Reliability and validity of the respiratory threshold method.

PURPOSE: To evaluate the test-retest reliability, sensitivity to detect meaningful change, and validity of a respiratory threshold (RT) method based on time-dependent breakpoints in the ratio between minute ventilation and end-tidal partial pressure of carbon dioxide (V̇E/PETCO2) during ramp exercise.

METHODS

Healthy male participants were recruited for a reliability (n = 42) and validity study (n = 143). Participants performed ramp cycling tests (30 W·min-1) on separate days. For validity analyses, participants were classified as untrained, physically active, or trained according to peak oxygen uptake (V̇O2peak) values. The gas exchange threshold (GET) and respiratory compensation point (RCP) were identified using segmented regression in the carbon dioxide output-oxygen uptake (V̇CO2 - V̇O2) and V̇E - V̇CO2 relationships, respectively. Respiratory thresholds (RT1 and RT2) were determined from time-dependent breakpoints in V̇E/PETCO2.

RESULTS

RT1 and RT2 demonstrated excellent test-retest reliability for both V̇O2 and power output (intraclass correlation coefficient = 0.98 - 0.99; coefficient of variation ≤ 2.6%). Typical error was lower than the smallest worthwhile change for all outcomes, indicating good sensitivity. RT1 and RT2 closely aligned with the GET and RCP, respectively, showing trivial mean bias, nearly perfect correlations (r = 0.97 - 0.99), narrow limits of agreement, and a low standard error of the estimate (≤ 2%). RT1 and RT2 clearly differentiated untrained, physically active, and trained participants, with large to very large between-group effects (p < 0.001).

CONCLUSION

The RT method provides a reproducible, sensitive, and physiologically grounded approach for identifying exercise thresholds and may serve as a complementary tool for threshold detection and performance monitoring.