Reactive Strength and Dynamic Rebound: A Drop Jump Index Comparison

Which Index Tells the Full Story? Comparing Reactive Strength and Dynamic Rebound Index in Drop Jump Performance

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.

Study Overview

• Status: Not yet recruiting
• Conditions: Physical Performance Parameters
• Intervention / Treatment: Other: Drop Jump Protocol

• Study Type: Interventional
• Enrollment (Estimated): 50
• Phase: Not Applicable

Contacts and Locations

Study Locations

• Portugal
  • Porto District
    • Maia, Porto District, Portugal, 4475-690 Maia
      • University of Maia
      • Contact: Henrique S Sousa, MSc; Phone Number: +351 22 986 6000; Email: hsousa.dcd@umaia.pt

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• 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).

Exclusion Criteria:

• Any musculoskeletal injuries at the time of data collection.
• Any medical condition that could compromise physical performance or safety during testing.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Other
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: None (Open Label)

Number of Arms

4

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Drop Jump 20 cm; Participants perform 3 valid Drop Jump trials from a 20 cm drop height	Other: Drop Jump Protocol; Participants 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.; Other Names: DJ
Experimental: Drop Jump 30 cm; Participants perform 3 valid Drop Jump trials from a 30 cm drop height	Other: Drop Jump Protocol; Participants 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.; Other Names: DJ
Experimental: Drop Jump 40 cm; Participants perform 3 valid Drop Jump trials from a 40 cm drop height	Other: Drop Jump Protocol; Participants 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.; Other Names: DJ
Experimental: Drop Jump 50 cm; Participants perform 3 valid Drop Jump trials from a 50 cm drop height	Other: Drop Jump Protocol; Participants 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.; Other Names: DJ

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
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).	Baseline
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)	Baseline

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Jump Height	Vertical jump height (m) recorded via contact mat during the Drop Jump.	Baseline
Ground Contact Time	Time (s) spent in contact with the ground during the Drop Jump, recorded via contact mat.	Baseline

Collaborators and Investigators

• Sponsor: University Institute of Maia

Study record dates

Study Major Dates

• Study Start (Estimated): March 1, 2026
• Primary Completion (Estimated): July 1, 2026
• Study Completion (Estimated): July 1, 2026

Study Registration Dates

• First Submitted: March 10, 2026
• First Submitted That Met QC Criteria: March 13, 2026
• First Posted (Actual): March 16, 2026

Study Record Updates

• Last Update Posted (Actual): March 16, 2026
• Last Update Submitted That Met QC Criteria: March 13, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Other Study ID Numbers: DRI_HS_01

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: UNDECIDED

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No