Control of knee stability after ACL injury or repair: Interaction between hamstrings contraction and tibial translation

Objective. The aim of this study was to investigate the interaction between hamstrings contraction and tibial translation during isometric quadriceps contraction (IQC) and their effects on cruciate ligament forces. Methods. A mathematical model of the knee in the sagittal plane was used with simple geometric representation for the model muscles, ligaments and bones. A mechanical analysis was used to calculate muscle, ligament and contact forces. Results. The results show that isolated IQC requires a posterior force applied to the tibia at the knee over the range 0-85°flexion, usually applied by the anterior cruciate ligament (ACL). The model hamstrings can provide this posterior force, except near extension, resulting in reduced anterior tibial translation (ATT) and, therefore, reduced ACL force. ATT varied inversely with the hamstrings force. The interaction between the two was highly dependent on flexion angle, the hamstrings force being least effective in reducing ATT near extension. Conclusions. Flexing loads placed distally on the tibia can stretch an injured or repaired ACL. The analysis suggests that modest hamstrings forces during IQC can help protect the ACL. In the complete absence of an ACL, hamstrings contraction as well as ATT may be inevitable while using quadriceps near extension.