Rethinking Acute Sports Injuries: Evidence for an Overuse Mechanism in Hamstring and ACL Injuries.

Sports injuries have traditionally been classified as acute or overuse based on their onset and associated circumstances. Hamstring strain injuries and anterior cruciate ligament (ACL) injuries are two common sports injuries that are typically implicitly considered to represent acute injuries. This brief review, however, argues that hamstring and ACL injuries may at least partly present as overuse injuries resulting from a mechanical fatigue phenomenon, rather than acute injuries. Human, animal, and cadaveric studies are discussed to support this view. For example, human studies show no kinematic deviation in the stride during which the hamstring injury occurs as compared to the preceding strides. Further, the location of injury and ultrastructural damage of hamstring injuries is largely comparable to that seen in repetitive muscle-tendon unit lengthening experiments in animals. For the ACL, repetitive simulated jump landings have been shown to lead to ACL failure despite the ACL load being well below its ultimate strength. Furthermore, analyses of ACL explants obtained from noncontact ACL-injured patients during reconstruction surgery indicate similar damage to cadaveric studies that repetitively loaded the ACL. In summary, studies with diverse methodological approaches support the view that mechanical fatigue may predispose hamstring and ACL tissues to failure at submaximal loads during seemingly normal movements. Although further research is needed to substantiate these hypotheses, recognizing mechanical fatigue as a factor in these injuries can inform training and rehabilitation protocols and open opportunities to use modeling approaches and wearable sensors to monitor tissue load and damage, ultimately reducing injury rates.