## 4 Discussion  
The CMJ and SJ tests have been strongly recommended to researchers and health practitioners. However, there is a great variety of testing methods and devices, and the majority of them are expensive and nonportable. The present study examined the concurrent validity and test-retest reliability of My Jump 2 installed on an iPhone X compared to a validated Optojump instrument for measuring jump performance during SJ, CMJ, and CMJAS in recreationally active males and females. My Jump 2 was found to be highly valid and reliable in determining the jump height of an SJ, CMJ, and CMJAS in comparison with an Optojump. Moreover, CMJ and CMJAS tests showed to be practically useful to assess and monitor vertical jump performance in recreationally active adults. Furthermore, the data presented in Bland-Altman plots (Figures 2-4), show that most of the values are close to the mean of the differences between instruments, thereby representing a high level of agreement [27]. The plot shows a systematic bias (Figures 2-4) such that, across all jump heights, values derived from the Optojump tended to be slightly higher than those from My Jump 2 app (resulting in positive difference scores). The mean bias between My Jump 2 and the Optojump for jump height was less than 0.9 cm. According to the authors knowledge, this is the first study to compare these two instruments. However, the low bias obtained in our study is in agreement with previous studies (mean bias: 0.2-1.1 cm) that compared My Jump app with force platform [15; 28; 29]. Higher bias (1.37 cm) was found only in females for CMJAS, which could be due to higher variability influenced by the lack of proper technique among females. Our test-retest design in the group of recreationally active males and females revealed that SJ, CMJ, and CMJAS appear as reliable assessment outcomes (ICC \(>\) 0.90), with slightly greater variability (CV \(>\) 5%) for SJ outcomes between two sessions. The current results showed mean differences of 0.3-2.3 cm in all jumps for both males and females. This is in line with a mean difference of 0.43 cm for CMJ reported in recreationally active adults on My Jump app [22]. The concurrent validity of SJ, CMJ, and CMJAS was assessed by comparing outcome measures to the Optojump, which is already validated for estimating vertical jump. Very large correlations were observed between My Jump 2 app and Optojump in both, the male (r = 0.95-0.98) and female (r = 0.94-0.97), recreationally active adults. Most studies have compared My Jump app with force platform on several different jumps [8; 15; 28]. The abovementioned studies showed nearly perfect correlation (r = 0.97-0.99) for CMJ and SJ in trained athletes [15; 28], but also for drop jumps (r = 0.94-0.97) in sport science students [8]. The mean differences found in previous validity studies for CMJ performance that compared portable measurement devices with force plates were between \(-\)1.06 cm and 11.7 cm [18; 30; 31]. Regarding the My Jump app, [PERSON] et al. [28] found a small mean difference between devices (0.1 cm) when testing CMJ and SJ jump in both male and female athletes. In one recent study [22] on recreationally active males and females, the mean difference in CMJ between devices was 0.21 cm, which is slightly lower than the mean difference found in our study for SJ and CMJ (0.4-0.9 cm). As mentioned earlier, concurrent validity studies have compared My Jump to force plate data. However, it was also important to examine the validity of My Jump compared to a more frequently used field measurement tool. Optojump has also been found to be a valid and reliable vertical jump measurement tool [18], that is amenable to multiple testing locations and, thus, is more commonly used in different vertical jump test settings.