Summary

After undergoing ACL reconstruction, individuals have movement impairments related to re-injury risk. Identifying factors that impact these movement impairments is important for safe return to play. In this cohort, quadriceps strength symmetry was predictive of movement quality during a drop vertical jump whereas absolute quadriceps strength was not. Assessing bilateral quadriceps strength may pro

Abstract

Objective

Anterior cruciate ligament reconstructions (ACLR) have increased in incidence year over year since 2002. The primary goal for many patients is returning to sport. Unfortunately, 30% of individuals have a second knee injury within 2 years of returning to sport, which has been associated with abnormal movement patterns. Monitoring between limb comparisons of strength is recommended to understand readiness for return to sport. It is unknown if absolute involved limb quadriceps strength may provide additional information to activity performance or re-injury risk. The purpose of this study was to examine the relationship between absolute quadriceps strength and interlimb ratios of quadriceps strength with performance of a bilateral drop vertical jump (DVJ) after ACLR.

Methods

51 individuals who were 9.9±2.4 months post-ACLR, 49% female, 17.2±2.7 years old, had a BMI of 24.6±4.0 kg/m2 passed a clinic testing battery that included >90% limb symmetry during isometric quadriceps strength, 1-rep max from 90° to 0° of extension, and 4 single-legged hop tests, as well as >90% on the Global Rating Scale and IKDC 2000. Individuals then completed a laboratory assessment. Quadriceps strength was bilaterally assessed isometrically (90° of knee flexion) and isokinetically (60°/second) (Biodex System 4 Pro. Shirley, NY, USA). Peak isometric knee extension, isometric rate of torque development (RTD), and peak isokinetic knee extension were measured and normalized to body weight. Individuals then performed 5 instrumented bilateral DVJs. Kinematic (240 Hz) and kinetic (2160 Hz) data were collected using an 8-camera system (Qualysis, Goteborg, Sweden) with two embedded force plates (Bertec Corp., Columbus, OH, USA). Peak external knee flexion moment (KFM) and peak vertical ground reaction force (vGRF) during the first landing phase as well as peak knee extension power (KEP) during the propulsion phase during the first ground contact phase for each limb were calculated. Interlimb ratios (ILR, involved/uninvolved) were calculated for all strength and biomechanical variables. Three separate backwards stepwise linear regressions were used to identify strength predictors of DVJ biomechanical ILRs. Six strength indexes (ILRs and injured limb values normalized to mass) were entered as independent variables with a set to 0.05 a priori. Independent variables with a p-value <0.05 were removed from the model.

Results

ILRs of KFM, KEP, and vGRF were predicted by models including ILR of quadriceps strength but not with absolute strength values of the involved limb. A higher isokinetic quadriceps strength ILR predicted a higher KFM ILR during the landing phase of bilateral DVJ (R2 = 0.120, p = 0.013). Both higher ILR of knee extension RTD and isokinetic strength predicted higher KEP ILR. (R2 = 0.340, p < 0.001). Higher ILR of knee extension RTD predicted higher vGRF ILR. (R2 = 0.307, p <0.001).

Conclusion

ILRs of quadriceps strength provide more insight into biomechanical symmetry during a bilateral DVJ than absolute strength values on the involved side. When deciding on return to sport readiness to minimize reinjury risk, having bilateral measurements may provide the medical team with information that will better predict biomechanical variables associated with re-injury.