Strength does not influence knee function in the ACL-deficient knee but is a correlate of knee function in the and ACL-reconstructed knee.

Purpose: Knee function, whether anterior cruciate ligament (ACL)-deficient or ACL-reconstructed, is related to many conditions, and no single biomechanical variable can be used to definitively assess knee performance. The purpose of this study was to investigate the relationship between extension and flexion muscle strength and knee function in patients prior and following ACL reconstruction.

Methods: 44 ACL-deficient patients with a mean age of 26.6 years were tested between 3 and 6 months following an acute injury and 2 years following ACL reconstruction. All reconstructed patients underwent surgical reconstruction within 6 months of ACL injury using bone-patellar tendon and interference screws. The Cincinnati knee rating system was used to assess knee function. Muscle strength was assessed with the Biodex™ Dynamometer. Isokinetic concentric and eccentric flexion and extension peak torque (Nm/kg) was tested at three different speeds: 60°/s, 120°/s and 180°/s. Isometric strength was tested in 30° and 60° of knee flexion. Both the involved and non-involved legs were tested to calculate symmetry indices.

Results: The mean Cincinnati score in the ACL-deficient patient was 62.0 ± 14.5 (range 36-84) and increased to 89.3 ± 9.5 (range 61-100) in the ACL-reconstructed patient. Significant relationships between knee function and muscle strength in the ACL-deficient group were observed for knee symmetry indices (r = 0.38-0.50, p = 0.0001-0.05). In the ACL-reconstructed group significant relationships between knee functionality were observed for isometric and isokinetic peak torque of the involved limb (r = 0.46-0.71, p = 0.0001-0.007).

Conclusion: The findings of this study suggest that neither extension nor flexion peak torque were correlates of knee function in the ACL-deficient knee. However, leg symmetry indices were correlated to knee function. In the ACL-reconstructed knee, knee symmetry indices were not related to knee function but extension and flexion isokinetic concentric and isometric peak torque were.