## 1 Introduction  
Coronavirus Disease 2019 has limited onsite sports training and assessments. The quarantine is more detrimental than the usual transition phase and has induced significant unfavorable effects on physical conditioning [1]. The countermovement jump (CMJ) test is a popular and valid test to monitor neuromuscular status in sports performance. Indeed, jump height is sensitive to acute fatigue [Hedges' g = \(-\)0.27 (95% CI: \(-\)0.48, \(-\)0.05)] and chronic adaptation [Hedges' g = 0.37 (95% CI: 0.32, 0.43)] [2]. Jump height has also been applied to determine physical conditioning in different athletic populations during the pandemic [1, 3, 4]. Comparing other assessments, for instance, sprint tests, CMJ only requires limited space. Its simplicity and effectiveness have been widely recognized by 61% of sports practitioners in individual and team sports as well as fundamental strength training programs [5]. The impaired CMJ performance after a match helped determine soccer players' readiness and implicate recovery [6]. Moreover, jump height is an indicator that is easy to understand and communicate between practitioners and researchers with a comparable coefficient of variation (CV) and sensitivity, similar to other CMJ parameters, for example, peak power, so the gap between researching and practicing would be easier to bridge. [PERSON] et al. [7] reviewed the concurrent validity, test-retest reliability, and applications of portal CMJ devices for field testing. The authors found that applying some measurements to athletes in a remote setting was not feasible due to the equipment's cost and size. In the last decade, mobile applications and cost-friendly portable devices were developed for the sports industry. More importantly, it has been necessary to scientifically validate the tools for collecting physiological and performance data. [PERSON] et al. [8] commented that without validation with the criterion method of assessing CMJ jump height, the new measurement devices should not be assumed to yield valid values. For instance, the Takei Vertical Jump Meter (TVJM), which is a portable wire-type linear encoder and feasible to use in remote assessment, was criticized for its accuracy due to the lack of validation study. Mobile applications and portable apparatus for assessments have the potential to foster remote performance testing for athletes. Indeed, the practice of sports coaching has been changed by the COVID-19 pandemic, and it necessitates the rapid development of remote monitoring of functional performance in sports. Numerous cheaper, non-invasive, and safer instruments have recently been invented to serve this purpose. However, to apply emerging mobile applications in the practical sports field, accuracy and consistency need to be examined. Instead of a general comparison with an expensive and hardly transportable force platform, a concurrent validity study is acceptable to determine the validity of new measurements compared with a well-established test and a previously validated system [9; 10]. My Jump 2 achieved strong concurrent validity in comparison with the force platform (ICC > 0.99) [11; 12] and was recognized as a highly useful instrument for field-based CMJ assessments. It is valid and feasible to adopt My Jump 2 as a criterion instrument for a validity-reliability study in developing vertical jump measuring devices. Considering the availability of scientific equipment, [PERSON] et al. [13] responded that researchers and practitioners might adopt validated measurements where possible, with clear data presentation on the population-specific typical error and minimal detectable change (MDC) reliability data. Previous studies revealed the reliability of jump assessments but did not provide a meaningful value for practitioners [11; 14; 15]. MDC, a value calculated by the standard error of measurement (SEM), is suggested to provide in this test-retest study [16].