Metabolic, Cardiovascular, and Stress Biomarker Adaptations to Breath-Hold Training in a National-Level Swimmer: A Signal-Generating Single-Case Study

Background: Breath-hold training (BHT) has emerged as a novel strategy to enhance metabolic efficiency and autonomic resilience in national-level athletes. This signal-generating single-case study examined physiological and neuroendocrine adaptations to an eight-week BHT program in a nationally ranked competitive swimmer. Methods: A national-level 23-year-old female freestyle sprinter (50 m best time = 26.59 s; 100 m = 60.40 s) completed three weekly BHT sessions integrated into her regular training. Pre- and post-intervention assessments included an incremental Mader cycling test with measurements of blood lactate ($\left[{\mathrm{La}}^{-}\right]$), heart rate (HR), salivary cortisol (sCort), and salivary alpha-amylase (sAA). Blood chemistry and pulmonary function, including diffusing capacity of the lung for carbon monoxide (DLCO), were also evaluated. Results: Post intervention, the athlete demonstrated reduced $\left[{\mathrm{La}}^{-}\right]$ and HR at all workloads, a 20 W increase in power at 4 mmol·L⁻¹ $\left[{\mathrm{La}}^{-}\right]$, and an elevated final workload achieved during the Mader test. Salivary stress biomarkers showed blunted responses with significant reductions in area under the curve and large effect sizes. These changes were observed under standardized pre-analytical conditions and individualized training adjustments. Conclusions: This study highlights coordinated improvements in metabolic, cardiovascular, and stress regulation mechanisms following BHT in a swimmer with verified national-level performance benchmarks. BHT, when applied in sport-specific contexts, may serve as an effective adjunct to high-performance training.