Relationship Between Knee Muscle Strength Symmetry and Lower Limb Landing Mechanics Following ACL Reconstruction

BACKGROUND: Restoring muscle strength in the injured knee is a primary rehabilitation goal following anterior cruciate ligament reconstruction (ACLR). However, even when standard rehabilitation criteria, such as muscle strength and hop distance, are achieved, many patients still exhibit landing mechanical asymmetries, suggesting that the conventional limb symmetry index (LSI), although reflective of strength recovery in the involved limb, fails to capture neuromuscular control deficits during functional tasks. Currently, the relationship between knee muscle strength symmetry and landing mechanics symmetry remains unclear. The purpose of this study is to evaluate lower limb mechanic asymmetry during single-leg drop-landing, and to determine the relationship between symmetry in isokinetic knee strength and landing mechanic asymmetry upon return to sport (RTS) following ACLR. METHODS: The cross-sectional study entailed a total of 40 participants at the time of RTS following ACLR (Graft type: hamstring tendon; time since surgery: 9.7 ± 3.4 months) and 20 control participants. Isokinetic concentric contractions of the knee flexors and extensors were performed at an angular velocity of 60°/s on each leg. The LSI [LSI = (involved/uninvolved)[Formula: see text]100%] was calculated for isokinetic quadriceps and hamstring strength, which was then used to divide the ACLR participants into high-symmetry (ACLR-HS, LSI ≥ 85% for both the quadriceps and hamstring) and lower-symmetry (ACLR-LS, LSI < 85% for either the quadriceps or hamstring) subgroups. Three-dimensional kinematic data and ground reaction force (GRF) data were collected for the bilateral lower limbs of all participants during the single-leg drop-landing task. Group differences were compared by use of one-way ANOVA, and Pearson correlations were performed to examine the associations between muscle strength symmetry and landing mechanic asymmetry of the measured variables in three groups. RESULTS: Both the ACLR-HS [peak knee extension moments: p = 0.007; peak vertical ground reaction force (PVGRF): p < 0.001)] and ACLR-LS (peak knee extension moments: p = 0.001; PVGRF: p < 0.001) groups demonstrated lower peak knee extension moments and PVGRF in the involved limb during landing compared with the control group's dominant limb. Compared with the control group, the ACLR-LS group also demonstrated greater asymmetry during landing in knee extension moment (p = 0.001) and knee valgus moment (p = 0.007). No significant correlations were found between quadriceps and hamstring strength symmetry and landing mechanics asymmetry across all variables at RTS among three groups. CONCLUSION: Restoring strength symmetry alone does not guarantee comprehensive functional recovery. To optimize outcomes, clinical decision-making should systematically integrate multidimensional indicators to promote comprehensive functional recovery and ensure a safe RTS following ACLR.