In this Editor’s Choice post, Dr. Matt Schmitz highlights a new study in JBJS.
In the Feb 4, 2026 issue of The Journal, Alzobi et al. take an interesting look at muscular function after anterior cruciate ligament (ACL) tear—in particular, how thigh muscles change in ACL-deficient knees managed without reconstruction. While ACL research has traditionally focused on quadriceps weakness soon after injury, this longitudinal MRI study conducted at Johns Hopkins School of Medicine suggests that the longer-term story may center elsewhere: the hamstrings.
Access the study and visual abstract at JBJS.org: Thigh Muscle Changes in the ACL-Deficient Knee. A 4-Year Longitudinal MRI Study of 1,207 Patients
Using data from the Osteoarthritis Initiative (a large, multicenter, prospective cohort study), the authors compared 1,207 thighs: 92 in individuals with MRI-confirmed ACL-deficient knees and no history of reconstruction and 1,115 controls (ACL-intact). The average age of the participants was 61 years. To make the groups as comparable as possible, the investigators used propensity-score matching (1:2 to 1:3), controlling for baseline covariates. Changes were analyzed over 4 years.
A key strength of the study is its measurement approach. The researchers applied a validated deep-learning-based MRI segmentation model (a 2D U-Net model trained on manually annotated scans) to quantify the cross-sectional area of major thigh muscle groups—quadriceps, hamstrings, adductors, and sartorius—along with muscle-quality markers. They estimated intramuscular adipose tissue and “contractile percentage” (the proportion of non-fat muscle tissue), which can reflect functional muscle health. They also incorporated strength testing with standardized isometric dynamometer protocols used across the Osteoarthritis Initiative.
Over 4 years, ACL-deficient thighs showed clear, progressive deterioration in the posterior thigh. The investigators found significant hamstring atrophy and a smaller but meaningful decline in the sartorius in the ACL-deficient thighs. Hamstring strength in the ACL-deficient thighs also dropped significantly over 4 years. In contrast, quadriceps and adductor cross-sectional area remained stable, and quadriceps strength and “specific force” were largely unchanged. Interestingly, the study did not find significant differences in intramuscular adipose tissue or contractile percentage between the groups.
The takeaway for clinicians and patients is practical: long-term rehabilitation after ACL injury may need to look beyond the quadriceps and prioritize strategies that preserve hamstring mass and function. The authors note that this is among the first longitudinal matched-cohort studies to track both thigh-muscle morphology and fat infiltration in unreconstructed ACL-deficient knees. The study has limitations, which the authors acknowledge, but I nonetheless applaud them for helping us to better understand the long-term effects of ACL deficiency—while reminding us not to forget the hamstrings!
Access the study and visual abstract at JBJS.org: Thigh Muscle Changes in the ACL-Deficient Knee. A 4-Year Longitudinal MRI Study of 1,207 Patients
Additional perspective is provided in a new commentary article by John J. Elias, PhD: Thigh Muscle Changes at Long-Term Evaluation Following ACL Injury Differ from Early Changes Associated with ACL Injury and Reconstruction
Matthew R. Schmitz, MD
JBJS Senior Editor for Pediatrics and Social Media
