Oxygenation Threshold Derived from Near-Infrared Spectroscopy: Reliability and Its Relationship with the First Ventilatory Threshold

<div><p>Background</p><p>Near-infrared spectroscopy (NIRS) measurements of oxygenation reflect O<sub>2</sub> delivery and utilization in exercising muscle and may improve detection of a critical exercise threshold.</p><p>Purpose</p><p>First, to detect an oxygenation breakpoint (Δ[O<sub>2</sub>HbMb-HHbMb]-BP) and compare this breakpoint to ventilatory thresholds during a maximal incremental test across sexes and training status. Second, to assess reproducibility of NIRS signals and exercise thresholds and investigate confounding effects of adipose tissue thickness on NIRS measurements.</p><p>Methods</p><p>Forty subjects (10 trained male cyclists, 10 trained female cyclists, 11 endurance trained males and 9 recreationally trained males) performed maximal incremental cycling exercise to determine Δ[O<sub>2</sub>HbMb-HHbMb]-BP and ventilatory thresholds (VT1 and VT2). Muscle haemoglobin and myoglobin O<sub>2</sub> oxygenation ([HHbMb], [O<sub>2</sub>HbMb], SmO<sub>2</sub>) was determined in m. vastus lateralis. Δ[O<sub>2</sub>HbMb-HHbMb]-BP was determined by double linear regression. Trained cyclists performed the maximal incremental test twice to assess reproducibility. Adipose tissue thickness (ATT) was determined by skinfold measurements.</p><p>Results</p><p>Δ[O<sub>2</sub>HbMb-HHbMb]-BP was not different from VT1, but only moderately related (r = 0.58–0.63, p<0.001). VT1 was different across sexes and training status, whereas Δ[O<sub>2</sub>HbMb-HHbMb]-BP differed only across sexes. Reproducibility was high for SmO<sub>2</sub> (ICC = 0.69–0.97), Δ[O<sub>2</sub>HbMb-HHbMb]-BP (ICC = 0.80–0.88) and ventilatory thresholds (ICC = 0.96–0.99). SmO<sub>2</sub> at peak exercise and at occlusion were strongly related to adipose tissue thickness (r<sup>2</sup> = 0.81, p<0.001; r<sup>2</sup> = 0.79, p<0.001). Moreover, ATT was related to asymmetric changes in Δ[HHbMb] and Δ[O<sub>2</sub>HbMb] during incremental exercise (r = -0.64, p<0.001) and during occlusion (r = -0.50, p<0.05).</p><p>Conclusion</p><p>Although the oxygenation threshold is reproducible and potentially a suitable exercise threshold, VT1 discriminates better across sexes and training status during maximal stepwise incremental exercise. Continuous-wave NIRS measurements are reproducible, but strongly affected by adipose tissue thickness.</p></div>