Quadriceps Strength and Landing Symmetry Following ACL Reconstruction

Abstract

PURPOSE: Quadriceps strength asymmetry is common following anterior cruciate ligament reconstruction (ACLR). However, quadriceps strength symmetry may not reflect symmetry in other tasks such as a landing. The purpose of this study was to 1) examine the relationship between quadriceps strength symmetry and landing symmetry and 2) to compare landing mechanics between limbs of individuals with ACLR. METHODS: Quadriceps strength and landing biomechanics were assessed in 46 individuals with primary unilateral ACL reconstruction (34 females, age=22.0±2.8 years, height=1.70±.09m, mass=71.9±16.1kg, IKDC=86.1±9.3). Participants completed 3 drop jump landings from a 30cm height located at a distance of 50% of their height onto 2 force plates. Quadriceps strength was assessed via isometric (peak and rate of torque development (RTD)) and isokinetic knee extension at 60°, 180°, and 240°/second. Limb symmetry index (LSI) was calculated as the ratio of the involved divided by the uninvolved limb for strength and landing biomechanics (knee flexion angle (KFA) (at ground contact, peak, and excursion), peak external knee flexion moment (KFM), and vertical ground reaction force (GRF)). Pearson correlation was used to assess the relationship between quadriceps LSIs and KFM and GRF LSIs. Spearman rho was used to examine the relationship between quadriceps LSIs and KFA LSIs. Paired samples t-tests were used to compare dependent variables between limbs (α=0.05). RESULTS: Isometric strength LSI (r=0.30, p=0.05) and RTD LSI (r=0.37, p=0.01) were associated with KFM LSI. Isometric strength LSI (rho=0.34, p=0.02) and isokinetic strength LSI at 60° (rho=0.40, p=0.01) and 180° (rho=0.31, p=0.05) were associated with knee flexion excursion LSI. Isokinetic strength at 180° (r=0.39, p=0.01) was associated with GRF LSI. Uninvolved limbs had greater GRF (2.48±0.77 vs. 2.23±0.69 BW, p=0.034) and KFM (0.17±0.04 vs. 0.14±0.04 %BW*height, p<0.001) compared to involved limbs. CONCLUSIONS: ACLR limbs had smaller GRF and KFM compared to uninvolved limbs. This may indicate a compensatory strategy to underload the involved limb during landing. The weak correlations between quadriceps strength LSI and landing LSI may indicate that other factors such as impaired neuromuscular control or fear of re-injury influence landing symmetry.