Intra-day and Inter-day reliability and validity of the Reactive Strength Index derived from unilateral drop jumps measured on the My Jump 2 app and a force platform

Background: The unilateral drop jump has been proposed as a test for measuring single-limb reactive strength index (RSI), a metric for the ability to rapidly absorb and produce force. RSI is considered important for performance and for guiding rehabilitation in athletes and physically active patients. RSI can be obtained clinically using a simple smartphone app based in video analysis. However, no data exists on the reliability or validity of deriving single- limb RSI from the MyJump2 application.
Purpose / Aim of Study: This study aims to investigate the reliability and validity of MyJump2 compared to a force platform, when measuring
Materials and Methods: Thirty-seven participants (Tegner >5) aged 18-35 years attended two sessions and performed UDJs from three different box heights (15, 20, 25 cm) down onto a force plate in a random order whilst being recorded on a smartphone camera. Minimal detectable change (MDC) was established, and Bland-Alman plots and ICC (intraclass correlation coefficient) scores between instruments were examined for systematic bias.
Findings / Results: Excellent validity was found across all three heights; 15, 20 and 25 cm, respectively (ICC = 0.986, 95%CI:0.976-0.989, p<0.001). However, MyJump2 underestimated the RSI by approximately 0.05 RSI. Inter-rater reliability within MyJump2 showed excellen to near to perfect correlation (ICC = 0.989, 95%CI:0.952- 0.996, p<0.001). Intra-day reliability showed moderate-excellent correlation across all three heights (ICC = 0.810-0.887, p<0.001). Inter-day reliability showed moderate-excellent correlation across all three heights (ICC = 0.805-0.865, p<0.001). Low SB was found between the two instruments. The MDC of the RSI extracted from MyJump2 ranged 0.08-0.18 (10.4-24.25%), with the 25 cm box height having the lowest MDC.
Conclusions: MyJump2 app is valid and reliable compared to a force platform when measuring the RSI of UDJs from different jump heights. The 25 cm box height had the best results indicating that this height would be the best option when testing UDJs. Systematic bias is present between the app and force platform; therefore, practitioners should not compare results across these two instruments.