Smartphone-Based Assessment of the Stretch-Shortening Cycle: Validity and Reliability of the My Jump Lab App for Measuring the Dynamic Rebound Index.

The aim of the present study was to analyze the validity and reliability of the My Jump Lab (My Jump Lab, Madrid, Spain) smartphone application for measuring the Dynamic Rebound Index (DRI) during drop jump testing. Seventeen physically active participants completed two testing sessions separated by 48 h. In each session, six drop jumps from a 40 cm box were performed while jump height, contact time and DRI were simultaneously recorded using a force platform and the app. Very large to nearly perfect correlations were observed between devices for all variables (r > 0.98). Agreement between methods was excellent, as indicated by the intraclass correlation coefficient (ICC > 0.97) and the concordance correlation coefficient (CCC > 0.93). Bland-Altman analysis revealed small systematic differences and narrow limits of agreement. The root mean square error (RMSE) was low, indicating minimal prediction error. Test-retest reliability between sessions was good for both devices (ICC = 0.825-0.925), and within-session reliability across attempts was high (ICC = 0.705-0.870). These findings indicate that My Jump Lab provides valid and reliable measurements of drop jump performance, including DRI, relative to a force platform, with potential utility in applied settings.