Comparison of foot kinematics and the morphology of intrinsic musculature of the foot using a foot-type classification based on function.

[Purpose] The purpose of this study was to investigate a correlation between the morphology of the intrinsic musculature of the foot and foot kinematics during gait using a foot type classification based on the windlass function. [Participants and Methods] We examined 67 feet of 35 healthy participants in this study. We collected three-dimensional foot kinematic data during gait from the Oxford Foot Model and assessed the morphology of the flexor digitorum brevis, abductor hallucis, adductor halluces (oblique head), and abductor digiti minimi muscles using B-mode ultrasound. Using the Foot Posture Index (six-item version), we divided static foot postures into two groups: normal arch and flatfoot. Subsequently, we compared foot kinematics and the morphology of the intrinsic musculature among the four groups using the analysis of variance with the Bonferroni test. [Results] Foot kinematics of the flatfoot-adduction type during gait significantly differed from that of the normal arch-abduction type. The abductor digiti minimi of the flatfoot-adduction type was significantly thinner than that of the normal arch-abduction type. [Conclusion] There may be some variations in flatfoot, and the flatfoot-abduction type might not be a risk factor for overuse injuries.