Purpose: Anterior cruciate ligament reconstruction (ACLR) is common in adolescents, however, there is little information highlighting the differences between adults and adolescents’ post-surgery. This study aims to identify differences in quadriceps muscle quality between adults and adolescents post-ACLR and how self-reported physical activity is associated with muscle quality. Methods: A cross-sectional study was conducted with 24 participants aged 14-36 years, 4-9 months post-ACLR. Adolescents and adults were compared. Quadriceps muscle quality was assessed using static ultrasound for rectus femoris cross-sectional area (CSA) and echo-intensity, and dynamic ultrasound for vastus lateralis pennation angle changes during treadmill walking. A limb symmetry index (LSI) was calculated for CSA and echo intensity. Physical activity was assessed using the International Physical Activity Questionnaire and Muscle Strengthening Exercise Questionnaire. Independent t-tests compared CSA and echo intensity LSI, and pennation angle change between groups. Spearman’s rank correlation quantified the association between physical activity and muscle characteristics. Results: Participants included 12 adolescents (age: 16.0±1.4 years, height: 171.7±8.5 cm, weight: 70.8±12.5 kg, maturity offset: 2.4±1.3 years) and 12 adults (age: 24.4±7.4 years, height: 174.0±7.0 cm, weight: 79.0±14.4 kg). No significant differences were found in rectus femoris CSA LSI (t22=0.22, p=0.83), echo intensity LSI (t22=-1.42, p=0.17), or vastus lateralis pennation angle change (t22=0.41, p=0.68). Associations between muscle quality and physical activity were low to negligible. Discussion: Adolescents and adults show similar muscle quality 4-9 months post-ACLR. Associations between muscle quality and self-reported physical activity were minimal. Future studies should explore longitudinal changes post-ACLR in both populations.
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