**4. Discussion**

The CMJ and SJ tests have been strongly recommended to researchers and health practitioners. However, there is a grea<sup>t</sup> 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 agreemen<sup>t</sup> [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 agreemen<sup>t</sup> 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, Gallardo-Fuentes 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.

From a practical perspective, the use of healthy recreational adults from across the general population, iPhone X with a 240 Hz high-speed camera, the relatively large number of participants, and field-testing conditions rather than a precise laboratory space all signify strengths of the current research. However, the main limitation was that we did not use force plate, which is considered as the "gold standard" in measuring vertical jump in various populations. Nevertheless, comparing My Jump app with Optojump is more appropriate because both use the flight time to measure jump height. Additionally, different methods for determining the height of the vertical jump exist, which can also impact the validity of instruments [32]. Most of the research has compared methods that calculate jump height to methods that calculate flight distance. Struzik and Zawadzki [33] mention a method based on a force–displacement curve. The method used to calculate jump height should be determined by the equipment available and the definition of jump height used by the practitioner [34].

Furthermore, a possible limitation of our study was that some participants might not have been familiar with the SJ jump style and the usage of hands in CMJAS test, especially among female participants. Relatively high variability obtained in SJ may be due to a lack of proper technique among recreational athletes, while previous research was conducted on elite athletes [28] with greater experience performing these jumps.

Additionally, in comparison to male participants, females have a little di fference in achieved jump height between CMJ and CMJAS. We can speculate that females did not swing with their arms correctly and use them to enhance their jump performance. During this jump, the arms reduce the pressure on the ground by moving downward toward the ground, which creates a negative e ffect, and later the arm swing creates a positive e ffect by moving upward and increasing the pressure on the ground [35]. Optimal jump is performed when the arms move in the jumping movement direction [36]. Additionally, female athletes show the trend for the increased di fferences in jump height between the two devices with increasing jumping height, which was confirmed by Attia et al. [37].

Furthermore, another limitation was that we did not check for the inter-rater reliability because some factors could contribute to di fferences in scores (i.e., the experience of the tester, the di fferent variability of scores, testers' seat position, and assessment view angle) [38]. Therefore, future study should include a larger number of observers to compare results and to account for probable human error. Nevertheless, our results support the usage of smartphone apps in measuring vertical jump in recreationally active males and females. Due to its advanced technology, popularity, low cost, and portability, smartphone apps will soon be commonplace for measuring variables associated with physical fitness and health with grea<sup>t</sup> precision [39].
