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Background and study aimCombat sports involve repeated high-intensity efforts separated by short recovery periods, creating substantial demands on aerobic function and recovery. However, little is known about whether resting metabolic and redox-related biomarkers are associated with cardiorespiratory fitness in combat sport athletes. This pilot cross-sectional study examined the associations of resting plasma adenosine triphosphate (ATP), total antioxidant capacity (T-AOC), superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione peroxidase (GPX) with maximal oxygen uptake (VO2max) and relative VO2max.Materials and methodsResting venous blood samples were collected once after at least 8 h of fasting and before a graded treadmill exercise test; no post-exercise samples were obtained. Cardiorespiratory fitness was evaluated using a graded treadmill exercise test, with VO2max, relative VO2max, respiratory exchange ratio, exercise heart rate, and exercise duration recorded. Statistical analyses included descriptive statistics, Spearman and partial correlations, multiple linear regression, and decision tree modeling, with additional robustness checks using Bootstrap confidence intervals and permutation tests.ResultsTwenty-one combat sport athletes were included. Mean VO2max was 2937.19 ± 643.93 mL/min, and mean relative VO2max was 42.38 ± 8.80 mL/kg/min.s. T-AOC was strongly positively correlated with both VO2max (Spearman’s ρ = 0.723, p < 0.01) and relative VO2max (Spearman’s ρ = 0.678, p < 0.01), and these associations remained significant in partial correlation analyses adjusted for age, sex, height, and body mass. MDA was moderately correlated with relative VO2max (ρ = 0.454, p < 0.05). In multivariable models, T-AOC was the only biomarker significantly associated with VO2max and, together with MDA, was associated with relative VO2max. Partial correlation, decision tree, Bootstrap, and permutation analyses consistently supported T-AOC as the most stable signal across analytical approaches, suggesting that its association with cardiorespiratory fitness was not model-specific. By contrast, ATP, SOD, and GPX showed limited independent explanatory value.ConclusionsCombat sport athletes in this pilot sample demonstrated high cardiorespiratory fitness. In this pilot sample, resting T-AOC showed the most consistent positive association with VO2max and relative VO2max, while MDA showed an exploratory association with relative VO2max. Because biomarkers were measured once before exercise, these results represent baseline cross-sectional associations and do not establish causality, acute exercise responses, or recovery status. Larger longitudinal studies are needed to confirm these findings.