Evaluation of Reactive Strength Index During Single-Limb Vertical Jumps and Isokinetic Strength Five to Eight Months After Anterior Cruciate Ligament Reconstruction.

Background: Reactive strength index (RSI) derived from single-leg vertical continuous jumps (SVCJ) is a low-cost method to assist in making return-to-sport (RTS) decisions after anterior cruciate ligament reconstruction (ACLR); however, its relationship with isokinetic knee strength at different postoperative time points remains unclear. The purpose of this study was to explore the relationship between the RSI during SVCJ and isokinetic knee muscle strength in patients five to eight months after ACLR. Study design: Retrospective cohort study. Methods: Patients who had undergone primary ACLR with hamstring tendon autografts were enrolled. At five, seven, and eight months postoperatively, the participants performed 15 SVCJ recorded on an iPad at 240 fps. RSI was calculated as jump height divided by ground contact time, with flight time and contact time derived from video analysis using a validated smartphone/tablet application. Isokinetic peak knee extension and flexion torques were measured bilaterally at 60°/s using the Biodex System 3. Limb symmetry index (LSI) was calculated for both isokinetic knee strength and RSI as (operated limb value / non-operated limb value) × 100 (%). Time effects were analyzed using repeated-measures analysis of variance, and Pearson correlations assessed associations between RSI and knee strength. Results: Twelve competitive athletes (mean age, 34.3 ± 11.3 years), participating in cutting- and jumping-based sports were analyzed. Isokinetic knee flexion peak torque significantly improved from five to eight months (p < 0.05) postoperatively. The extension-strength LSI was 75% at eight months, whereas the flexion-strength LSI increased from 84.5% at five months to 93.5% at eight months. The RSI LSI reached 83.1% at eight months. Correlations between RSI and knee extension strength were strong and significant at five months (r = 0.81, p < 0.01) and seven months (r = 0.91, p < 0.01) but were weak and not significant at eight months (r = 0.36, p = 0.26). Conclusion: RSI may serve as a surrogate for isokinetic knee extension strength during midterm rehabilitation after ACLR, later reflecting higher-order explosive performance; smartphone/tablet-based assessment supports staged RTS decision making. Level of Evidence: 2b.