Why Do Patients Have Lingering Weakness in the Quadriceps After ACL Reconstruction?

    After anterior cruciate ligament (ACL) reconstruction, it’s common for patients to experience quadriceps weakness, which is thought to be caused primarily by muscle atrophy. 

    That may not be the only factor, however. In a small study, researchers at the University of Michigan School of Kinesiology found that besides muscle loss, the quadriceps muscle – specifically, the fascicles within that muscle – contract differently after ACL reconstruction. Taken together, these deficits result in a muscle that is weaker and behaves like that of a much older individual, the researchers said. 

    These findings, published online ahead of print by the Journal of Biomechanics, could help clinicians design more effective rehabilitation programs for post-ACL reconstruction patients. 

    Key New Discovery

    “This is the first human-based paper that is focused on proving that muscle is not just smaller after injury, but it also contracts differently,” said Lindsey Lepley, PhD, ATC, assistant professor of Athletic Training and co-director of the Orthopedic Rehabilitation and Biomechanics Laboratory at the University of Michigan School of Kinesiology.

    “This is a key new discovery that helps explain the persistent weakness that is so commonly observed.” 

    Many of the factors that affect aging muscle also emerge after ACL reconstruction, Dr. Lepley and her colleagues contend. “Generally, our group has been saying that an ACL injury prematurely ages the limb – the joint itself often shows signs of arthritis within 10 years and the muscle also exhibits factors like aged muscle tissue,” she said. 

    Altered Fascicle Behavior

    For the study, they compared 18 patients (9 males, 9 females) recovering from ACL reconstruction with 9 healthy subjects (3 males, 6 females). The researchers used ultrasonography to image the quadricep muscle during 3 maximal isokinetic knee extensions in an isokinetic dynamometer. Their goal was to measure and evaluate vastus lateralis fascicle behavior, including fascicle length, angle, and shortening velocity. These parameters were calculated and analyzed from rest to peak torque.

    The researchers found that the affected leg contracted differently: The fascicles in the quadriceps were slower and lengthened less and exhibited reduced strength compared with the surgical patients’ contralateral leg and the legs of the healthy subjects. This altered fascicle behavior following ACL reconstruction may be an underlying factor in prolonged post-surgical muscle weakness. 

    “Muscle atrophy can play a role, but it is also possible that other non-contractile elements of muscle are clogging up the system and not allowing it to contract effectively,” Dr. Lepley said. “The tissue left behind may be more fibrotic or have more fat infiltration, hence the contractile nature of the muscle may change.” 

    Next Steps

    Dr. Lepley’s team will next evaluate subjects with a history of ACL reconstruction while they’re walking. 

    “We’ll build on this by taking what we learned in this more stationary experiment, where we looked at participants on a strength machine, and move it to a more dynamic condition with people walking,” she said. “We will be actively recruiting for this walking study in the coming weeks.”


    Davi SM, Brancati RJ, DeStefano LJ, Lepley AS, Lepley LK. Suppressed quadriceps fascicle behavior is present in the surgical limbs of those with a history of ACL reconstruction. J Biomech. 2021 Oct 11;129:110808. doi: 10.1016/j.jbiomech.2021.110808. Online ahead of print.