## 4 Discussion  
This study aimed to test the criterion validity of the IR photocell _ADR jumping_ device against a MoCAP system in the estimation of the CMJ, as well as to determine the reliability in the within-session test-retest of the device. To the best of our knowledge, this is the first study to validate the _ADR jumping_ device against a 3D MoCAP system. The main results of this research show that the _ADR jumping_ photocell is a valid instrument for measuring CMJ. In terms of criterion validity, the Wilcoxon test for paired samples showed significant differences concerning the MoCAP system (10.6 cm), with moderate ES. Likewise, near perfect correlations (\(r_{s}=0.95\)) between the two devices were also observed. The correlations obtained were in the same line as those obtained by other studies comparing photoelectric sensors with jumping mats or force platforms [37; 38; 42]. Otherwise, the differences obtained in these studies (systematic error) were smaller than those found by [PERSON] et al. and [PERSON] et al. [20; 41]. However, these studies did not use MoCAP as a reference system, and, therefore, the comparisons would not be very adequate. Although all the methods have been validated and their reliability tested, a systematic error between the force or jumping platforms and the MoCAP systems was observed [49], which can add to or subtract from that already obtained from other sources. The results of [PERSON] et al. (2022) revealed a substantially reduced systematic error of 0.84 cm, compared to the systematic error found in the present study, despite having similar correlation values (\(r=0.98\)) and both studies utilizing the flight-time method to calculate jump height. This could indicate that systematic errors are lower in studies utilizing the flight-time method compared to those utilizing force plate as a criterion instrument [40; 68]. Similarly, the degree of validity found is in line with that found for other devices that use FT to estimate V] height such as jump mats [32; 69] or photogrammetry [26; 28; 70; 71]. Regarding the analysis of the Bland-Altman plots, LoA ranging from 8 cm to 13 cm were observed, and all points were within the LoA, indicating a small dispersion in the differences. Furthermore, the LoA were within the maximum allowed differences between methods, and no heteroscedasticity was observed, discarding the existence of proportional error in the measurement. It should be added that the correlations obtained from the Passing-Bablok regression model between _ADR jumping_ and MoCAP were almost perfect, with random error values of SEE 0.95 cm, which can be quantified as small, according to the standardized SEE of 0.2 [62]. Similarly, a fitting equation was obtained showing an intercept of 10.92 cm (systematic error) and a slope of 0.991, very close to linearity meaning proportional error, similarly to other photocells [32; 37]. The CCC values indicated poor correlations (0.25), largely due to the lack of precision (\(\rho\)) derived from the systematic error. The differences found between _ADR jumping_ and MoCAP may be due to the way the two devices perform the estimation. While MoCAP tracks a marker located close to the centre of mass, photocells are based on FT, and, therefore, the detection of the exact landing and take-off point is crucial. Therefore, factors such as horizontal displacement during the flight phase; the flexion of the ankle joint [38; 40]; hip or knee in the landing phase; thickness and flexibility of the contact surface [49]; and the angle of incidence of the photocell with the foot, which can cause the signal to continue to be captured after take-off [40], can be sources of error. Either way, the _ADR jumping_ can be considered, in general terms, as a valid tool, interpreting this criterion validity according to the values observed and the coherent results with other studies carried out on photocells.