## 4 Discussion
### Reliability  
The level of agreement between devices showed significant values above 0.91, confirming the Fitnessmeter app as a highly reliable tool. A significant reliability in the measurements using the Fitnessmeter was also identified from the ICC analysis regardless of the leg used for the change of direction (ICC = 0.66-0.72). These values were quite similar to those observed here in comparison to the photocells (ICC = 0.57-0.76). Accordingly, previous research has reported moderate reliability in the use of COD timer app to determine change of direction times in the 505 test [33] and the 5 + 5 m test [35]. Furthermore, reproducibility analysis of the Fitnessmeter also showed CV values not exceeding 2.94%, which were similar to those from the Photocell (CV = 2.21-2.60%). Nevertheless, it is worth noting that the photocell was more sensitive in detecting variations between trials than the Fitnessmeter, as indicated by the larger CV values in both legs. This difference may be attributed to the infrared light sensors used by the photocell, which can detect any body part crossing the light beam, even small ones such as a hand. In contrast, the mobile camera used by Fitnessmeter likely requires a larger body part, such as the head or trunk, to be detected in a particular area.  
The reliability findings of the Fitnessmeter app align closely with those reported by [PERSON] et al. [39] in their study on the CODTimer app (v.1.0). In their investigation, the CODTimer app demonstrated moderate to good reliability (ICC = 0.671-0.840) for measuring change of direction times in the 5 + 5 test, with CV values ranging from 2.2% to 3.2%. These results are comparable to the ICC and CV values obtained in the current study, further supporting the reliability of smartphone-based applications for COD assessment. However, the Fitnessmeter app showed slightly higher ICC values (0.66-0.72) for inter-limb measurements compared to the CODTimer app, which reported ICC values of 0.671 for the left leg and 0.840 for the right leg [39]. In addition, the level of variability is well within the smallest worthwhile change (SWC) for COD performance in adolescent athletes, which is typically considered to be around 2-3% of the mean performance time [39; 45].  
While moderate, the ICC values for the right leg were lower than for the left in both instruments. Slight differences in how participants initiated or executed the change of direction (COD) with their right leg might have introduced more variability in the recorded times. The ICC values were within an acceptable range for field-based performance tests, especially given the practical nature of the study. Additionally, the low CV values (<3%) observed in both studies indicate that both apps provide reproducible results, making them suitable for practical use in field settings.  
Therefore, considering the high validity and moderate reliability demonstrated by the Fitnessmeter in comparison to the gold standard and similar mobile apps, both professional and amateur users can confidently utilize it at a very affordable cost. It should be noted that the way of measuring the result in the different applications consists of calculating the times in relation to the frame per second, in comparison with Fitnessmeter, which detects the movement by means of the image provided by the cell phone camera, obtaining the result at the same moment of the execution of the test.