Objective To investigate differences in the relative sizes of the ankle-stabilizing muscles in individuals with versus without flexible flat feet and to determine predictors of symptom severity.Methods This cross-sectional study included 30 patients with symptomatic flexible flat feet and 24 normal controls. The following were evaluated: foot posture index, resting calcaneal stance position angle, radiographic findings (calcaneal pitch, Meary’s angle, talocalcaneal angle, talonavicular coverage angle [TNCA]), foot function index (FFI), and cross-sectional areas (CSA) of the tibialis anterior (TA), tibialis posterior (TP), and peroneus longus (PL) upon ultrasonographic examination. To address morphometric differences among participants, individual muscle measurements were normalized to proportions of total muscle CSA. Between-group differences were evaluated with independent t-tests. Correlations between muscle ratios, radiographic parameters, and FFI scores were investigated. Logistic regression analysis was performed to determine which parameters predicted severe symptoms.Results The relative size of the TP was significantly greater and those of the TA and PL were significantly smaller in patients with flat feet than in normal controls. Correlations were found among relative muscle CSA ratios, radiographic parameters, and FFI score. Linear regression analysis confirmed that the TNCA and the relative CSA of the PL were independent predictors of symptom severity.Conclusion This study found significant differences in the relative CSAs of the ankle muscles in patients with flexible flat feet versus individuals without flat feet; these differences were significantly correlated with anatomic abnormalities. Symptoms were more severe in patients with relatively greater forefoot abduction and relatively smaller PL.

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