Validity and Reliability of the Measurement of the Reactive Strength Index during One-leg Continuous Hopping Using an Inertial Measurement Unit

Abstract

[Purpose] This study aimed to verify the validity and reliability of the measurement of the Reactive Strength Index (RSI) during one-leg continuous hopping (OLCH) using an inertial measurement unit (IMU). [Participants and Methods] Seven healthy adults participated in this study. Force plates and inertial sensors were synchronized to analyze RSI during OLCH. The RSI measured by the force plate (RSI-fp) was used as the gold standard, and the RSI obtained by the inertial sensor (RSI-IMU) were analyzed using three original criteria. [Results] The RSI-0G based on the gravitational acceleration in free fall (0G) showed a significant positive correlation with RSI-fp; however, Bland-Altman analysis showed a systematic error. The intra-class correlation coefficient ICC (1,1) of RSI-0G was very high and no systematic errors were found. [Conclusion] RSI-0G demonstrated high reliability but lacked validity, thus it should be utilized as the inertial sensor’s original RSI value.