Intra-Session Reliability of Eccentric Countermovement Jump Kinetics in Individuals Six Months After Anterior...

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.

To evaluate the intra-session reliability and variability of selected eccentric-phase CMJ variables in physically active individuals six months after ACLR.

Crossectional reliability study.

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).

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.

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.

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