# Intra-Reliability of a Wearable Near-Infrared Sensor for Monitoring the Intensity of Exercise
## 4 Discussion  
The findings of this study indicate that the Q-LAC device is a reliable instrument for measuring the oxygen saturation of venous hemoglobin during low-to-moderate-intensity cycling exercise. Consistent with our hypothesis, the relative reliability (the ability to assess differences between subjects while taking the measurement error into account) was substantial to almost perfect, and absolute reliability (the similarity of repeated measures) was excellent in SmO\({}_{2}\). This study is the first to provide reliability data on oxygenation for the Q-LAC portable device. Numerous prior studies investigating the intra-rater reliability of NIRS during exercise have demonstrated that reliability varies based on exercise modality and intensity, muscle group, NIRS device, and the training status of participants [11; 12; 13; 14; 15; 30; 31; 32]. Therefore, it is imperative to establish reliability for specific NIRS-derived variables during exercise protocols intended to assess the effect of any intervention. Consistent with available studies, the Q-LAC sensor exhibited a high level of relative reliability in measuring SmO\({}_{2}\) in the VL muscle of the subjects, with an ICC range of 0.72 to 0.91. These findings align with earlier research that demonstrated strong test-retest reliability (0.81-0.90) for SmO\({}_{2}\) across various exercise intensities during a multi-stage cycling test, conducted on the right VL using another wearable NIRS sensor [14]. Furthermore, [PERSON] et al. [13] reported good to excellent reliability (ICC: 0.77-0.99). Additionally, previous reports by [PERSON] et al. [12] indicated excellent reliability for the NIRS sensor device at low intensity (rest and first ventilatory threshold (VT1)) and at high intensity (second ventilatory threshold (VT2) and max): 0.95, 0.97, 0.95, and 0.95, respectively. The abovementioned studies evaluated the  
Figure 3: Agreement of SmO\({}_{2}\) values obtained in Test 1 (T1) and Test 2 (T2) for five workloads: 50 W (**a**), 100 W (**b**), 150 W (**c**), 200 W (**d**), and 250 W (**e**). The central solid line in each subgraph indicates the systematic bias, shown as the mean difference between T1 and T2. The dotted lines above and below this mean depict the random error via limits of agreement (LoAs), quantified as mean \(\pm\) 1.96 SD. The 95% confidence intervals for these measurements are explicitly detailed [lower limit, upper limit]. Additionally, a dashed line in each subgraph represents the linear regression of the differences against their mean, depicting any proportional bias. The regression equation, along with Pearson’s correlation coefficient (r) and the associated \(p\)-value, are shown in the upper right corner of each subgraph. Moxy Monitor sensor [12; 13; 14]. In contrast, [PERSON] et al. [15] investigated the relative reliability of the Portamon device during a continuous IET, showing an ICC range of 0.06-0.76 for the TSI in patients with flow limitations of the iliac arteries (FLIA) and 0.16-0.70 in healthy cyclists. It is noteworthy that some studies also included women [13; 14; 15], which may influence higher adipose tissue thickness (ATT) at the quadriceps muscle measurement site, potentially limiting the quality of NIRS signals collected in female subjects. This study exclusively examined males; future analyses should encompass physically active females and individuals of varying ages for a comprehensive assessment. Research on the reliability, minimum detectable change, and sensitivity of testing items in NIRS data used in combination is scarce. Unlike previous examinations into the absolute reliability of wearable NIRS devices during low-to-moderate-intensity exercise, our results demonstrate a notably higher level of absolute agreement (5.1-9.9%) [13; 14]. In these studies, an increase in the coefficient of variation (CV) was observed with an escalation in workload. For instance, [PERSON] et al. [13] reported CV values of 4, 5, 12, and 20% for 100, 150, 200, and 250 W, respectively. Furthermore, [PERSON] et al.