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