Exploring the Potential to Non-Invasively Assess Lactate Threshold Using Near-Infrared Spectroscopy (NIRS)

Abstract

Lactate is a versatile molecule that plays a pivotal role in whole-body energy metabolism — a process wherein fuel sources are burned down and converted into energy. Our understanding of lactate has changed considerably over time, notably, low oxygen (O2) was once considered to be the primary driver of lactate accumulation in human cells, however, lactate accumulation has been found to occur in the absence or presence of O2; and yet, a relationship between tissue O2 availability exists. The relationship between skeletal muscle oxygenation (SmO2) and thresholds of blood lactate concentration (BLC) constitutes the basis of previous attempts to use changes in SmO2 via near-infrared spectroscopy (NIRS) as a proxy for BLC. The objective of this study was to 1) confirm the relationship between changes in muscle oxygenation (SmO2) using NIRS and the lactate threshold (LT) determined via traditional blood sampling; and 2) further explore the relationship between SmO2 kinetics and exercise thresholds of respiratory gas exchange. Sixteen recreationally active runners (n = 16; age 25.4 ± 8.3yrs; body fat 14.9 ± 5.5%) performed an incremental, progressive treadmill running protocol consisting of 4-min running stages, separated by 1-min rest intervals. Heart rate, 𝑉̇ O2, 𝑉̇ CO2, 𝑉̇ E, total hemoglobin, and SmO2 were continuously monitored; blood lactate concentration and rating of perceived exertion were obtained during rest intervals and following each exercise stage, respectively. The SmO2 control ratio revealed a distinct breakpoint near, but not necessarily coincident with, the lactate and gas exchange thresholds. Bland-Altman analysis resulted in a mean bias of 2.3 bpm HR and 0.1 L/min 𝑉̇ O2 between the blood lactate-determined LT and NIRS-determined LT, suggesting an acceptable level of agreement between methods. These findings suggest that NIRS may be a valid non-invasive and practical alternative to estimate lactate threshold intensity in endurance athletes.