Cardiorespiratory coupling tightens with workload during graded exercise: A pilot study with adolescent athletes

Cardiorespiratory coupling (CRC) reflects coordination between heart and lung function, but how it changes with increasing intensity during graded exercise remains unclear. We investigated CRC as real-time covariation between cardiac timing and breath-by-breath pulmonary oxygen uptake (VO₂) to test whether coupling strengthens with workload in adolescent athletes. We conducted an observational, within-subject analysis of the ACTES cycling dataset. Eighteen adolescents cycled at 50, 110, and 140 W. Beat-to-beat RR intervals and pulmonary VO₂ time series were examined in ultra-short 60 s segments. Fluctuations were summarized using SDRR/RMSSD and SDVO₂/RMSSDVO₂. CRC was measured using joint symbolic dynamics (JSD; Shannon entropy, SE, and Miller-Madow-corrected entropy, CSE) and the highest normalized cross-correlation (X-Corr). Mean RR decreased and mean pulmonary VO₂ increased with workload (both p < 0.0001). SDRR and RMSSD were lower at 110 and 140 W versus 50 W; SDVO₂ declined from 50 to 110/140 W, whereas RMSSDVO₂ was unchanged. X-Corr increased from 50 to 110/140 W (p ≤ 0.0014). JSD indices decreased as workload increased (SE: global p = 0.139; CSE: global p = 0.029), suggesting tighter CRC. CRC becomes more pronounced with increased workload, aligning with reduced heart rate variability and reflecting vagal withdrawal and reflex responses that improve heart-lung integration in adolescent athletes.