Vertical jumping (VJ) is a multi-joint movement that requires complex motor coordination, and it has been identified as one of the fundamental movement skills [1]. The maximum height achieved by an athlete in this activity is a functional evaluation parameter frequently used by professionals [2] due to the simplicity and richness of outcome information [3]. Specifically, athletic trainers use the maximum jump height as an indirect parameter for assessing the strength and muscle power of the lower limbs [4]. The significant effect of lower limb exercises in the form of jump squats on the development of explosive strength has also been validated [5].

Judo is a sport characterized by large intensity intervals alternated with periods of low intensity [6]. For judokas, strength in the lower leg, particularly the knee and thigh is of great importance for staying in balance and performing specific techniques in a correct manner [7-9]. In competitive judo, the use of techniques that require balance and explosion movements to throw your opponent are common practice [10]. In this technique, lower-body muscle groups are essential to success [11], thus suggesting that lower-body muscle strength is paramount to success. Vertical jump tests have been used as assessment tests of lower body strength in judokas, often using force platforms for such evaluation [12-14], since a relationship appears to exist between strength-velocity curve and jump tests [11]. Therefore, jump height estimation appears to be important in this sport. Additionally, jump height allows the estimation of lower limb power, which could be essential in the performance of projection techniques (eg.Nage-waza), where the athlete produces a lot of strength during the execution of the technique (this technique can take a while given time to be achieved) [15]. Due to its importance, the analysis of the jump height in judo athletes has been performed previously as part of a physical fitness performance assessment [16].

The Countermovement Jump (CMJ), or vertical jump with countermovement, is one of the jumps mostly used to assess the manifestation of explosive reactive force (power) in the lower limbs [17] and as been used previously for testing judokas [12-14]. CMJ consists of a jump based on the stretching-shortening cycle (a mechanism where the function is to provide greater mechanical efficiency of movements that use eccentric muscle actions followed immediately by explosive concentric actions) [18]. The CMJ is an easy-to-apply test, requiring only the jump platform or a mobile application to measure the jump height. Usually, for assessing the jump, contact platforms are used and considered the gold standard [19]. These contact platforms have vertical and pressor sensors to determine when athletes lose contact with the ground and help determine the flight time and the height of the jump. Despite the reliability of the mentioned equipment, they are expensive and often not portable, resulting in difficulties in assessing the athletes in the training field.

In the search for less expensive and more portable solutions, mobile / tablet applications (app) [20-22] have been developed to measure vertical jump height. The My Jump 2 (MJ2) application is an app already used to measure the height of the jump, and is an updated version of My Jump application [20]. This application allows the calculation of the jump height, flight time, initial velocity, force and power of vertical jumps. Additionally, provides contact time, vertical stiffness, and reactive force index of the drop jumps and gives Force-Speed profile. The app My jump has already been demonstrated as valid by other authors, such as Gallardo-Fuentes et al. [23], in which the recruited sample was 21 athletics athletes (men and women) and Yingling et al. [24] whose sample was 21 active sports science students. Additionally, Sharp et al. [25] carried out a brief review of studies on the validity and reliability of the My Jump application. Despite stating that the MJ2 appears to have strong evidence supporting its use, it also stated that further studies are needed, particularly with athletic populations [20, 21]. Besides that, none of the previous studies assessed Judo players or athletes in the training field.

Given the importance of determining the height of the jump in Judo athletes, it has an established relationship with lower limb strength, it is also essential to check the validity of the use of an inexpensive and fast application system, that can be used in the training for improving the performance of athletes. Considering that, this study aimed to analyse the validity and reliability of MJ2 for measuring jump height in an athletic population, particularly judo players.

The sample composed of 21 judo players, where six were female and 15 were male (age = 27.41 ± 7.52 years, height = 153.20 ± 21.35 cm, and body mass = 66.13 ± 6.34 kg) recruited for this study. The result of the sample size estimation when setting power at 80%, alpha at 0.05, expected reliability at 0.9 and a drop-out rate of 10% was 20 participants.

Data collection and testing procedures were explained to the athletes and an informed consent was filled for all athletes. All judo players had more than 2 years of practice of the sport. Data collection was carried out adhering the ethical standards for human subjects research and it was approved by the Instituto Piaget Ethics Committee with the number 3/2020.

