###### Abstract  
The reliability and validity of the Jumpster app was assessed via a comparison to the Vertec during a countermovement jump. Using both tools simultaneously, 36 participants completed 100 total jump trials. Validity was assessed using correlation and tolerance analyses. Reliability was assessed using 95% predictive intervals (PI\({}_{95}\)) and tolerance limits (TL\({}_{95}\)) between the measures, comparing the standard error of the measure (SEM) and coefficients of variation (CVs) for each tool and by examining the intraclass correlation coefficient (ICC 2,K; upper and lower 95% CI) comparing both tools. The Jumpster app was weakly related to the Vertec (\(r=0.24;p<0.01\)). The tolerance analysis showed a moderately strong proportional bias of the Jumpster app to the Vertec (\(r=0.45;p<0.01\)). While all data fell within the calculated PI\({}_{95}\)\(\pm\) TL\({}_{95}\), the Jumpster app SEM (14.7 cm) and CV (40.30%) exceeded the Vertec SEM (3.57 cm) and CV (7.22%), and the ICC was 0.55 [0.79, \(-0.08\)]. These results paired with an overall app failure rate of 15.97% indicate that the Jumpster app is neither reliable nor valid for measuring the countermovement jump.  
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all settings or cost-conservative. The Vertec Vertical Jump Trainer (Sports Imports, Hilliard, OH, USA) has been shown to be valid and reliable when compared to both \"gold standard\" devices [15,17]; however, it too is expensive and requires significant ceiling clearances for proper use. As such, low-cost tools requiring minimal equipment would broaden the use of VJ height as a clinically relevant measure of overall fitness and lower extremity power.  
The advent of smart phones has resulted in a range of new fitness apps in the last decade, including apps designed to measure the VJ. Arguably one of the most popular and well-studied mobile apps used to assess VJ is My Jump 2 ([PERSON], Spain), which is part of a suite of apps included in the My Jump Lab app ([PERSON], Spain). In a range of studies, various versions of this app have consistently been shown to possess high validity and reliability when compared to both the Vertec and electronic timing and video systems [18, 19, 20, 21, 22, 23, 24, 25, 26]. However, the My Jump Lab app's cost may still be a barrier to some would-be adopters. More economical options include the What's My Vertical? app ([PERSON], USA) [27,28] and the JumPo 2 app (University of Brasilia, Lptf, Brazil) [29,30], both of which have also been shown to produce valid and reliable results. Compared to the My Jump 2 app, these options cost considerably less money; however, like the My Jump 2 app these more cost-effective tools also rely on the use of a properly arranged video camera native to the device running the app. This unfortunately adds another possible barrier to their use. To our knowledge, no free VJ apps relying on built-in smart phone accelerometers have been scientifically tested, making such assessments novel. The ability to use a free app that requires no outside help or special set-up procedures could broadly increase use of the VJ test in various clinical and non-clinical sport settings.  
Therefore, the purpose of this study was to assess the validity and reliability of the free Jumpster app (version 1.1; Skyhawk Media, LLC, Palmdale, CA, USA) by comparing the VJ height results obtained during a countermovement jump (CMJ) to that of the Vertec, a common field-accessible tool. Based on pilot data, we hypothesized that the Jumpster app could provide moderate validity and reliability when compared to the Vertec when using appropriately standardized methods.