Muscle Oxygen Saturation as a Marker of Internal Load in Elite Soccer: A Case Series

ABSTRACT: Herzog, S, Philippi, M, Feldmann, A, Asian, J, Filter, A, Requena, B, Batterson, PM, and Bogdanis, GC. Muscle oxygen saturation as a marker of internal load in elite soccer: A case series. J Strength Cond Res 40(9): e991-e998, 2026-Accurate monitoring of internal and external load during training is essential for optimizing performance and managing fatigue. In soccer, heart rate (HR) is commonly used to assess internal load. This study examined the use of muscle near-infrared spectroscopy, which measures muscle oxygen saturation (SmO 2 ), as a potential alternative to HR by providing real-time insights into local muscular effort. Seventeen elite male professional soccer players (21.2 ± 4.01 years) performed an incremental treadmill test to exhaustion, of whom 14 had complete data sets suitable for threshold analyses. During the test, HR, expired gases, and SmO 2 of the vastus lateralis were continuously measured to identify ventilatory, lactate, and SmO 2 breakpoints. On a separate day, HR and SmO 2 were recorded in a subset of five players during small- and large-sided games (SSG and LSG). A repeated-measures analyses of variance comparing HR corresponding to the three different thresholds indicated that HR at BP2 (178 ± 5 beats·minute -1 ) was similar with HR at LT2 and VT2 (177 ± 7 beats·minute -1 and 181 ± 6 p = 1.000 and p = 0.0920, respectively). Bland-Altman analysis indicated acceptable agreement between HR thresholds, while SmO 2 thresholds showed wider limits of agreement and greater individual variability. During small-sided game and LSG, players spent significantly more time above SmO 2 -based thresholds compared with HR-based thresholds ( p < 0.05), with the largest differences observed during LSG. These findings suggest that SmO 2 is more responsive to high-intensity, intermittent activity and may better reflect local muscular demand than HR. Incorporating SmO 2 into training and match monitoring may enhance internal load assessment and support more individualized performance and recovery strategies.