Association Between Reactive Strength Index During Single-Limb Vertical Continuous Jumps and Isokinetic Knee Extensor Strength Following Anterior Cruciate Ligament Reconstruction

OBJECTIVE: To analyze the association between asymmetry in the reactive strength index (RSI) during single-limb vertical continuous jumps (SVCJs) and isokinetic knee extensor strength in athletes who have undergone anterior cruciate ligament reconstruction (ACLR). DESIGN: Cross-sectional study. METHODS: This study included 75 athletes (40 female athletes) aged 23.7 ± 8.6 years who had undergone ACLR (mean time from ACLR to measurement: 8.8 ± 4.1 months). The ground contact time and flight time during SVCJs were measured using a force plate, and jump height was calculated from the flight time. The RSI during SVCJs was calculated as follows: RSI = jump height (cm)/contact time (s). The knee extensor strength was measured using an isokinetic dynamometer at 60°/s. The limb symmetry index (LSI) of RSI during SVCJs and knee extensor strength were calculated by dividing the values of the involved and uninvolved limbs. Age, sex, modified Tegner activity scale, and subjective knee function were also recorded. Univariable and multiple regression models were used to assess the association between these and the knee extensor strength. RESULTS: The involved limb showed significantly lower isokinetic knee extensor strength ( P<0.001, effect size = 0.55) and RSI during SVCJs ( P<0.001, effect size = 0.70) than did the uninvolved limb. In the multiple regression model, only LSI of RSI during SVCJs remained significant (B = 0.440, SE = 0.068, β = 0.604) and accounted for 36.4% of the variance in knee extensor strength recovery ( P<0.001, r 2 = 0.364). CONCLUSION: RSI asymmetry during SVCJs was strongly associated with isokinetic knee extensor strength asymmetry. JOSPT Open 2025;3(2):154-161. Epub 5 November 2024. doi:10.2519/josptopen.2024.0052