A NEW REACTIVE STRENGTH INDEX FOR TRACK AND FIELD SPRINTING: DIFFERENCES BETWEEN AGE GROUPS.

INTRODUCTION: The Reactive Strength Index (RSI) represents a commonly accepted measurement of the reactive strength, defined as the ability to perform fast stretch-shortening cycles (SSC). Track and field sprinting requires a fast SSC, especially during the maximum velocity phase. The RSI is usually assessed as the ratio between jump height (JH) and contact time (CT) in specific vertical jumps, but some studies suggest that an RSI calculated by a horizontal jump test may be a better predictor of sprint performance [1]. These studies, however, present some methodological limitations in the calculation of the RSI, such as the use of non-standardized horizontal jumps. Therefore, the aim of the present study was to assess a new RSI (RSI RUN) based on CT and stride length (SL) measured in sprint running and then to correlate this index with sprint performance. METHODS: One hundred and five athletes, divided in three groups based on age (Group 1 [G1] elite and sub elite athletes [n=16, age 23.9 ± 3.6 y], Group 2 [G2] junior sprinters [n=22, 15.5 ± 0.6 y] and Group 3 [G3] youth athletes [n= 67, 12.6 ± 1.2 y]), performed a Vertical Stiffness test (VJ, 6 stiffed leg jumps ) and a 60-m sprint. Subsequent analysis considered the best RSI for VJ (RSIVJ), calculated as JH divided by CT, and the RSI for the sprint (RSIRUN), calculated as the average SL divided by the average CT of five steps performed at top speed. RESULTS: A nearly perfect significant correlation was detected between RSI RUN and sprint performance in G1 and G2 (r=0.945 and 0.923, respectively) and a very large correlation was detected in G3 (r=0.754). No significant correlations were observed between RSIVJ and max speed in the three group. Combining the three groups, RSIRUN showed a nearly perfect correlation with performance (r=0.902). In addition, RSIVJ showed a moderate correlation with performance (r=0.481). CONCLUSION: The new horizontal-based RSIRUN showed a greater correlation with sprint performance in athletes of different age compared to vertical-based RSI. On the contrary, RSIVJ calculated by the stiffness test, was moderately correlated with max speed when the whole sample was considered, but not when each group was separately analyzed. In conclusion, the RSIRUN is a better predictor of sprint performance than the stiffness test and the calculation of this parameter may help coaches and sport scientists to assess the characteristics of the athletes, to identify talents and to monitor the influences of training on CT and SL.