Monitoring skeletal muscle oxygen saturation kinetics during graded exercise testing in NCAA division I female rowers.

Abstract

Purpose: The purpose of this study was to analyze changes in skeletal muscle oxygen saturation (SmO₂) kinetics during exercise in female rowers both acutely and longitudinally in relation to blood lactate (BLa). We also aimed to determine the agreement and statistical equivalence between physiological thresholds derived from SmO₂ and BLa kinetics.

Methods: Twenty-three female NCAA Division I rowers were tested throughout the 2023-2024 academic year. Of these, 11 athletes completed at least two near-infrared spectroscopy (NIRS)-equipped GXTs, with physiological data analyzed for longitudinal changes. A 7x4-min discontinuous GXT protocol was performed by all athletes. First and second SmO₂ breakpoints (SmO₂BP1 and SmO₂BP2) were estimated via piecewise linear regression modeling, and BLa thresholds (LT₁ and LT₂) were calculated using ADAPT software. Paired-samples t-tests assessed differences, and equivalence was tested using two one-sided tests (TOST). Agreement was determined using Bland-Altman analysis yielding mean differences (MD) and 95% limits of agreement (LoA). Intraclass correlation coefficients (ICC_2,1) were also calculated.

Results: No difference was found between SmO₂BP2 and LT₂ (MD = -5.76W [95% LoA = -38.52 to 22.25W], p = 0.134), moderate-to-good levels of agreement (ICC_2,1 = 0.67 [95% CI: 0.36-0.85], p < 0.001), and no statistical equivalence (p = 0.117). This was not the case for SmO₂BP1 and LT₁, with NIRS significantly underestimating LT₁ (MD = -8.14W [95% LoA = -38.90 to 27.37W], p = 0.026), poor-to-moderate agreement (ICC_2,1 = 0.24 [95% CI: -0.13-0.58], p = 0.10), and no statistical equivalence (p = 0.487). Additionally, SmO₂ recovery kinetics (SmO₂resat) during 1-min rest intervals increased in response to graded increases in exercise intensity (p < 0.001, η² _p = 0.71), with higher intensities appearing to blunt this effect (step 6 - step 7: MD = -0.16%⋅s^-1, p = 0.69). No statistically significant changes were observed in LT's or SmO₂BP's throughout the 2023-2024 season.

Conclusion: In female collegiate rowers, NIRS may be a tool that compliments BLa testing when determining the second lactate threshold (i.e., LT₂). However, significant inter-individual variablility exists between SmO₂BP2 and LT2 paired with a lack of statistical equivalence suggest the two are not interchangeable. While not a standalone replacement, if used in combination with traditional BLa testing methods NIRS may be a complimentary tool that helps inform individual athlete training zone prescription.