## 4 Discussion  
To our knowledge, this is the first study to examine if the Jumpster app, which relies on the built-in accelerometer of a smart phone, provides comparable results to the Vertec Vertical Jump Trainer system in assessing CMJ height. Based on prior pilot data, we hypothesized that the Jumpster app would prove to be moderately reliable and valid compared to the Vertec. Unfortunately, with an overall failure rate just over 15% and significantly greater internal and external inconsistency (when compared to itself and the Vertec respectively), our results indicate that the app is neither reliable nor valid. When measuring jump height, one expects that different devices will provide similar (i.e., non-significantly different) results that are closely correlated to each other. In our study, the Jumpster app failed to show the significantly lower jump height for women that was measured by the Vertec. Moreover, the weak correlation between the two device measures paired with comparisons of our assessed proportional bias to the Vertec limits of variability all indicate poor validity of the Jumpster app. Absolute reliability appears similarly impaired, noting a more than 4-fold larger SEM and CV for the Jumpster app. A close inspection of these data indicates that the app underestimates on the low-end of the scale and overestimates on the high-end of the VJ height scale. Whether such data clustering is random or indicative of our specific study population is, however, beyond the scope of this work. And while the ICC value (0.55) initially indicated at least moderate relative reliability of the Jumpster app when compared to the Vertec, the wide 95% CIs (upper bound: 0.79; lower bound: \(-\)0.08) immediately undercut this interpretation providing additional evidence that the app is not reliable. These combined results lead us to discourage the use of the Jumpster app at this time to assess VJ height as it was found to be invalid and unreliable with each planned analysis utilized in this study. While there is a continuing need in sports science to innovate simple and affordable field tools to measure performance, it is essential that such devices are adequately reliable and valid. A significant body of literature exists for movement scientists to endorse the use of various video-based VJ apps [19; 20; 21; 22; 23; 24; 25]. The relative \"gold standard\" among this class of VJ apps appears to be My Jump 2, which was developed more than a decade ago. This app relies on positioning a smart phone, iPad, or Mac computer to record live video with a native camera, allowing users to ascertain flight time, which is then used to calculate jump height [23]. In a recent meta-analysis, [PERSON] et al. [23] synthesized 21 studies consisting of 839 participants and revealed a high level of agreement between the My Jump app and criterion measures with near perfect reliability. More recent data using the What's My Vertical? and Jumpo apps have also shown good validity and high reliability. In contrast, the Jumpster app relies on indwelling accelerometers and is advertised as an accurate and easy to use tool, allowing one to simply slip their phone in their pocket and measure their VJ. Our study was designed to test the app under more rigorous standardized conditions with a more secure harness than would be used in the field. Despite these efforts, the app failed to produce either reliable or valid results. As noted in our methods, during pilot testing, placing the phone in a pocket could produce excess movement or involuntarily ejection of the phone, so a more uniform restraint was employed, which itself needed to be further secured after some additional pilot test failures. Even after minimizing these extra movements of the device, the app proved unsuccessful in producing any result several times during the study. The initial question of whether the accelerometers in the iPhone are valid and reliable was posed.