## 1 Introduction  
Physical fitness is important for older adults to maintain their independence and enhance wellbeing [1]. Therefore, it is of great importance to measure physical fitness in adults regularly. Vertical jump tests were recognized as the most common means for assessing physical fitness in various populations [2; 3; 4]. Moreover, a fundamental step in jump training studies is a vertical jump test. It is also a common method for assessing lower limb power in a physical education class, gym, or other sports programs [5]. Furthermore, it serves as an indicator of athletes fatigue during in-season [6]. Due to its simplicity and important outcome information, vertical jump tests are broadly used by coaches, strength and conditioning professionals, and professionals in health care. The most frequently used vertical jumps are squat jump (SJ), counter-movement jump (CMJ), and drop jump (DJ).  
The most commonly used instruments for measuring the vertical jump characteristics have been photoelectric cell systems, force platforms, linear position transducers, infrared cells, contact mats, and video recording [7; 8; 9; 10; 11; 12]. The great majority of mentioned instruments presents good validity and reliability in measuring different jumps with the force plate considered as the \"gold standard\". However, most of the above-mentioned instruments are not cheap and not widely accessible for different populations. Accordingly, due to the fact that these tools are expensive and not easy for transport, practical value for measuring vertical jumps in recreationally active adults is questionable.  
Technology improvements led to the integration of high-speed cameras in mobile phones. The mobile application My Jump 2 takes advantage of these cameras to record slow-motion videos of different jump tasks. It gives us information about jump height by selecting the take-off and landing frame. Its validity and reliability were previously reported in male sport science students for drop jumps [8], elderly people [13], and in professional cerebral palsy football players for SJ and CMJ [14]. To the authors' knowledge, there is only one study [15] that analyzed the concurrent validity and reliability of a My Jump app for measuring vertical jump in recreationally active adults. However, the participants were younger men (22 years), and only CMJ was evaluated. There is evidence that the reliability of jumping explosiveness in physical performance tests might vary between men and women [16]. Therefore, it is necessary to check the validity and reliability of the My Jump app with recreational male and female adults. Moreover, in the study mentioned above, My Jump has recorded videos with iPhone 5 s app at 120 fps. As mentioned earlier, the key limiting factor to the accuracy of the app is the frame rate [8]. Therefore, the 240 fps camera on iPhone X was expected to make a significant improvement in the app's performance regarding reliability and validity.  
Due to smartphone apps popularity, portability, affordability, and advanced technology, it is important to check the accuracy of these apps for measuring variables related to physical performance and health. Therefore, the present study aimed to investigate the reliability, validity, and usefulness of the My Jump 2 app in comparison to reliable and validated Optojump photoelectric cells system in measuring SJ, CMJ, and CMJ with arm swing (CMJAS) in recreationally active adults. The current research covered a heterogeneous sample with a bigger age range as contrasting to the homogeneous sample in most studies. Our goal was to reassess the app validity in a more heterogeneous sample that has diverse jumping capabilities in order to overcome possible errors in measurements.