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 duration of exercise needed to cause changes in laxity, the magnitude of changes 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 maintained at a similar level during exercise. The levels before exercise were restored gradually over 1 hour after exercise. As the muscle strength showed no changes during exercise, we consider these changes in laxity are not associated with a decrease in muscle strength. Furthermore, since the changes in knee laxity after triathlon (in which the knee is subjected to prolonged excessive exercise) are similar to those after running at 7 km/hr, we believe that the magnitude of changes in laxity after exercise is fixed, regardless of the level of the exercise load. Moreover, the threshold of exercise that causes the maximum change is low.