Aizu Naoki

This study aimed to elucidate the effects of the tibialis anterior (TA) muscle contraction on the medial longitudinal arch (MLA) and hallux valgus (HV) angle, primarily focusing on the potential functional role of the TA in foot alignment. Twenty-five healthy adults (mean age: 20.7 ± 1.0 years) participated. Electrical stimulation was applied to the TA without ankle dorsiflexion. A triaxial accelerometer was attached to the skin over the navicular bone, and electrogoniometers were placed over the interphalangeal joints of the hallux and ankle. The navicular displacement was calculated through double integration of the acceleration data. Contraction of the TA caused navicular displacement in the supination direction (upward, posterior, and medial), suggesting a temporary elevation of the MLA. The hallux showed an average varus shift of 1.36°; the shift was observed in 24 of 25 participants (p < 0.001). A significant positive correlation was found between upward displacement of the navicular bone and the extent of varus change in the HV angle (r = 0.463, p = 0.020). Additionally, a greater initial HV angle was associated with a larger varus shift in the hallux after TA contraction (r = 0.433, p = 0.030). No significant correlations were observed between the HV angle change and ankle dorsiflexion, suggesting that the medial hallux shift may result from structural changes in the MLA rather than isolated joint motion. These findings provide novel evidence that TA contractions can elevate the MLA and induce varus displacement of the hallux. These findings further suggest that the TA contributes to foot biomechanics not only as an isolated muscle but also as part of a coordinated muscle system involved in arch dynamics and toe alignment. This suggests the potential clinical applications of TA activation in noninvasive interventions for mild-to-moderate HV, such as exercise therapy or neuromuscular electrical stimulation.