Associations between countermovement jump impulse and running kinetics following ACL reconstruction

Persistent deficits in lower-limb loading are commonly observed following anterior cruciate ligament reconstruction (ACLR) and may influence running biomechanics. Countermovement jump (CMJ) impulse can provide a phase-specific measure of force-time characteristics to reflect limb loading capacity; however, its relationship with running kinetics remains unclear. This study aimed to quantify phase-specific CMJ impulse following ACLR and examine associations with vertical ground reaction force (vGRF) metrics during treadmill running at the return-to-running phase. Forty-two participants performed three unilateral and bilateral CMJs on dual force plates and ran at 8 km/hr on an instrumented treadmill. Concentric, eccentric braking, and landing impulses were calculated. Running variables included weight acceptance rate, weight acceptance peak force, and maximum applied force. Inter-limb differences were assessed using Wilcoxon/paired t-tests. Impulse-vGRF relationships were analysed using Pearson/Spearman correlations and multiple linear regression. The ACLR limb exhibited significant deficits across all unilateral and bilateral CMJ phases, except for unilateral landing impulse. Deficits were greatest for unilateral concentric impulse (Δ1.3Ns·kg^-0.67, d = 1.49, p < 0.001). Maximum applied force limb symmetry index (LSI) was explained by unilateral concentric impulse LSI (adjusted R² = 0.25, p < 0.001). Maximum applied force of the affected limb was explained by unilateral concentric impulse (adjusted R² = 0.38, p < 0.001). Weight acceptance peak force of the affected limb was explained by unilateral concentric and landing impulses (adjusted R² = 0.37, p < 0.001). No meaningful relationships were observed for eccentric impulses or weight acceptance rate metrics. These findings indicate that jump impulse testing may provide clinically relevant indicators of running kinetics.