Friday, November 21, 2008

Cutting-Edge Research: Fatigue Increases ACL Injury Risk

As posted previously on the Science of Soccer Online, females are far more susceptible to non-contact anterior cruciate injuries (click here). There are a number of factors that increase this risk. One of those factors is the biomechanics of landing and cutting. During these maneuvers, women tend to land with a position of knee valgus (a knock-kneed position) which increases stress on the ACL.

A recent study shows that the combination of neuromuscular fatigue and unanticipated movement alters hip and knee biomechanics in female athletes. Fatigue results in less hip flexion during single leg landing. This is compensated by increased inward hip rotation and greater knee valgus (the knee tends to run inward), biomechanical alterations that are exaggerated during unanticipated maneuvers. This landing position places the ACL at greater risk of injury. Technical and neuromuscular training may help avoid this scenario.

College-aged, female athletes were asked to perform a jump and land in one of three randomly selected positions designed to simulate movements on the field. They were asked to land on the left foot and push to the right, land on the right foot and push to the left or land on both feet and jump vertically. A signal determining the landing position was given either 5 sec before the jump (anticipated) or during the jump, before landing (unanticipated). These jump / landing maneuvers were performed before and after a fatiguing bout of exercise (designed to simulate a soccer match). Hip, knee and ankle movements were measured using 3-D computerized video analysis.

The researchers found that after inducing fatigue, the athletes landed with much different biomechanics. In the fatigued state, the women landed with less hip flexion, increased inward hip rotation and greater knee valgus. Essentially, when the athletes were fatigued, they landed and turned their knee inward (for a figure of this landing position, click here). The unanticipated jumps / landings exaggerated this position. This inward hip rotation coupled with knee valgus (a knock-kneed position) increases stress on the ACL and places is at great risk of damage.

Thus, when female athletes are fatigued, they alter their biomechanics of landing and cutting. This, in turn, places increased stress on the ACL, increasing the risk of injury.

How can this situation be corrected? There are a few strategies that should be affective. The first strategy is to focus technical training. That is, continued instruction of proper execution of landing and cutting maneuvers. This will help the athlete avoid the “danger” position of hip rotation and knee valgus. The second strategy is neuromuscular training. Using the training programs described in earlier posts will strengthen the muscles supporting the knee, develop proper proprioceptive reflexes. The final strategy is to improve strength and neuromuscular fitness. Increased strength and endurance should help the athlete avoid the biomechanical changes that place the ACL at risk. A summary of neuromuscular training programs designed to lessen the risk of ACL injuries can be found in an earlier post (click here).

Coaches and athletes should be aware of the risk factors for ACL injury. This study adds two additional risks – fatigue and unanticipated movements. It cannot be stressed enough that proper neuromuscular training should be a regular part of any soccer training program. An overwhelming body of research shows that these training programs are very effective at reducing the incidence of ACL injury in female athletes.


Borotikar BS, Newcomer R, Koppes R, McLean SG (2008) Combined effects of fatigue and decision making on female lower limb landing postures: Central and peripheral contributions to ACL injury risk, Clinical Biomechanics, 23:81-92.