Letters to the Editor

Dear Editor: It was with great interest that we read the article by Chappell et al, “Effect of Fatigue on Knee Kinetics and Kinematics in StopJump Tasks” (July 2005, pages 1022-1029). The article addresses an important problem, given the anecdotal evidence from athletes, clinicians, and coaches, suggesting that ACL injuries and many other injuries occur most often when athletes are fatigued. However, although the experiments presented in this article appear to be well designed, there are inconsistencies in the terminology used to define the loads calculated from the inverse dynamics approach. The inconsistent definitions of loading in this article appear to have led to a conclusion regarding fatigue and ACL injury that is not supported by the results of this study. The inconsistencies in terminology used in this article lie in the reporting of internal versus external loading. Studies that report loads calculated from inverse dynamics maintain a convention of reporting either net externally applied loads from ground reaction, gravity, or inertial forces or net internal forces from the muscles, passive soft tissues, and bone that must balance these external loads. However, in this article, the authors appear to have mixed these 2 conventions. For example, the knee flexion/extension moment during landing is reported by the authors as an extension moment (Figure 8). Because the ground reaction force and inertia of the body center of mass must act to flex the knee, this extension moment must be the internal moment that the knee extensor mechanism must generate to counteract the external flexion moment. In contrast, the authors report a knee valgus moment in female subjects during landing. It appears they are reporting an externally applied valgus moment in this case, apparently to be consistent with the literature the authors cite and the results of other studies. However, changing the convention from internal to external load without a clear definition can lead to a misinterpretation of the results. In fact, the inconsistencies in defining the load convention appear to contribute to a misinterpretation of the results in this study. The meaning of the anterior-posterior shear forces existing at the knee during landing appears to have been misinterpreted, as a result of not consistently defining internal versus external loads. The force shown in Figure 5 in the article does not reflect the force in the ACL, as suggested in this article, but rather the opposite. The posterior ground reaction force due to braking results in a loading situation at the knee that protects the ACL rather than endangering it. The externally applied force at the ground is transmitted up through the ankle, resulting in an external force that pushes the tibia backward. Intuitively, this posterior force from the ground is analogous to a posterior drawer test, which causes the ACL to become slack and the posterior cruciate ligament to become taut. However, the authors interpret an increase in this force as a worse condition for ACL injury. Thus, the conclusion that fatigueinduced increases in this shear force may increase strain on the ACL and risk of injury in both female and male subjects is not supported based on the results of this article. In fact, the opposite conclusion fits these results, that the increase in shear force due to fatigue, as shown in Figure 7, protects the ACL from injury. Future studies should focus on the other components of loading that change owing to fatigue to determine which ones would lead to a greater risk of ACL injury in fatigued athletes.