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 recovery k of m. triceps brachii was non-invasively estimated by NIRS through repeated intermittent occlusions under two conditions: well-oxygenated (kHIGH) and low O2 availability (kLOW). The difference between kHIGH and kLOW (delta-k) 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). kHIGH 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). kHIGH correlated with FINA points, 100 m performance and training volume. delta-k was not different among tiers and was not associated with training volume or performance. M. triceps brachii oxidative capacity (kHIGH) was positively associated with performance and training volume in swimmers. delta-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. Highlights What is the central question of this study? Do highly trained swimmers have greater muscle oxidative capacity but higher sensitivity to reductions in O2 availability, that is, are relatively more `O2 diffusion limited`, than recreational swimmers? What is the main finding and its importance? We found that m. triceps brachii oxidative capacity varied significantly among swimmers of different competitive levels, and that it was associated with both training volume and performance. However, relative resistance to O2 diffusion was not different among swimmers of all levels, suggesting that training-induced adaptations to support muscle oxidative capacity were matched well to adaptations to support muscle O2 diffusion.
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