaturation has good reliability over various occlusion durations, with the highest reliability during longer occlusions of 3 to 5 min compared to 30 s and 1 min [63]. Additionally, a 3.4% SmO\({}_{2}\) error with an MDC of 9% SmO\({}_{2}\) was found, similar to our study with an MDC > 7% SmO\({}_{2}\)[62]. Although no studies have evaluated reliability under acute fatigue conditions, our results align with studies using VOT-NIRS, where desaturation and resaturation data range between 30% and 70% and an MDC > 11% of SmO\({}_{2}\). Hyperemia had a high CV of 40%, possibly contributing to the high variability and errors exceeding the reported mean values of TE = 6% SmO\({}_{2}\). HRV data showed higher ICCs of 0.91 to 0.99 and low CV variations of 1.8% to 2.9% compared to all SmO\({}_{2}\) variables. Overall, fatigue reduced data reliability (see Table 3), probably due to increased muscle oxygen consumption, supporting this study's hypothesis.