The stretch-shortening cycle (SSC) is a fundamental mechanism in explosive human movement, enabling musculotendinous units to store and release elastic energy, thereby allowing the body to produce higher forces and velocities with greater mechanical efficiency. Movements such as sprinting, jumping, hopping, and sudden changes of direction, which depend on rapid force application, are supported by an effective SSC. Because of this, identifying a simple and interpretable index of SSC function has long been a priority in both research and applied sport settings.
The Reactive Strength Index (RSI), most commonly defined as the ratio of jump height to ground contact time, has become the most widely used metric for quantifying SSC performance. However, this ratio presents notable methodological limitations: it combines variables of incompatible dimensions and is insensitive to drop height, thereby ignoring the eccentric load imposed on the musculotendinous system. The Dynamic Rebound Index (DRI), recently proposed as a dimensionless and mechanically grounded alternative, is calculated as DRI = (box height + jump height) / (9.8 × ground contact time²). Despite its theoretical advantages, a direct comparison between the RSI and DRI remains unexplored in the literature, which justifies the relevance of the present study.
This study aims to compare the RSI and the DRI in physically active adults of both sexes during the execution of the Drop Jump at four different drop heights (20, 30, 40, and 50 cm). The specific objectives are: (a) to characterize performance across the four height conditions; (b) to analyze differences between sexes in both indices; (c) to examine the effect of drop height on both indices; and (d) to explore the informative complementarity between the two indices.
Aged 18 years or older.
Physically active (engaging in regular physical activity at least 3 times per week - minimum of 180 min of physical activity).
Any musculoskeletal injuries at the time of data collection.
Any medical condition that could compromise physical performance or safety during testing.
Crossover
Drop Jump Protocol
Other interventionParticipants perform 3 valid Drop Jump trials at each of four drop heights (20, 30, 40, and 50 cm) in a randomized order, with 1 minute of rest between trials and 5 minutes between conditions. Jump height and ground contact time are recorded using a contact mat (ChronoJump, Boscosystem, Spain) for subsequent calculation of the RSI and DRI.
Reactive Strength Index (RSI)
Calculated as RSI = jump height (m) / ground contact time (s), measured during the Drop Jump at four drop heights (20, 30, 40, and 50 cm).
Dynamic Rebound Index (DRI)
Calculated as DRI = (box height (m) + jump height (m)) / (9.8 × ground contact time² (s)), measured during the Drop Jump at four drop heights (20, 30, 40, and 50 cm)
Jump Height
Vertical jump height (m) recorded via contact mat during the Drop Jump.
Ground Contact Time
Time (s) spent in contact with the ground during the Drop Jump, recorded via contact mat.
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