6th vital sign app: Testing validity and reliability for measuring gait speed.

Background: Gait speed tests are useful predictors of different health outcomes in people. These tests can be administered by the convenience of one's smartphone.

Research Question: Is the 6th Vital Sign app valid and reliable for measuring gait speed?

Methods: The study used a prospective test-retest design. Fifteen college subjects were asked to walk at their normal pace for 2 min. Each subject performed two trials. Speed was recorded by the 6th Vital Sign app, Brower timing gates, and by hand-measurement of distance walked divided by the 2 min. Criterion validity was assessed by paired t-tests, Cohen's D effect sizes, and Pearson correlation tests. Inter-trial reliability within each device was assessed with Pearson correlation tests.

Results: Speed measured by the app was significantly lower than speed measured by gates (p = 0.004) and by hand-measurement (p = 0.009). The difference between gates and hand-measurement was not significant (p = 0.684). The speed measured by gates and hand-measurement were very highly correlated (r = 0.974), but speed measured by app was only moderately correlated with gates (r = 0.370) and hand-measurement (r = 0.365). The inter-trial reliability was fairly high with correlations r = 0.916, 0.944, and 0.941 when speed was measured by the app, gates, and hand-measurement, respectively.

Significance: The app tended to underestimate speed when compared to gate and hand-measurements. Therefore, we conclude that the 6 Vital Sign app is not valid for use for clinical diagnosis or prognosis.