Injury to the anterior cruciate ligament (ACL) is one of the most common knee injuries, with approximately 80,000 to 250,000 ACL injuries occurring each year. Despite considerable attention and intervention efforts, ACL injuries continue to occur in alarming numbers. There is a need to better understand ACL injury mechanisms and develop more effective injury prevention interventions. We hypothesize that some non-contact anterior cruciate ligament (ACL) ruptures result from microtrauma accumulating over repeated bouts of strenuous exercise. We also hypothesize that non-failure ACL trauma can be detected by changes in knee laxity. However, before we can test these hypotheses, we must understand the normal temporal response of knee laxity during and following exercise so that we can discern normal viscoelastic ACL responses from responses indicative of trauma. Therefore, the objectives of this study were to (1) quantify the temporal response of anterior knee laxity to strenuous exercise, (2) test the hypothesis that anterior knee laxity will increase during strenuous exercise in young adults, (3) identify the time needed for knee laxity to return to pre-exercise values following strenuous exercise in young adults, and (4) explore the variability in knee laxity changes and recovery times among young adults completing a strenuous bout of exercise.

A total of 32 subjects, 16 exercise and 16 controls, were tested. A custom knee arthrometer was used to quantify the subjects’ right knee laxity prior to, immediately after, and every hour for five hours following a 50 minute strenuous leg exercise protocol. Knee laxity was also measured 24 hours post exercise. Leg exercises consisted of jumping rope, lunges, squats, running, and various cutting moves. Knee laxity was defined as the anterior displacement of the tibia relative to the femur with the knee flexed 30 degrees and an applied anterior force of 133 N.

Knee laxity did not change in the control group, but did change over time in the exercise group. The exercise group demonstrated a significant mean increase in anterior knee laxity following exercise of 0.63 mm, or 32%. There were varied knee laxity temporal responses following exercise. Three subjects demonstrated a decrease in laxity post exercise, 2 subjects increased post-exercise but didn’t recover within 24 hours, and 3 subjects demonstrated an increase in laxity post exercise, but then recovered to laxity values below their pre-exercise condition. These results highlight the varied temporal response of knee laxity following exercise and the need to understand their mechanisms before knee laxity can be used as a diagnostic tool for ACL injury prevention.