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 degrees 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. Relevance Some ACL-deficient patients can maintain a high level of functional activity; others fail to cope with the effects of deficiency. Understanding the possible mechanisms for the varied outcome has implications for both conservative treatment and rehabilitation after ACL reconstruction. The analysis suggests that the patients who cope may be those who have learnt the appropriate balance between ATT and hamstrings co-contraction. (C) 1998 Elsevier Science Ltd. All rights reserved.