Introduction: Anterior cruciate ligament (ACL) injuries are frequent among injuries incurred to the knee joint and are affected by extrinsic as well as by intrinsic factors, one of which is anterior knee joint laxity. The stability of the knee joint depends on the function of passive and active stabilizers. While knee flexors may reduce the load imposed on the ACL, knee extensors stretch the ACL. Evidence shows that through various mechanisms, placing a precise and suitable load on the ligament can improve the ligament stiffness. Purpose: Our aim is to elucidate, whether and how a 12-week knee-extensor strengthening program, coupled with passive anterior translation, affects anterior knee laxity. Methods: The Lysholm Knee Scoring Scale questionnaire was filled out before the start and immediately after the training program. We carried out isokinetic dynamometry of muscle strength and hop tests. In order to measure and analyze anterior knee laxity, we utilized the GNRB® arthrometer and carried out additional measurements in week 7 of the program. During the training program, the two patients included in the study, suffering from ACL injuries, performed three training sessions per week. Each session included exercises for strengthening active knee stabilizers and open kinetic chain exercises for strengthening the quadriceps of the injured lower limb with and without added passive anterior translation, produced with a force of 150 N. Results: We noticed the improvement of muscle strength in one patient. No differences were found according to the Lysholm Knee Scoring Scale, anterior knee laxity of the injured leg did not change between the first and third measurement, the average limb symmetry index calculated from hop tests changed from 97,5 % to 97,8 %. We observed that the second patient lost muscle strength, the average limb symmetry index calculated from hop tests changed from 35,8 % to 31,5 %. While the result of the Lysholm Knee Score increased, anterior knee laxity of the injured leg decreased. Discussion and conclusion: The results show that the 12-week knee-extensor strengthening program, coupled with passive anterior translation, may have caused the decreased anterior knee laxity. We believe that in order to make a more comprehensive evaluation of the simultaneous effects of active and passive ACL loading further research in this field should be carried out, focusing on the quantification of induced force on the ACL, on an optimal ratio between active and passive force and on the number of repetitions.
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