A Mixed-Methods Approach to Evaluating the Internal Validity of the Reactive Strength Index

Area of Science:

• Biomechanics
• Human Movement Science
• Sports Science

Background:

• The reactive strength index (RSI) is a key metric for assessing muscle-tendon complex reactive capacity.
• Conventional RSI calculation relies on assumptions about linear mechanics and jump height estimation, potentially impacting validity.
• Previous research suggests a mixed-methods approach may provide a more accurate RSI estimation.

Purpose of the Study:

• To investigate the internal validity of RSI scores computed using conventional and mixed-methods approaches.
• To examine the impact of different drop heights on RSI calculations.
• To compare estimated impact velocities with theoretical values.

Main Methods:

• Seventy physically active young adults performed depth jumps from three different heights (0.51, 0.66, 0.81 m).
• RSI was calculated using both a conventional method and a mixed-methods approach incorporating 2D videography, body segment parameters, and force platform data.
• Statistical analyses included linear regression, ANOVA, and one-sample t-tests to compare the methods and assess impact velocities.

Main Results:

• No significant main effects or interactions were found between computational approaches in the ANOVA (p = 0.467–0.938).
• Linear regression showed moderately strong associations (R² = 0.685–0.741) between the conventional and mixed-methods RSI scores.
• The conventional method tended to overestimate RSI for scores below 1.0 and underestimate for scores above 1.0, with estimated impact velocities up to 13% lower than theoretical (p < 0.001).

Conclusions:

• Researchers with access to motion capture and force platforms should consider the mixed-methods approach for enhanced RSI validity.
• Practitioners should exercise caution when comparing conventional RSI scores across individuals due to potential over/underestimation.
• The findings highlight the importance of computational methodology in accurately assessing reactive strength.