# Intra-Reliability of a Wearable Near-Infrared Sensor for Monitoring the Intensity of Exercise
## 1 Introduction  
The use of muscle near-infrared spectroscopy (NIRS) technology is gaining popularity in both laboratory and applied sports settings to monitor changes in muscle metabolism and muscle oxygen saturation (SmO\({}_{2}\)) during and after exercise or training interventions [1]. This rise in popularity is partly due to the availability of commercially accessible portable wireless muscle oximeters. NIRS offers a non-invasive means of gathering information about oxygenation and hemodynamics in muscle tissue. It relies on the oxygen-dependent properties of near-infrared light. The NIRS signals represent a weighted average of the oxygen saturations of hemoglobin (Hb) in the vascular bed (including small arteries, arterioles, capillaries, venules, and small veins) and myoglobin (Mb) in muscle fibers [2]. As capillaries contribute to approximately 90% of the total blood volume in muscle [3], and assuming that all muscle regions receive adequately oxygenated arterial blood under normal conditions, changes in oxygenation detected by NIRS primarily reflect alterations in capillary (Hb-related) and intracellular (Mb-related) oxygen levels. Recent scientific reports suggest that this approach enables the discrimination of aerobic and anaerobic workloadduring incremental exercise tests [4] and provides insights into the physiological responses to exercise [5]. The incremental exercise test (IET) to volitional exhaustion is commonly used in laboratory conditions to assess aerobic performance and evaluate key cardiorespiratory fitness parameters, including oxygen consumption (VO\({}_{2}\)), ventilation (VE), heart rate (HR), and oxygen pulse (VO\({}_{2}\)/HR) [6]. Additionally, the IET provides valuable insights into metabolic thresholds (aerobic and anaerobic), substrate utilization, maximal work rate (Wmax), and maximal aerobic speed/power (MAS/MAP) [6]. These data enable the determination of individual exercise intensity and the optimization of training prescriptions. Previous research has indicated that a ramp test with a linear load, characterized by a lack of steady-state phases, is effective in determining the aforementioned variables [7]. Unfortunately, ergospirometry analysis has limitations, such as the inability to comprehend peripheral responses like SmO\({}_{2}\) in skeletal muscles [4]. In endurance training, which is based on low- and moderate-intensity exercises, power meters and heart rate monitors are used to monitor status [4]. However, environmental conditions (temperature, humidity, time of day, altitude, etc. ), the degree of rest, hydration status, and emotional state may influence HR responses [8]. Hence, the monitoring of local muscle oxygenation by modern portable oximeters presents an intriguing perspective for athletes and the general population. Prior to introducing a new device for monitoring SmO\({}_{2}\) in sports field conditions, it is imperative to scrutinize both absolute and relative reliability. Absolute test-retest reliability pertains to the consistency of repeated measures, while relative reliability encompasses the capacity to discern differences between subjects while considering measurement error [9]. Consequently, these analyses play a crucial role in establishing the ability to obtain valid measurements, enabling the monitoring of meaningful results and facilitating the interpretation of an athlete's progression across acute responses in single sessions and long-term adaptations [10]. Therefore, reliable and valid measurements with sufficient sensitivity are vital. Previous studies have evaluated the reliability of portable and wireless NIRS devices in assessing SmO\({}_{2}\) or the TSI (tissue saturation index) in healthy adults [11], male marathon runners [12], trained cyclists [13; 14], patients with flow limitations of the iliac arteries, and healthy cyclists [15].