Anterior-posterior knee laxity increased by exercise - Quantitative evaluation of physiologic changes

To quantify physiologic changes in anterior-posterior laxity of the normal knee caused acutely by exercise, we evaluated the exercise level and the du ration of exercise needed to cause changes in laxity, the magnitude of chan ges in laxity, and the time needed for laxity to return to the level before exercise, by continuously applying a fixed exercise load. After a 20-minute run at 7 km/hr, anterior laxity with a displacement force of 133 N increased by about one third, and the compliance increased by one half with anterior drawer between 0 and 67 N. These changes were maintaine d at a similar level during exercise. The levels before exercise were resto red gradually over 1 hour after exercise. As the muscle strength showed no changes during exercise, we consider these changes in laxity are not associ ated with a decrease in muscle strength. Furthermore, since the changes in knee laxity after triathlon (in which the knee is subjected to prolonged ex cessive exercise) are similar to those after running at 7 km/hr, we believe that the magnitude of changes in laxity after exercise is fixed, regardles s of the level of the exercise load. Moreover, the threshold of exercise th at causes the maximum change is low.