DEFINING THE RELATIONSHIP BETWEEN ISOKINETIC QUADRICEPS STRENGTH AND GROUND REACTION FORCE ASYMMETRIES IN THE DROP VERTICAL JUMP AT SIX MONTHS FOLLOWING PRIMARY ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

Compensatory movement patterns, whereby patients shift loading to the unaffected limb to shield their affected knee, can be persistent following anterior cruciate ligament reconstruction (ACLr). These compensations may result in reduced affected limb vertical ground reaction forces (vGRF) during movements such as jumping. The contribution of underlying impairments in quadriceps strength to these compensations during the bilateral drop vertical jump (DVJ) is unknown. The purpose of this study was to: 1) compare the effect of graft type on vGRFs during the DVJ and 2) assess the relationship between isokinetic, concentric quadriceps strength and vGRF symmetry during the DVJ in patients at six-months following primary ACLr. Participants (n = 164, 27.2 ± 8.2 years old) were assessed at 6.36 ± 0.52 months following primary ACLr (bone-patellar-tendon-bone (BPTB, n = 57), quadriceps tendon (QT, n = 54) and hamstring tendon (STG, n=53)) as a component of a prospective, multi-year trial. The functional assessment included a standardized DVJ protocol (Landing Error Scoring System – Real Time (LESS-RT)) with dual force plates measuring vGRFs. Five repetitions of maximal, isokinetic knee extension (90 degrees per second) were completed after the DVJ. Peak vGRFs were determined in the initial landing, take-off, and secondary landing DVJ phases. vGRFS and peak quadriceps torque were expressed as a limb symmetry index (LSI). One- and two-way ANOVAS were completed to assess the impact of graft type on vGRFS by DVJ phase and isokinetic peak torque. Linear regressions were completed on concentric quadriceps LSI and vGRF LSI in each phase to define the relationship between quadriceps strength and vGRF asymmetries. Quadriceps strength was reduced on the affected limb (mean difference -0.826 nm/kg (95%CI -0.932 - -0.719 nm/kg), p < 0.001) and pairwise comparisons indicated reduced affected limb torque in BPTB (mean difference -0.21 nm/kg (95%CI -0.396 - -0.028 nm/kg), p = 0.03) and QT (mean difference -0.252 nm/kg (95%CI -0.44 - -0.07 nm/kg), p = 0.008) relative to STG. Affected limb vGRFS were reduced regardless of graft type in the initial landing (mean difference -594.0 N, p < 0 .001), take-off (mean difference –162.6 N, p < 0 .001) and secondary landing (-368.6 N, p < 0 .001) DVJ phases. LESS-RT score differed by graft (p = 0.042) as it was marginally higher for BPTB (median 7, range 1-12) or QT (median 7, range 0-12) grafts relative to STG (median 6, range 0-13). Quadriceps LSI accounted for 16.4%, 30.3%, and 11.1% of vGRF asymmetry in the initial landing, take-off and secondary landing DVJ phases respectively (p < 0.001 for all). While quadriceps strength is a contributor to DVJ vGRF symmetry and varied with graft type, vGRF asymmetries were comparable regardless of the graft used. Restoration of quadriceps strength should be prioritized in rehabilitation to improve DVJ performance symmetry. Future work should consider eccentric quadriceps strength, as this may have a stronger relationship with the landing phases of the DVJ due to the heightened eccentric demands. The contribution of other factors that may influence compensatory movement strategies in the DVJ, including kinesiophobia and confidence, should be evaluated.