Evidence of break-points in breathing pattern at the gas-exchange thresholds during incremental cycling in young, healthy subjects

Abstract

The present study investigated whether ‘break-points’ in breathing pattern correspond to the first ($G_{{{\text{EX}}_{1} }}$) and second gas-exchange thresholds ($G_{{{\text{EX}}_{ 2} }}$) during incremental cycling. We used polynomial spline smoothing to detect accelerations and decelerations in pulmonary gas-exchange data, which provided an objective means of ‘break-point’ detection without assumption of the number and shape of said ‘break-points’. Twenty-eight recreational cyclists completed the study, with five individuals excluded from analyses due to low signal-to-noise ratios and/or high risk of ‘pseudo-threshold’ detection. In the remaining participants (n = 23), two separate and distinct accelerations in respiratory frequency (f _R) during incremental work were observed, both of which demonstrated trivial biases and reasonably small ±95% limits of agreement (LOA) for the $G_{{{\text{EX}}_{1} }}$ (0.2 ± 3.0 ml O₂ kg⁻¹ min⁻¹) and $G_{{{\text{EX}}_{ 2} }}$ (0.0 ± 2.4 ml O₂ kg⁻¹ min⁻¹), respectively. A plateau in tidal volume (V _T) data near the $G_{{{\text{EX}}_{1} }}$ was identified in only 14 individuals, and yielded the most unsatisfactory mean bias ±LOA of all comparisons made (−0.4 ± 5.3 ml O₂ kg⁻¹ min⁻¹). Conversely, 18 individuals displayed V _T-plateau in close proximity to the $G_{{{\text{EX}}_{ 2} }}$ evidenced by a mean bias ± LOA of 0.1 ± 3.1 ml O₂ kg⁻¹ min⁻¹. Our findings suggest that both accelerations in f _R correspond to the gas-exchange thresholds, and a plateau (or decline) in V _T at the $G_{{{\text{EX}}_{ 2} }}$ is a common (but not universal) feature of the breathing pattern response to incremental cycling.