Author(s): UMEHARA, J., MAEO, S., KINOSHITA, M., NISHIZAWA, N., KOBAYASHI, Y., EIHARA, Y., KANEHISA, H., ISAKA, T., Institution: KANSAI MEDICAL UNIVERSITY, Country: JAPAN, Abstract-ID: 2276

INTRODUCTION:
Skeletal muscle exhibits hypertrophic adaptations in response to mechanical loading. A large body of literature has demonstrated that strength training induces muscle hypertrophy, with some studies reporting regional differences within individual muscles. In particular, regional hypertrophy has been observed in the quadriceps femoris following knee extension training. These regional hypertrophic differences suggest that training may induce changes in three-dimensional (3D) muscle shape. However, whether strength training systematically alters muscle shape beyond changes in volume remains unclear. Therefore, this study aimed to determine whether knee extension training induces 3D changes in muscle shape in the quadriceps femoris.
METHODS:
Seventeen untrained adults (23.3 ± 4.0 yrs) completed 12 weeks of twice-weekly knee extension training at 70% of one-repetition maximum. Eleven additional untrained adults (24.3 ± 2.4 yrs) served as a non-training control group. Magnetic resonance images were obtained before and after the intervention to assess morphological changes in the four heads of the quadriceps femoris. Muscle volumes were calculated to verify training-induced hypertrophy. Muscle shapes were quantified using statistical shape modeling. Principal component (PC) analysis was applied to pre–post shape changes of each muscle head, with individual shape changes represented as PC scores. Changes in muscle volume and PC scores were compared between training and control groups.
RESULTS:
Muscle volume significantly increased in all quadriceps heads in the training group, confirming muscle hypertrophy. Shape change variability after training was smallest in the rectus femoris (RF), followed by the vastus medialis (VM), vastus lateralis (VL), and vastus intermedius (VI), as reflected by the contribution rates of the first PC (42%, 38%, 31%, and 21%, respectively). These results indicate more systematic shape changes in the RF and VM, whereas shape changes in the VL and VI were more variable across individuals. Following training, the RF demonstrated changes in anteroposterior curvature, the VM exhibited enlargement in the anteroposterior and mediolateral directions, and the VL and VI showed localized protrusions in different regions that varied among individuals. The primary PC scores in the VL, VM, and VI were significantly greater in the training group than in the control group, whereas the RF showed no between-group difference. Shape change in the VL was moderately correlated with its hypertrophy (r = 0.50, p = 0.041), whereas shape changes in the other heads were not associated with volume changes.
CONCLUSION:
This study demonstrated that knee extension training induced 3D shape changes in the quadriceps femoris in addition to muscle hypertrophy. These findings suggest that training-induced adaptations may involve coordinated structural remodeling beyond simple volume increases, potentially influencing functional capacity.