Cruciate Ligament Strain in an In Vitro

There are nearly 100 000 anterior cruciate ligament (ACL) injuries annually in the United States, 70 % of which are defined as noncontact.13 Noncontact ACL injuries, which involve foot-ground reaction and segmental gravitoinertial forces but no other external contact forces, commonly occur during unipedal jump landings.16 To arrest downward body momentum during a jump landing, a lengthening quadri-ceps contraction is required to resist excessive knee flexion after ground contact. Typically, such lengthening contrac-tions can lead to large muscle forces, in excess of 140 % of isometric values.23 Could these large forces increase the risk of ACL injury and, if so, by what mechanism? There are 4 lines of evidence suggesting that a large quadriceps force can increase ACL strain, even to the point of ACL rupture. First, in vitro, in knees flexed less than 60°, a simulated increase in quadriceps force increases ACL strain.17,18 Second, increasing the quadriceps force in vitro up to 4 kN causes anterior tibial translations of greater than 20 mm, and ACL rupture could result.5,6 Third, ACL strain is significantly affected by an increase in quadriceps activit