The influence of gender and body mass index on the FPI-6 evaluated foot posture of 10- to 14-year-old school children in São Paulo, Brazil: a cross-sectional study

BACKGROUND Adolescence is marked by changes to the body, including the feet. The Foot Posture Index (FPI-6) stands out from other foot type classification methods as valid, reliable, and multidimensional. However, the current literature differs according to age group, with little consolidation of normative data in school children, largely due to the influence of such factors as sex, age and body mass index (BMI). Thus, this study assesses foot posture in adolescents according to age, sex and BMI. METHODS The study evaluated 1.394 adolescents from Amparo and Pedreira regions in São Paulo, Brazil. Subjects were positioned barefoot on a wooden base and each foot was assessed by FPI-6 criteria. Each criterion was scored on a scale of -2 to +2, negative for supinated and positive for pronated posture. Initially the data were assessed for normality using the Shapiro-Wilk test and descriptive statistics were calculated. To investigate and compare the scores of FPI-6 with regards to age and body mass index, analysis of variance (ANOVA) was used, followed by post hoc Tukey. To compare the FPI-6 with regard to gender, an independent student t test was used. All data were analyzed using SPSS version 21.0 and the 5% significance level. RESULTS Boys had higher scores than girls (p = 0.037) for the right foot, and the group with normal BMI values scored higher than the obese group (p = 0.001). For the left foot, 11- and 13-year-olds differed (p = 0.024) with respect to age in general. The overweight and obese group scored lower than the normal BMI group (p = 0.039; p = 0.001, respectively). CONCLUSIONS Overall, the feet in this study were classified as normal, with a tendency to pronation, particularly in boys. There were differences between the 11 and 13 year groups and, with regard to BMI, there were higher scores for the group with normal BMI. Therefore, a higher BMI in adolescence is not indicative of a pronated foot type.