Relationship Between Quadriceps Neuromuscular Activity and Knee Extensor Strength at Four Months After ACL Reconstruction in High-Level Young Athletes.

Background: Restoring knee extensor strength after anterior cruciate ligament (ACL) reconstruction using a quadriceps tendon (QT) autograft remains challenging. Whether altered quadriceps neuromuscular activity contributes to this weakness is unclear. Hypothesis: Quadriceps strength recovery after ACL reconstruction using QT would be impaired owing to altered quadriceps neuromuscular activity. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 31 patients (Tegner activity scale score ≥7) who had undergone ACL reconstruction 4 months earlier were enrolled into either the QT group (n = 15) or hamstring tendon (HT) group (n = 16). Knee extensor strength was assessed using maximum voluntary isometric and isokinetic (60 deg/s) force. Neuromuscular activity of the vastus lateralis (VL) and vastus medialis (VM) during isometric contraction was recorded with high-density surface electromyography to evaluate motor unit discharge rate (MU DR). A generalized linear mixed-effects model was used to test side (involved and uninvolved limbs) and task (25%, 50%, and 70% of isometric contraction) effects, and regression analysis was used to examine associations between side-to-side isometric strength and MU DR differences. Results: Baseline characteristics did not differ significantly between the QT (9 males, 6 females; mean age, 17.1 ± 3.9 years) and HT (10 males, 6 females; mean age, 18.2 ± 2.7 years) groups. The mean time from surgery to testing was also similar (QT group: 120.8 ± 8.0 days; HT group: 121.3 ± 7.2 days). The limb symmetry index for isometric strength showed no difference (QT group: 75.0% ± 19.6%; HT group: 78.2% ± 14.3%; P = .63). In contrast, isokinetic strength at 60 deg/s was significantly lower in the QT group (70.5% ± 13.2%) than in the HT group (83.9% ± 11.6%) (P < .01). In the QT group, MU DR showed a side × task interaction, with higher discharge in the involved (11.7 ± 3.2 pluse per second [pps]) versus uninvolved (10.4 ± 1.9 pps) limbs during 70% isometric contraction in VM (P < .01). Correlations were observed between isometric strength differences and MU DR in both VL (r = 0.65; P = .01) and VM (r = 0.67; P = .01). Conclusion: Neuromuscular changes were evident in the QT group, particularly in the VM, but not in the HT group. Notably, in the QT group, altered MU activity was correlated with muscle strength decline.