(56) COMPARISON OF THE REACTIVE STRENGTH INDEX AND THE MODIFIED REACTIVE STRENGTH INDEX IN DIVISION III ATHLETES

Background: For strength and conditioning professionals, it is important to have measurements that can help determine the needs of the individual athlete as it pertains to strength and speed. The reactive strength index (RSI) is a measurement of the ratio between jump height or flight time, and ground contact time while performing a maximal effort drop jump (e.g., off a 30cm box). Additionally, the modified RSI (mRSI) provides the ratio between jump height or flight time, and time-to-takeoff when performing a maximal countermovement vertical jump (CMJ). However, there is minimal data representing how these values can be interpreted among Division III athletes.

Purpose: The purpose of this study was to evaluate the relationship between RSI and mRSI and provide descriptive data reflective of Division III athletes.

Methods: One-hundred sixteen athletes, (males, n=65; female, n = 51), who participate in regular strength and conditioning and sports that involve reactive strength, such as football (n = 57), soccer (n = 23), tennis (n = 19), lacrosse (n = 17) participated in this study. Participants performed a standardized warmup that was consistent with their typical strength and conditioning routine. Then, participants were instructed to perform a series of jumps (CMJ and drop jumps off a 30cm box) while holding a Polyvinyl chloride (PVC) pipe on their shoulders to eliminate arm swing. Prior to performing any jumps, participants were provided with a demonstration and allowed several practice attempts. All jumps were performed on a set of force plates and vertical ground reaction force was collected at a sampling rate of 1000 Hz. The average drop jump data was used to calculate RSI by flight time (RSI_FT) and jump height derived from takeoff velocity (RSI_JH). Similarly, the countermovement jump was used to calculate mRSI by flight time (mRSI_FT) and jump height (mRSI_JH). Pearson’s correlations were used to determine the associations between the RSI variables and descriptive statistics with quartiles to display the 25^th, 50^th, and 75^th percentile.

Results: All four measurements of RSI were significantly correlated (p < 0.001). The strength of the relationship between RSI_JH and mRSI_JH, as well as RSI_FT and mRSI_FT was r = 0.78, and r = 0.87, respectively. Descriptive statistics of RSI performances are provided in Table 1.  

Conclusion: The data are consistent with previous findings that suggest the calculations of RSI and mRSI are not equivalent and the use of jump height or flight time are not interchangeable. PRACTICAL APPLICATION: When utilizing a version of the RSI for a measurement of reactive strength to assess the needs of an athlete, practitioners should be consistent with the way they choose to calculate RSI. Additionally, practitioners working with Division III athletes could utilize the current descriptive data to determine high and low performers and help determine training recommendations.

Acknowledgements: This project was supported by the University of Wisconsin- La Crosse Undergraduate Research & Creativity Grant