How Muscle Forces Contribute to ACL Tears
Abstract & Commentary
Synopsis: An imbalance of muscle activity during footstrike may be responsible for the force that actually causes the ACL to tear. This supports the potential for neuromuscular training to decrease the incidence of ligament injuries in athletes.
Source: Colby S, et al. Electromyographic and kinematic analysis of cutting maneuvers: Implications for anterior cruciate ligament injury. Am J Sports Med 2000;28:234-240.
Anterior cruciate ligament (acl) injuries have been estimated to occur in one out of every 3000 individuals. However, the majority of these injuries occur in individuals between the ages of 15 and 45, an age group encompassing approximately 47% of the population. With this new denominator, the incidence of injury in the susceptible population becomes one per 1750 individuals.1 Despite this frequency and despite the fact that the immediate and long-term consequences of ACL injuries are significant, the exact mechanism of this injury is yet unknown.
Colby and associates emphasize that we do know that these injuries occur "near footstrike, when the quadriceps muscles are eccentrically activated to resist flexion." Moreover, they occur during deceleration or while changing directions, as in cutting and landing. Colby et al theorize that internal forces generated by the leg muscles must be involved in creating sufficient force to tear the ACL during these maneuvers, since inertial forces involved in these activities (decelerating, cutting, and landing) would result in an anterior force on the femur and a posterior force on the tibia (i.e., forces that might injure the posterior cruciate ligament [PCL] but should not injure the ACL).
Therefore, to better understand muscle forces on the knee at footstrike, Colby et al evaluated electromyographic activity using surface electrodes of the vastus lateralis, vastus medialis, rectus femoris, biceps femoris, and medial hamstrings. Fifteen healthy collegiate and recreational athletes were assessed at various flexion angles while performing ACL "at-risk" activities of cross-cutting, sidestep cutting, stopping, and landing. In their study, footstrike of the athlete occurred at an average of 22° of knee flexion during all maneuvers and, just before footstrike, a high-level quadriceps muscle activation occurred. This activation peaked in mid-eccentric motion. During this same time period, the hamstring muscle activation was suboptimal.
Hence, from Colby et al’s data it would appear that on or about footstrike, the high quadriceps forces achieved by eccentric contraction coupled with the submaximum hamstring muscle activity could result in sufficient anterior displacement of the tibia on the femur to injure the ACL.
Comment by Letha Y. Griffin, MD, PhD
Colby et al add another piece to the puzzle of the mechanism of noncontact ACL injuries. Environmental, anatomic, hormonal, and neuromuscular factors have all been implicated as possible etiologic factors of noncontact ACL injuries. At present, alteration of neuromuscular risk factors appears to hold the most promise as the basis for formulating prevention strategies.
The fact that women land from a free fall, pivot, and cut all in a more upright position (i.e., less hip and knee flexion) than men may contribute to their higher rate of noncontact ACL injuries when compared with equally trained and conditioned males. It does appear, from this study and others, that the quadriceps mechanism can produce sufficient force to tear the ACL at low levels of knee flexion. Therefore, it would seem prudent to institute as part of preseason conditioning in all athletes, especially women athletes, drills designed not only to strengthen the hamstrings but also to teach athletes to land from jumps, to cut, and to pivot with the hip and knee flexed, and by so doing, minimize strain on the ACL.
Hewett et al, in an article reviewed in the March 2000 issue of Sports Medicine Reports, showed that such a program for neuromuscular training for female athletes reduced their incidence of ACL tears to a rate equivalent to that of male athletes.2 More prospective studies like this would need to be done to verify the hypothesis that neuromuscular retraining to increase hamstring and quadriceps co-contraction can decrease the incidence of noncontact ACL injuries and therefore are worthwhile prevention techniques.
References
1. Garrick J. Epidemiology of ACL injuries. Hunt Valley Conference on ACL Prevention Strategies, June 1999.
2. Hewett TE, et al. The effect of neuromuscular training on the incidence of knee injury in female athletes. Am J Sports Med 1999;27(6):699-706.
ACL noncontact injuries:
a. occur primarily in the elderly.
b. frequently occur during deceleration or while changing directions.
c. have a higher rate of occurrence in men soccer players than women.
d. have not been found to be associated with any significant long-term sequelae.
e. are frequently caused by wearing too large of a shoe.
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