A complete rupture of the anterior cruciate ligament (ACL) results in static and often in functional instability. After an ACL-injury, most patients have to decrease their activity level, while some can cope with their injury and manage to keep a high activity level. A better tmderstanding of the influence of muscle activation and external load on the non-injured and injured knee, but also of the adaptive motion strategies employed by the patients with an unstable knee, is of importance for the construction of rehabilitation programs.

The general purpose of this thesis was to study certain knee motions during activities focused on rehabilitation, in individuals with anterior cmciate ligament injury and uninjured controls. The sagittal tibial translation and knee flexion angle were registered during motion with the CA-4000 computerized electrogoniometer and EMG was used for analysis of muscle activity. The injured knee had increased static laxity compared to the contralateral non-injured knee and the non-impaired group.

The amount of tibial translation increased with increased quadriceps torque during nonweight bearing activities (isokinetic and isotonic contractions). In the non-injured knees, the restraining mechanism was engaged above 50% of isokinetic concentric quadriceps torque and during the eccentric contractions already at low quadriceps torques (<10% of maximum). The motion pattern was similar in the injured knees and the amount of individual translation was larger during the eccentric contractions, compared to the non-injured knees.

During weight bearing, tibia translated anteriorly and remained in that position. In the non-injured knee, different loads and performance of weight bearing activities produced different amount of translation. Level walking and squat with the center of gravity behind the feet produced somewhat smaller amount of translations, while other weight bearing activities produced similar amount of translation compared to the non-weight bearing isotonic knee extension. In the injured knee, all weight bearing activities produced similar amount of translation, smaller compared to the non-weight bearing isotonic knee extension.

Quadriceps - hamstrings co-activation was not present during non-weight bearing quadriceps dominant activities and therefore, it was not shown to be a factor limiting anterior tibial translation. Also, during the weight bearing activities, hamstrings eo-contraction could not limit the anterior tibial translation. Quadriceps and gastrocnemius seems to work synergistically to stabilize the knee by maintaining an anterior position of tibia.

In the injured knee, isokinetic concentric quadriceps contraction below 50% of maximum torque produced the smallest amount of tibial translation. Weight bearing exercises produced similar ammmt of translation despite of load and performance, indicating that when level walking is allowed after an injury, other weight bearing activities can be performed without increasing the amount of translation. Eccentric isokinetic quadriceps contractions above 50% of maximum torque and isotonic quadriceps exercise with and without weights at flexion angles less that 40°, should be avoided in order protect the knee joint.