Intra-Session Reliability of Eccentric Countermovement Jump Kinetics in Individuals Six Months After Anterior Cruciate Ligament Reconstruction

Background: Neuromuscular impairments, including reduced eccentric force capacity and persistent limb asymmetries, are commonly observed after anterior cruciate ligament reconstruction (ACLR), even during later stages of rehabilitation. Eccentric-phase countermovement jump (CMJ) metrics derived from force plates are increasingly used to monitor neuromuscular recovery after ACLR; however, the intra-session reliability of these eccentric variables in individuals after ACLR remains insufficiently described. Purpose: To evaluate the intra-session reliability and variability of selected eccentric-phase CMJ variables in physically active individuals six months after ACLR. Study Design: Crossectional reliability study. Methods: Eighty physically active participants were assessed six months after ACLR. Intra-session reliability was evaluated using three CMJ trials performed within a single testing session on dual force plates. Eccentric-phase variables were calculated separately for the operated (OP) and non-operated (NOP) limbs: Eccentric Peak Force (EPF), defined as the peak vertical ground reaction force during the braking phase; Eccentric Impulse (EI), defined as the impulse from bodyweight threshold to the instant of zero center-of-mass velocity; and eccentric Rate of Force Development (RFD), defined as the slope of force development over the first 200 ms after exceeding bodyweight. Intra-session reliability was assessed using intraclass correlation coefficients (ICC 3,1) and standard error of measurement (SEM). Measurement variability was evaluated using the coefficient of variation (CV). Results: EPF demonstrated good intra-session reliability for both limbs (ICC = 0.868 [OP], 0.895 [NOP]) with acceptable variability (CV = 7.8% [OP], 6.4% [NOP]). EI also showed good reliability (ICC = 0.868 [OP], 0.840 [NOP]; SEM = 4.3 N·s [OP] and 6.2 N·s [NOP]), although variability was higher (CV = 10.2% [OP], 11.3% [NOP]). Eccentric RFD demonstrated moderate reliability (ICC = 0.620 [OP], 0.715 [NOP]) and high variability (CV = 30.1% [OP], 22.4% [NOP]; SEM = 217 N·s⁻¹ [OP] and 184 N·s⁻¹, [NOP]). Between-limb differences are reported descriptively and were not a primary outcome of the study. Conclusion: In individuals assessed six months after ACLR, eccentric CMJ-derived EPF and EI demonstrated acceptable intra-session relative reliability, whereas eccentric RFD showed lower reliability and greater variability. These findings indicate that EPF and EI may provide more consistent intra-session measurements than eccentric RFD when assessing eccentric-phase CMJ performance after ACLR. Results should be interpreted within the context of a single-session reliability design. Level of Evidence: 3.