Patients with anterior cruciate ligament (ACL) injury often suffer knee instability, which can be visualized as a wobbling knee during weight bearing exercises, such as the single leg squat. We propose to use the single-leg-squat-and-hold (SLSH) task with kinematic analysis to objectively evaluate dynamic knee stability in ACL injured patients. The aim of this study is (1) to compare knee kinematic variations capturing knee wobbling during SLSH between ACL subjects and healthy controls; and (2) to detect the changes in knee kinematic variations during SLSH following ACL reconstruction.

Injuries to the anterior cruciate ligament (ACL) are common in pivoting sports such as soccer, handball, basketball and rugby. The ultimate goal of ACL reconstruction for most athletes is return-to-play. Despite successful surgery and a demanding rehabilitation process, some athletes still failed to return to play, and for those who returned to play, some would suffer a second ACL injury. With surgical reconstruction of the ACL, static knee stability can be restored, but dynamic knee stability may not . Patients with ACL injury often experience symptoms such as knee instability, feelings of "giving way" during activities. Biomechanically, dynamic knee stability is considered as the ability to control the relative tibiofemoral displacements during loading.

Regular monitoring of the knee stability before and after ACL reconstruction is necessary for medical personnel to design the most suitable rehabilitation program. In contrast to jumping and hopping tasks, squatting tasks present a similar weight bearing profile during knee flexion, but without the potentially risky landing phase. As poor knee dynamic knee stability can be visualized as a wobbling knee (frequent knee movements). In order to quantitatively and objectively evaluate dynamic knee stability from observing the knee wobbling motion, we propose a single leg squat and hold task(SLSH) with kinematic analysis. Kinematic curve derived from the SLSH involves both the magnitudes and the frequency of the motion fluctuation. This study aims (1) to compare knee kinematic variations capturing knee wobbling during SLSH between ACL-deficient patients who are scheduled for ACLR and healthy controls; and(2)to detect the changes in knee kinematic variations during SLSH following ACLR. We hypothesize that the ACL-deficient patients demonstrate greater knee kinematic variation than the healthy subjects and the knee kinematic variation will reduce after ACLR.

Inclusion Criteria:

1. Aged 18-35;
2. Participate in level I or level II sports with a Tegner score of more than 6 before injury(level I: sports involving jumping, cutting and pivoting, e.g.football, basketball etc; Level II: sports involving lateral movements, less pivoting than Level I eg. racket sports)
3. Scheduled for ACLR because of unilateral ACL injury;
4. Failure to meet any single criteria for a potential coper;
5. Contralateral knee without history of injury.

Exclusion Criteria:

1. Concomitant fracture, meniscus injury or full-thickness chondral injuries;
2. preoperative radiographic signs of arthritis;
3. Revision ACL surgery;
4. With ankle pain, hip pain, low back pain or spine pathology;
5. Woman with pregnancy.

Elite or recreational athletes with ACL injury who suffer knee instability and require surgical reconstruction