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Background: Our lab previously demonstrated that the manual assessment (with a cell phone stopwatch) of the 30-s Chair Stand, 8-ft Up-and-Go, and 6 Minute Walk Test was as or more valid and reliable than using body-worn sensors. Based on this finding, we developed an Android cell phone application (App) to improve the feasibility of data collection and to automatically calculate outcomes. The main purpose of the present study was to validate the App against slow-motion video analysis for the assessment of outcomes from the 30-s Chair Stand, 8-ft Up-and-Go, and 400 m Walk Test. Methods: A subset of 15 subjects were randomly selected from our larger Steps Taken Against Neuromuscular Decline (STAND) Initiative cohort for this validity assessment. Each subject completed a single trial of the 30-s Chair Stand, 8-ft Up-and-Go, and 400 m Walk Test (in that order). One investigator assessed the subject using the App, while another filmed the test at 240 frames per second. The videos were played back frame-by-frame to manually timestamp the same events that were timestamped live using the App. The mean difference between the App and video (bias) was assessed with paired t-tests, and 95% Limits of Agreement were established to represent the random error between the two measures. Results: No significant bias between the App and slow-motion video was detected for any 30-s Chair Stand or 400 m Walk Test outcome, or for the total time taken to complete the 8-ft Up-and-Go. Paired t-tests did, however, suggest a numerically small but systematic negative bias for all other 8-ft Up-and-Go outcomes, indicating that subcomponents of the 8-ft Up-and-Go are slightly faster when being assessed by the App, compared with slow-motion video. The 95% Limits of Agreement were most narrow for the 30-s Chair Stand and 400 m Walk Test components, as well as total time on the 8-ft Up-and-Go, with a broader limits for other 8-ft Up-and-Go components. Conclusions: The App validly assessed all 30-s Chair Stand and 400 m Walk Test outcomes, as well as the total time taken to complete the 8-ft Up-and-Go. Given that the App is as or more valid than existing technologies, with respect to the outcomes it reports, its greater feasibility may make it an appropriate addition to clinical and fitness settings.

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