Comparison of expired gas and muscle oxygenation measures of aerobic-anaerobic transition in well-trained cyclists

Introduction: Previous data suggests that an individual’s ventilatory threshold (VT) measured through expired gas analysis represents a transitional point between aerobic and anaerobic metabolism during exercise. Near infrared spectroscopy (NIRS) has also been reported to detect the VT within the muscle, perhaps providing a more direct representation of energy metabolism. This study aimed to confirm whether VT was similarly detected by expired gas and NIRS measures, given that differences in the estimation of VT may alter exercise prescription. Methods: Fourteen (age: 28.6 ± 6.2 yr; body mass: 78.1 ± 11.3 kg; VO₂max: 4.18 ± 1.35 L^.min^-1) well-trained cyclists completed an incremental cycling test to exhaustion (25 W^.min^-1) on a LeMond evolution ergometer. During cycling, all expired gases were collected through a Jaeger Oxycon metabolic cart and NIRS responses from the vastus lateralis were monitored using a wireless Portamon device. Each individuals VT from expired gases was determined using the V-slope method and the break point and nadir of the NIRS response were also identified. Pearson correlations determined the strength of the relationships between all measures. Results: The power output at VT from the expired gas (338 ± 71 W; 75.3 ± 10.4% MAP) and NIRS (270 ± 51 W; 60.3 ± 7.6% MAP) measures were moderately correlated (r=0.59). Further, the nadir in muscle oxygenation occurred well before VO₂max was determined (361 ± 62 vs. 448 ± 61 W). Strong relationships (r>0.8) were present between the power output that related to the nadir in muscle oxygenation and expired gas measures of both VT and VO₂max. Conclusion: The data demonstrates that the detection of VT using NIRS technology provides an earlier indication of the transition between aerobic-anaerobic metabolism during incremental exercise. Its suggested that the work intensity aligned with VT using expired gas measurement may be blunted by various chemical and respiratory buffering processes. Lastly, the observed nadir in muscle oxygenation appears to be an important indicator of muscle oxidative capacity as it is strongly related to both power output at VT and VO₂max.