Reactive Strength Index Modified Differentiates Starters and Non-Starters in Female Volleyball National Team Players

Jumping ability is crucial in volleyball, where both jump height and execution speed significantly influence performance. Previous research has yielded mixed findings on the role of jump force-time characteristics in determining starting status among female volleyball players. This study examined the relationship between countermovement jump (CMJ) force-time metrics and starting status in elite female volleyball players to identify key performance indicators. Nine national team athletes (five starters, four non-starters) performed CMJs on a force platform. Force-time variables from distinct jump phases were analyzed. Reactive strength index modified (RSImod)—the ratio of jump height to time to take-off—was used to assess lower-body explosiveness and reactive strength. Starters demonstrated significantly higher RSImod scores and relative propulsive mean force, indicating superior ability to generate force rapidly. Moderate-to-large effect sizes favoring greater relative strength (force and power normalized to body mass) were observed but were not statistically significant. Reactive strength, as reflected by RSImod, is a key differentiator of starting status in elite female volleyball players. Relative strength also appears to contribute to performance but requires further exploration. Training programs should prioritize lower-body power and reactive strength through targeted plyometric and resistance exercises to enhance volleyball performance.