["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Competitive breath‐hold divers exhibit skeletal muscle characteristics indicative of enhanced blood‐muscle exchange capacity and increased lactate efflux potential. Top left: Participant characteristics, matched for age, height, body mass, and maximal oxygen uptake (V̇O2max${\\dot{\\mathrm{V}}}\\rm{O}_{\\rm{2max}}$). Bottom left: Muscle biopsy methodology using a standard Bergstrom needle with suction to sample the m. vastus laterallis. Right panel: Analytical approaches applied to muscle samples and the key findings of the study.\n\n\n\n\n\n\n\n\n\nAbstract\nThe characterization of skeletal muscle phenotypes in diving populations remains one of the least explored domains of breath‐hold physiology, representing a critical gap in our understanding of how skeletal muscle adapts to the unique demands of breath‐hold diving. Accordingly the present study investigated specific markers of skeletal muscle structure and metabolism in competitive breath‐hold divers. Twenty males volunteered to participate in this study (10 competitive breath‐hold divers; 10 non‐divers), matched for age, body size and whole‐body aerobic capacity (V̇O2max${\\dot{\\mathrm{V}}}\\rm{O}_{\\rm{2max}}$). A percutaneous skeletal muscle biopsy was obtained from the m. vastus lateralis to quantify capillarization, fibre‐type distribution (i.e. types I, IIa and II other), protein content of mitochondrial complexes, monocarboxylate transporter (MCT) isoforms and citrate synthase activity. MCT4 content was 28% higher in breath‐hold divers compared to non‐divers (P = 0.020), whereas MCT1 and citrate synthase activity showed no between‐group differences (P ≥ 0.161). Complex V content was higher in the non‐divers (P = 0.049), whereas no between‐group differences were noted for complexes I, II, III and IV (P ≥ 0.253). Capillarization was significantly higher in breath‐hold divers (P ≤ 0.048), whereas fibre‐type distribution did not differ between groups (P = 0.999). Competitive breath‐hold divers exhibited skeletal muscle characteristics indicative of enhanced blood–muscle exchange capacity and augmented lactate efflux potential. Such adaptations may confer an advantage during prolonged breath‐holds by preserving glycolytic function and maintaining redox homeostasis. In recovery these traits likely facilitate more efficient clearance of metabolic byproducts.\n\n\n\n\n\n\n\n\n\n\nKey points\n\nThe skeletal muscle phenotype of breath‐hold divers remains poorly characterized, limiting understanding of how skeletal muscle adapts to the physiological demands of prolonged breath‐holding.\nThis study compared skeletal muscle structure and metabolic markers between competitive breath‐hold divers and non‐diving controls matched for age, body size and whole‐body aerobic capacity.\nBreath‐hold divers exhibited greater skeletal muscle capillarization, indicating an enhanced capacity for blood–muscle exchange.\nThey also showed an increased potential to remove lactate from skeletal muscle tissue.\nThese adaptations likely support sustained glycolytic function and redox balance during prolonged breath‐holds, while facilitating more efficient clearance of metabolic byproducts during recovery.\n\n\n\n"]