(7) PRELIMINARY EVALUATION OF AN UPPER BODY REACTIVE STRENGTH INDEX

Purpose: The purpose of this study was to examine trial-to-trial consistency of a novel upper body reactive strength index (UB-RSI). Methods: 11 males (age: 23.7 ± 2.5 years, mass: 91.0 ± 9.4 kg, height: 1.83 ± 0.05 m) and 8 females (age: 22.5 ± 1.2 years, mass: 71.7 ± 14.4 kg, height: 1.65 ± 0.10 m) volunteered to participate in this cross-sectional study. Participants needed to be between 18-30 years of age, engage in regular exercise, not have a history of significant upper body injury or surgery, and be able to perform a plyometric pushup. Prior to testing, participants cycled on an upper-body ergometer for 3 minutes (90 seconds forward, 90 seconds backward) and then completed five standard bodyweight pushups, as a warm-up. After 2 practice trials, subjects completed 5 plyometric pushups on a force plate that recorded ground reaction force data at 600 Hz. For each trial, subjects started in a pushup position with their hands shoulder-width apart. They then performed a rapid upper body countermovement to their preferred depth and forcefully executed a pushup so that their hands left the surface of the force plate. Subjects were instructed to focus on maximizing their pushup flight height. A 45-second rest period was included between trials to limit fatigue. The UB-RSI metric was generated for each trial using the vertical ground reaction force (vGRF) data by dividing pushup height (based on flight time) by the time-to-takeoff, which is the time from initiation of the countermovement until takeoff (UB-RSI = pushup height [cm] / time-to-takeoff [s]). Countermovement initiation was identified by finding the point where the vGRF dropped below a threshold of 7 times the standard deviation of the vGRF signal as the subject remained still in the initial pushup position for at least 1 second prior to a trial. Takeoff was identified as the point where the vGRF dropped below 20 N. To examine the trial-to-trial consistency of the UB-RSI an intraclass correlation coefficient was calculated (ICC [2,1]), along with its 95% confidence interval (95% CI). The following guidelines were used to interpret the ICC value: < 0.50 = poor consistency, 0.50-0.75 = moderate consistency, 0.75-0.90 = good consistency, >0.90 = excellent consistency. An independent t-test was also conducted to compare the UB-RSI values between males and females (alpha level = 0.05). Results: The ICC value for the UB-RSI was 0.94 (CI 95% = [0.89, 0.97]), which reflects excellent trial-to-trial consistency. Males produced higher UB-RSI values compared to females (p< 0.001; 10.4 ± 4.1 cm/s vs. 3.8 ± 1.2 cm/s), indicating that males had greater upper body reactive strength. Conclusions: The UB-RSI exhibited excellent trial-to-trial consistency and detected sex differences in upper body reactive strength. Practical Applications: The modified reactive strength index (mRSI) is commonly used to assess lower body reactive strength during a countermovement jump. The mRSI is calculated by dividing jump height by the time from countermovement initiation until takeoff. The UB-RSI is an analogous metric to assess upper body reactive strength. The UB-RSI could be used to assess upper body reactive strength among athletes who participate in sports that require “explosive” upper body movements, such as football linemen who need to hand strike opponents. It could also be a useful metric for monitoring recovery of upper body reactive strength for athletes recovering from upper body injury. Acknowledgements: None