Dynamic Stability (Agility) Dynamic knee stability was assessed by agility, which was measured by a 6-meter left-and-right shuttle run. A good dynamic knee stability is necessary for changing the movement direction. Each subject started running from the midline to the left terminal and then stepped on the square with his left foot. Then, the subject used the left leg as the axis and turned around to the right side. The subject then ran to the right terminal. Once the front foot was crossing the midline, this was the first round for testing the agility of left knee. The subject kept running to the right terminal without stopping on the midline, stepping on the square with his right foot, turning around to the left side, and running toward the left terminal. This was the first round of right knee test as the subject's front foot crossed the midline. Whenever the front foot crossed the midline, a timer was clocked. The time interval in seconds between the 1st and the 2nd time clocked was the 1st measure of the left knee agility, and the time interval between the 2nd and the 3rd was the 1st measure of the right knee agility, and so forth. There were five-round tests for the agility of right and left knees respectively. Three raters were assigned respectively at the midline, right and left terminals. Single-blind method was adopted. That is, all the three raters did not know which group the subject was assigned to. The square at the right or left terminal ends was 10 inches wide and 3 meters away from the midline. It would be recognized as a qualified round if the plantar surface of the foot ever contacted any part of the square; otherwise, the subject would be asked to repeat the whole set consisting of five rounds. Three sets were collected and averaged. The rest interval between each set should be long enough for the examinee to feel relaxed before the next set. The subject was asked to run as fast as possible. The average time difference of 3 sets between the affected and the sound side was regarded as the dynamic stability of the affected side. The greater the difference was, the poorer the dynamic stability or the agility would be. A practice session was given prior to the actual beginning of the study to avoid learning effect.