Motor task and muscle strength followed different adaptation patterns after anterior cruciate ligament reconstruction

The aims of this study were: 1) to investigate the adaptations in walking, running and muscle strength after anterior cruciate ligament (ACL) reconstruction and 2) to examine the interaction between muscle strength and walking or running kinematics at different time intervals following an ACL reconstruction.

Eleven patients 3-6 months postsurgery, 11 patients 6-12 months postsurgery and 13 patients 12-24 months post surgery were filmed using two cameras (50 Hz) whilst walking (1 m/s) and running (2.5 m/s) on a treadmill. In addition, the strength of the muscles surrounding the knee, hip and ankle joints was tested using an isokinetic protocol (60 degrees/s).

The comparison between the injured and the non-injured limbs revealed significant lower knee extension and flexion angles during the stance phase of the injured limb (P<0.05). This was found in the 3-6 month postsurgery period when walking and running and in the 6-12 month postsurgery period when mainly running. The analysis of the muscle strength revealed lower (P<0.05) maximal joint moments for the knee extensors, the knee flexors and the hip flexors of the injured limb during all the observed post surgery periods.

The results of this study indicate that the adaptation of the motor task and the muscle strength follows different time patterns. This suggests that a decrease in muscle strength capabilities can be tolerated up to a certain extent by the patients when walking and during sub maximal running. However, when the decrease in muscle strength exceeds a certain threshold the biological system, being flexible, changes its locomotion strategy.