Diving into the unknown: Evidence of enhanced skeletal muscle lactate efflux potential and blood-muscle exchange in competitive breath-hold divers.

The 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 ( V ̇ O 2 max ${\dot{\mathrm{V}}}\rm{O}_{\rm{2max}}$ ). 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. KEY POINTS: The 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. This 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. Breath-hold divers exhibited greater skeletal muscle capillarization, indicating an enhanced capacity for blood-muscle exchange. They also showed an increased potential to remove lactate from skeletal muscle tissue. These adaptations likely support sustained glycolytic function and redox balance during prolonged breath-holds, while facilitating more efficient clearance of metabolic byproducts during recovery.