Toward a Better Understanding of Hip Adductor Function: Internal Rotation Capability Revealed by Anatomical and MRI Evaluation.

Background: At present, the rotational function of the hip adductor muscle group remains unclear. This study aimed to clarify the rotational function and stabilizing role of the pectineus, adductor longus, and adductor brevis (adductor muscle group) based on anatomical findings and T₂ values (ms) obtained from magnetic resonance imaging (MRI). T₂ values are prolonged in tissues with higher water content, and in skeletal muscle, it has been demonstrated that T₂ values increase in proportion to exercise intensity. Methods: Using fixed specimens (n = 6, aged 61-96 years), we observed the three-dimensional arrangement of muscles in the neutral position of the hip joint and observed the extension or shortening of muscles associated with passive maximum internal and external rotation of the hip joint. In addition, we evaluated the activity of the adductor muscle group by T₂ values (ms) from MRI pre- and post-internal rotation (forward step with the left leg) and pre- and post-external rotation (backward step with the left leg) movements of the right hip joint in a standing position (n = 8, healthy adult subjects, mean age 29.1 ± 5.3 years). Results: Regarding functional anatomy, the arrangement of the gluteus minimus and adductor muscle groups was almost parallel across the femoral neck. In the evaluation of adductor muscle group activity using MRI, the percent change in T₂ values (%) of the pectineus was 6.38 ± 1.35 pre- and post-internal rotation and 1.35 ± 0.71 pre- and post-external rotation, whereas that of the adductor longus and brevis was 4.84 ± 1.31 pre- and post-internal rotation and 1.31 ± 0.68 pre- and post-external rotation. The percent change in T₂ values pre- and post-internal rotation exercise was significantly greater than that pre- and post-external rotation exercise in the pectineus, adductor longus, and brevis muscles (p < 0.05). Conclusions: The adductor muscle groups are suggested to contribute to joint stability in the coronal plane and provide joint internal rotation in the standing position.