INTRODUCTION: Cross-country skiing and biathlon are endurance sports that require an accurate exercise intensity prescription to optimize performance. Traditional methods for monitoring exercise intensities (e.g., lactate or heart rate) have limitations in terms of comfort and precision during on-field training. Recently, near-infrared spectroscopy (NIRS) has emerged as a new method for non-invasive monitoring of muscle oxygenation. The Moxy Monitor, a portable NIRS device, was shown to enable accurate determination of thresholds, when compared to lactate thresholds [1]. However, to date, the optimal location of the sensor remains unknown, particularly in sports as Nordic ski using both upper- and lower limbs. Therefore, this study aimed to evaluate the feasibility and accuracy of using NIRS to determine breakpoints (i.e. BP1 and BP2) compared to first (LT1) and second (LT2) lactate thresholds and to compare different muscle sites in male and female elite cross-country skiers and biathletes. METHODS: Fifty-two athletes (29 males, 23 females) from the French national teams were recruited performed an incremental treadmill test on roller skis, with 3-minute work intervals at increasing speeds until exhaustion, interspersed with 30-s rest for lactate measurement (Lactate Pro 2). NIRS sensors were located simultaneously on four muscle sites: vastus lateralis, hamstrings, biceps, and triceps. Oxygen saturation (SmO2) was collected and analyzed to detect BP1 and BP2 [2] and compared to LT1 and LT2. Statistical analysis included repeated measures ANOVA and Chi-Square tests. RESULTS/DISCUSSION: There are three key findings: First, the detection of BP1 is not effective due to insufficient precision, making it unsuitable for practical use. Second, it was possible to detect efficiently BP2 in vastus lateralis (88.5%), hamstrings (96.2%) and biceps (86.5%) but not in the triceps (24.1%). Third, there was a very good accuracy (i.e., bias [95% confidence intervals] in heart rate between BP2 and LT2) in the three muscles: vastus lateralis (-0.55 bpm [-8.85; 7.75]), hamstrings (1.3 bpm [-2.8; 4.2]) and biceps (1.0 bpm [-7.5; 9.5]). Finally, no significant differences were found between male and female athletes in the second threshold determination, likely due to the athletes' low adipose tissue thickness. CONCLUSION: NIRS technology shows potential as a practical field tool for detecting the second lactate threshold in elite Nordic skiers. Its non-invasive nature and ease of use make it a valuable addition to traditional methods like lactate testing. However, improvement in the precision of BP1 detection is necessary for broader application. Further investigation should focus on NIRS as a device for monitoring training intensity during on-field training sessions, exploring its potential for real-time application in elite sport.
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