Abstract
The decreased internal knee extensor moment is a significant gait asymmetry among patients with anterior cruciate ligament (ACL) deficiency, yet the muscular strategy driving this altered moment for the injured limb is unclear. PURPOSE: This study aimed to determine whether patients with ACL deficiency and characteristic knee instability would demonstrate normal extensor and increased flexor muscle force to generate a decreased internal extensor moment (i.e., use a hamstring facilitation strategy). METHODS: Gait analysis was performed on 31 athletes with acute ACL rupture who exhibited characteristic knee instability after injury. Peak internal knee extensor moment was calculated using inverse dynamics, and muscle forces were estimated using an electromyography-driven modeling approach. Comparisons were made between the injured and contralateral limbs. RESULTS: As expected, patients demonstrated decreased peak knee flexion (P = 0.028) and internal knee extensor moment (P = 0.0004) for their injured limb but exhibited neither an isolated decrease in extensor force (quadriceps avoidance) nor an isolated increase in flexor force (hamstring facilitation) at peak knee moment. Instead, they exhibited decreased muscle force from both flexor (P = 0.0001) and extensor (P = 0.0103) groups. This strategy of decreased muscle force may be explained in part by muscle weakness that frequently accompanies ACL injury or by apprehension, low confidence, and fear of further injury. CONCLUSIONS: This is the first study to estimate muscle forces in the ACL-deficient knee using an electromyography-driven approach. These results affirm the existence of neuromuscular asymmetries in the individuals with ACL deficiency and characteristic knee instability.