Abstract
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 (VO(2)) 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 VO(2) time series were examined in ultra-short 60 s segments. Fluctuations were summarized using SDRR/RMSSD and SDVO(2)/RMSSDVO(2). 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 VO(2) increased with workload (both p < 0.0001). SDRR and RMSSD were lower at 110 and 140 W versus 50 W; SDVO(2) declined from 50 to 110/140 W, whereas RMSSDVO(2) 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.