Exercise intensity influences variability and test-retest reliability of pulmonary gas-exchange measurements during constant work-rate cycling

Introduction: The study examined how exercise intensity and participants' characteristics influence the variability and test-retest reliability of pulmonary gas-exchange while providing domain-specific reliability benchmarks for exercise testing. Methods: Forty-two healthy adults (VO2max = 49.6 +/- 8.8 ml & centerdot;kg(-1)& centerdot;min(-1)) completed a ramp incremental exercise test and six 6-min constant work-rate cycling trials distributed across moderate, heavy, and severe exercise intensities. Exercise intensities were set at 33%Delta and 66%Delta between baseline and gas exchange threshold (GET), GET and respiratory compensation point (RCP), and RCP and VO2max, respectively. Pulmonary oxygen uptake (VO2), carbon dioxide output (VCO2), and ventilation (VE) were recorded using two metabolic carts. Systematic differences between days and intensities were assessed with two-way repeated-measures ANOVA. Variability and participants' characteristics were examined with within-trial coefficient of variation (CV%), Pearson correlations, linear mixed-effects models (LMME), intraclass correlation coefficient (ICC), standard error of measurement (SEM), minimal detectable change (MDC), and Bland-Altman analysis. Results: VO2, VCO2, and VE increased with intensity (p <= 0.001), with no day-to-day differences (p > 0.05). CV% decreased progressively with intensity, from similar to 7-10% in the moderate to <5% for VO2 and VCO2 in the severe domain. ICCs were excellent (>0.95) in the heavy and severe domains. Correlations showed moderate negative associations between CV% and aerobic fitness (r -0.34 and -0.58, p < 0.05), while LMME identified exercise intensity as the main factor associated with variability. MDC for VO2 ranged from 166 to 415 ml & centerdot;min(-1), and Bland-Altman plots confirmed minimal bias. Discussion: exercise intensity influences the variability and reliability of breath-by-breath gas-exchange responses, with relative variability decreasing at higher metabolic loads. Fitness-related differences in CV% possibly reflect physiological scaling effects. These results provide practical reliability thresholds and improve the interpretation of repeated-measures testing in exercise physiology.