Regarding the CMJ performance of the subjects, a mean of 26.69 ± 8.14 cm was recorded (Table 1). When comparing evaluators’ measurements to the reference standard no statistical differences were obtained in the measurements (Average comparison p>0.05). When using ICC (Intraclass Correlation Index) and the CI (Confidence Interval), we attained an ICC of 0.94 and the 95% CI was 0.88-0.97 (p<0.001) for evaluator 1. For evaluator 2, the ICC=0.97 and its 95% CI was 0.94-0.98 (p<0.001). Regarding the measurements of the two evaluators (mean of the heights of jumps between 1 and 2), the ICC and the IC were 0.95 and 0.90-0.97 (p<0.001), respectively.

Fig. (2) shows the level of agreement using the Bland-Altman analysis for both evaluators. For evaluator 1, the dark gray line shows the mean of the differences (1.94 cm) and the light gray lines represent the 95% limits of agreement (9.48 to -5.60 cm). Regarding evaluator 2, the dark gray line shows the mean of the differences (2.19 cm) and the light gray lines represent the 95% limits of agreement (7.38 to -3.00 cm).

Table 2 presents the complimentary analysis for the results obtained in the Balnd Altmann analysis. The results of the t test against the null value (meaning zero as difference) showed that the mean difference between the Standard measures and the Evaulators analysis are not significantly different. The regression analysis between the average and the difference presented values of R²=0.12 and R²=0.08 for each evaluator, respectively, showing that the error is not proportional.

Fig. (3) shows the linear regression plot through Pearson's correlation between the height of the jump obtained with the Chronojump and the measurements of both evaluators with the My Jump 2 app, where r=0.97 and r²=0.85 (p<0.05).

Fig. (1). CMJ execution (images obtained during data collection).

Fig. (2). Level of agreement (Bland-Altman) with 95% limits of agreement (light gray lines) and the mean difference (dark gray line) between MyJump2 and the force platform for CMJ for both evaluators.

Fig. (3). Linear Regression of the jump height for Chronojump and App MyJump2 with combined evaluators. p<0.05.

The aim of this study was to validate the My Jump 2 application in judo athletes. To this end, the Intraclass Correlation Coefficient (ICC) was calculated between each evaluator and a contact platform that can be considered “the gold standard”. According to the literature, if the ICC values are greater than 0.8, they are considered to have a high level of reliability [31, 32]. The values found in this study are 0.94 and 0.97, respectively, demonstrating a high correlation between the values calculated by the app MJ2 and the gold standard for measuring CMJ height. These ICC values are very similar to those obtained by Balsalobre-Férnandez et al. [27] (ICC = 0.997, 95% CI: 0.996–0.998, P < 0.001 for both observers) and Carlos-Vivas et al. [33] obtained an almost perfect ICC between the MJ app and the force platform (ICC=0.996, 95% CI: 0.998-0.993, p<0.001).

According to the results of the Bland-Altman method (Figs. 2-4), the measurement differences between the instruments were (2.06 ±0.85 cm) and the inter-evaluator differences were (- 0.15 ± 3.77 cm), which suggests that the app MJ2 has acceptable error values in its use. The findings in the study are in agreement with other authors who have found that both MJ2 [34-36] and the first version Myjump [21, 37] are valid, reliable tools, despite not assessing judo athletes.

The results of the present investigation are less significant than those reported by Monteiro et al. when comparing jump height results with their normative value [38]. There are some reasons for the discrepancy in mean values, such as sample size, different training experiences by athletes, and above all, different procedures, since Monteiro, in his study, used a barbell attached to a linear transducer [38]. Similar differences in jump height can be recounted when comparing the present study with the studies of Detanico et al. [39] and Kons et al. [40], which can also be explained by the difference in training experience.

This study is not without some limitations., It was possible to demonstrate that the values obtained with the app My Jump 2 are quite reliable, but a larger sample of athletes could be used in future studies, to confirm the results of this study. Besides that, each athlete performed two jumps, which allowed an analysis of 42 jumps. Additional studies should endeavour to use a greater number of jumps per athlete. Another limitation is the level of the athletes, where some were in high competition at the national level, and others were only two years practitioners. Since the purpose of the study is to evaluate the reliability of the app and not the performance of the judokas, the maximum height achieved would not compromise the results obtained.

This study adds to the existing literature regarding the validity of a smartphone app to measure jump height, specifically validating the use of this app to measure leg strength in judo athletes. The app MJ2 seems to be an easy and practical tool to use in the context of sports training, and it can be an asset for clubs that do not have the conditions to acquire top-of-the-range equipment to assess their athletes. With this app, the trainer could not only measure the height of the jump but also use other aspects such as asymmetry tests, horizontal jump tests, and even strength speed tests. That said, it is recommended that coaches include the app My Jump 2 in their assessment materials so that athletes achieve better results both in the context of training and competition.