Muscle oxygen saturation rates coincide with lactate-based exercise thresholds, 2023, Batterson et al

[Andy]

Introduction

Monitoring muscle metabolic activity via blood lactate is a useful tool for understanding the physiological response to a given exercise intensity. Recent indications suggest that skeletal muscle oxygen saturation (SmO2), an index of the balance between local O2 supply and demand, may describe and predict endurance performance outcomes.

Purpose
We tested the hypothesis that SmO2 rate is tightly related to blood lactate concentration across exercise intensities, and that deflections in SmO2 rate would coincide with established blood lactate thresholds (i.e., lactate thresholds 1 and 2).

Methods
Ten elite male soccer players completed an incremental running protocol to exhaustion using 3-min work to 30 s rest intervals. Blood lactate samples were collected during rest and SmO2 was collected continuously via near-infrared spectroscopy from the right and left vastus lateralis, left biceps femoris and the left gastrocnemius.

Results
Muscle O2 saturation rate (%/min) was quantified after the initial 60 s of each 3-min segment. The SmO2 rate was significantly correlated with blood lactate concentrations for all muscle sites; RVL, r = − 0.974; LVL, r = − 0.969; LG, r = − 0.942; LHAM, r = − 0.907. Breakpoints in SmO2 rate were not significantly different from LT1 or LT2 at any muscle sites (P > 0.05). Bland–Altman analysis showed speed threshold estimates via SmO2 rate and lactate are similar at LT2, but slightly greater for SmO2 rate at LT1.

Conclusions
Muscle O2 saturation rate appears to provide actionable information about maximal metabolic steady state and is consistent with bioenergetic reliance on oxygen and its involvement in the attainment of metabolic steady state.

Paywall, https://link.springer.com/article/10.1007/s00421-023-05238-9

 

[SNT Gatchaman]

Response in The remarkably tight relationship between blood lactate concentration and muscle oxygen saturation (2023, European Journal of Applied Physiology) —

Unlike most LTTEs, this letter is primarily meant to commend and less to critique

The authors’ main finding of close negative relationships between blood lactate concentrations ([La]b ) and muscle oxygen saturation (SmO2) in various muscles, may not be deemed surprising. However, the study’s remarkable, and indeed surprising finding, is the strength of these relationships (r = 0.907‒0.974 in four discrete muscles). The 82.2‒94.8% of explained variance is particularly notable because [La]b is a whole‑body measure, while SmO2 is a local, muscle‑specific variable.

Although the authors rightly recognize the curvilinear nature of the relationships between running velocity and both [La]b and SmO2 and calculate their correlations as such, it is unclear why the curvilinearity of the ‘flagship’ [La] b SmO2 relationship (Batterson et al. 2023, Fig. 3) is not recognized and similarly dealt with.[...] Had the authors employed a curvilinear (e.g., 2nd ‑order polynomial) rather than linear regression analysis, the correlation coefficients would have been markedly stronger and the findings even more remarkable.

The authors have demonstrated the potential practicality of using the non‑invasive NIRS‑based SmO2 determinations as a valid alternative to the invasive lactate sampling, or to the more involved and cumbersome ventilatory threshold protocols. With the suggested improved analysis, this message could be even more convincing.

 

[SNT Gatchaman]