Validity and reliability of an iPhone App to assess time, velocity and leg power during a sit-to-stand functional performance test

The purposes of this study were: (i) Analyze the concurrent validity and reliability of an iPhone App for measuring time, velocity and power during a single sit-to-stand (STS) test compared with measurements recorded from a force plate; and (ii) Evaluate the relationship between the iPhone App measures with age and functional performance. Forty-eight healthy individuals (age range: 26-81 years) were recruited. All participants completed a STS test on a force plate with the movement recorded on an iPhone 6 at 240 frames-per-second. Functional ability was also measured using isometric handgrip strength and self-paced walking time tests. Intraclass correlation coefficients (ICC), Pearson's correlation coefficient, Cronbach's alpha (α) and Bland-Altman plots with 95% confidence intervals (CI) were used to test validity and reliability between instruments. The results showed a good agreement between all STS measurement variables; time (ICC = 0.864, 95%CI = 0.77-0.92; α = 0.926), velocity (ICC = 0.912, 95%CI = 0.85-0.95; α = 0.953) and power (ICC = 0.846, 95%CI = 0.74-0.91; α = 0.917) with no systematic bias between instruments for any variable analyzed. STS time, velocity and power derived from the iPhone App show moderate to strong associations with age (|r| = 0.63-0.83) and handgrip strength (|r| = 0.4-0.64) but not the walking test. The results of this study identify that this iPhone App is reliable for measuring STS and the derived values of time, velocity and power shows strong associations with age and handgrip strength.