## 2 Materials and Methods
### Design and Procedure  
The probe began at 7:00 a.m., with the participating athlete having his last meal before 9:00 pm the previous evening. In the hour preceding the start and immediately after the conclusion of the ultra-endurance event, we assessed the following parameters, drawing on previous research conducted using ultra-endurance probes [2; 4; 10]. Body mass was assessed using a SECA model 711 scale (SECA GmbH & Co. KG, Hamburg, Germany), which has a precision of 100 g and a measurement range of 0.1 to 130 kg. The scale was situated on a flat, smooth surface and calibrated to zero. Participants, barefoot and wearing minimal clothing, stood at the center of the platform. They avoided contact with any surrounding objects, ensuring their weight was evenly distributed on both feet while facing forward. Cortical arousal was evaluated using the Critical Flicker Fusion Threshold (CFFT) within a viewing chamber (Lafayette Instrument Flicker Fusion Control Unit Model 12021), adhering to established procedures from previous studies. An increase in CFFT indicated enhanced cortical arousal and information-processing capabilities, whereas values below the baseline suggested diminished efficiency in information processing and central nervous system fatigue. CFFT is a measure used to evaluate cortical arousal and information-processing capabilities. Variations in CFFT values were interpreted as follows: an increase in CFFT indicates enhanced cortical arousal and improved information-processing capabilities, reflecting central nervous system (CNS) activation. Conversely, a decrease in CFFT suggests diminished efficiency in information processing, which can be associated with CNS fatigue [20; 21]. Isometric handgrip strength was measured using a TKK 5402 dynamometer (Takei Scientific Instruments Co., Ltd., Niigata City, Japan). The measurement was taken on the athlete's dominant hand. The athlete was seated with the shoulder at 0\({}^{\circ}\) flexion, the elbow at 90\({}^{\circ}\) flexion, and the forearm in a neutral position. The highest value obtained from two trials was recorded. Lower limb strength was assessed using a horizontal jump test. The athlete stood behind a marked line on the ground, with feet shoulder-width apart. Three attempts were made, and the highest result was taken for analysis. Heart-Rate Variability (HRV) was monitored using a Polar V800 HRV monitor (Kempele, Finland). Measurements commenced a few minutes before the event started and concluded at the event's end. Following previous protocols [2; 4; 10], the following parameters were analyzed immediately after the event:  
Hydration status was evaluated using a colorimetry procedure with a urine color chart, which identified pH status and the presence of glucose, nitrites, protein, and glucose on the urine strip. Blood glucose concentration was determined by analyzing 5 uL of capillary blood from the finger using a portable analyzer (One Touch Basic, LifeScan Inc., Madrid, Spain). Blood lactate concentration was measured by collecting a 5 uL sample of capillary blood from the subject's finger and analyzing it with the Lactate Pro II system (Arkay, Inc., Kyoto, Japan). Thermography response: Thermal images were captured using a second-generation FLIR One iOS Thermal Camera Smartphone Module (FLIR Systems, Wilsonville, OR, USA) through the Thermal Camera+ for the FLIR One app installed on an Apple iPhone 6. The emissivity setting in the app was adjusted to \"matte: 95%\" to closely match the human skin emissivity of 98% [22]. Before capturing each thermal image, the camera was recalibrated using the app's built-in function to reduce noise. The thermal images captured with the FLIR One have a resolution of 160 \(\times\) 120 pixels in a \"compressed image format\" to reduce storage size, and the accompanying software automatically upscales them to 640 \(\times\) 480 pixels. The corresponding visible spectrum images are taken at a resolution of 1440 \(\times\) 1080 pixels.