Changes In Quadriceps And Hamstring Muscle Volume And Quality In The First Month Following ACL Reconstruction

PURPOSE: Diminished quadriceps and hamstring muscle function negatively impact functional outcomes and long-term joint health following anterior cruciate ligament reconstruction (ACLR). However, it remains unknown how quickly both structural (i.e., muscle volume) and muscle quality (i.e., fat fraction) muscular changes occur acutely following ACLR. The purpose of this study was to compare quadriceps and hamstring volume fat fraction between pre-operative and 1 month following ACLR. METHODS: Sixteen participants (56% Female, age: 23 ± 6 years, mass: 70.8 ± 10.3 kg, height: 1.70 ± 0.09 m; days between pre-operative measurement to ACLR: 9 ± 9 days) who underwent ACLR were included in this study. Quadriceps and hamstring muscle volume and fat fraction were assessed using magnetic resonance imaging (MRI) across the full muscle from origin to insertion, both preoperatively (baseline) and at 1 month since ACLR in the involved and uninvolved limbs and analysed with proprietary software developed by Springbok Analytics (Charlottesville, VA). Muscle volumes and fat fractions were evaluated across all four quadriceps and three hamstring muscles and then combined for each muscle group using weighted means. 2x2 repeated measures ANOVA was used to analyse differences in muscle volume and fat fraction between timepoints and limbs. RESULTS: A significant increase in fat fraction was observed in the involved quadriceps (8.8 ± 2.7 vs. 10.5 ± 3.6%; p < 0.001) and a significant decrease in muscle volume in the involved quadriceps (1605.5 ± 430.3 vs. 1304.8 ± 390.3 mL; p < 0.001) at 1 month compared to baseline. Hamstring fat fraction in the involved limb also increased from baseline to 1-month (10.3 ± 3.0 vs. 11.6 ± 3.6%; p = 0.002), while changes in muscle volume were not statistically significant (1001.0 ± 290.7 vs. 952.1 ± 263.4 mL; p = 0.78). CONCLUSIONS: Our findings indicate that significant structural and muscle quality changes occur in thigh musculature within the first month following ACLR. Changes in muscle volume and quality in the quadriceps and hamstrings highlight the need for targeted rehabilitation to preserve muscle mass and improve muscle quality in the first month post-ACLR. Future research is needed to determine the long-term impact of these changes on functional outcomes such as physical function and joint health. Supported by: National Institute of Arthritis and Musculoskeletal and Skin Diseases (5R21AR083163-02)