This study aimed to evaluate in vivo oxidative capacity and relative resistance to O₂ diffusion using near-infrared spectroscopy (NIRS) in the m. triceps brachii of recreational to world class swimmers and evaluate their relationships with swimming performance. Twenty-eight swimmers were enrolled and assigned into three subgroups according to their level: 'recreational/trained' (Tier 1/2; n = 8), 'national' (Tier 3; n = 12) and 'international/world class' (Tier 4/5; n = 8). Performance was evaluated by 100 m freestyle trials. Training volume was measured by self-reported distance (km/week). The mV̇O2${\mathrm{m}}{{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ recovery k of m. triceps brachii was non-invasively estimated by NIRS through repeated intermittent occlusions under two conditions: well-oxygenated (k_HIGH) and low O₂ availability (k_LOW). The difference between k_HIGH and k_LOW (Δk) was calculated as an index of relative resistance to O₂ diffusion. FINA points and 100 m performance differed among all groups. Training volume was greater in Tier 4/5 (34.0 ± 5.5 km week^-1) and Tier 3 (35.5 ± 11.6 km week^-1) than in Tier 1/2 (6.4 ± 1.8 km week^-1). k_HIGH was greater in Tier 4/5 and Tier 3 (3.18 ± 0.41 and 2.79 ± 0.40 min^-1) versus Tier 1/2 (2.10 ± 0.36 min^-1; all P < 0.002). k_HIGH correlated with FINA points, 100 m performance and training volume. ∆k was not different among tiers and was not associated with training volume or performance. M. triceps brachii oxidative capacity (k_HIGH) was positively associated with performance and training volume in swimmers. ∆k, which reflects relative resistance to O₂ diffusion, was not different among athletes. These data suggest that m. triceps brachii oxidative capacity is associated with swimming performance and that muscle O₂ diffusing capacity exerts a similar relative resistance to O₂ diffusive flow across swimmers.