Visually identified breakpoints in muscle oxygen saturation are not equivalent to ventilatory thresholds in...

Portable near-infrared spectroscopy (NIRS) enables cost-effective monitoring of muscle oxygen saturation (SmO₂) during exercise, presenting a promising alternative for delineating exercise intensity domains. This study investigated whether SmO₂ breakpoints (BP₁ and BP₂) can provide accurate estimates of ventilatory thresholds (GET and RCP).

Twelve male Tier 3 triathletes completed an outdoor incremental TestVAM. Vastus lateralis SmO₂ was recorded using NIRS, alongside pulmonary gas exchange measurements. BP₁ and BP₂ were identified visually. Heart rate (HR) and running speed at BP₁, BP₂, GET, and RCP were extracted. The intraclass-correlation coefficient (ICC) was calculated and equivalence between SmO₂ breakpoints and ventilatory thresholds was assessed with equivalence bounds set at 5 bpm for HR and 0.5 km h^-1 for speed.

The ICC(3,1)_abs was poor to moderate for all comparisons (range: 0.18-0.69). HR and speed between BP₁ and GET were not equivalent (p > 0.91), with GET occuring ealier in both HR (p = 0.014, Δ = - 9.92 bpm, 90% CI [ - 16, - 3.83]) and speed (p = 0.007, Δ = - 1.29 km h^-1, 90% CI [ - 1.99, - 0.59]). Similarly, BP₂ and RCP were not equivalent (p > 0.98), and both HR (Δ = - 9.3 bpm, 90% CI [ - 12.18, - 6.42]) and speed (Δ = - 1.75 km·h^-1, 90% CI [ - 2.33, - 1.17]) occuring earlier at RCP (p < 0.001).

In this sample, SmO₂ breakpoints did not fullfill criterion validity against ventilatory thresholds and therefore should not be used for determining exercise intensity domains. Further research should evaluate their equivalence using robust detection methodologies in larger samples.