The arch index: A measure of flat or fat feet?

Background: Studies using footprint-based estimates of arch height have indicated that obesity results in a lowered medial longitudinal arch in children. However, the potentially confounding effect of body composition on indirect measures of arch height, such as the arch index, has not been investigated. Methods: This study assessed the body composition of 12 male and 12 female adults (mean age: 39.9 γ 8.1 years, height: 1.724 ± 0.101 m; weight: 95.1 ± 13.7 kg, and BMI: 31.9 ± 3.0 kg/m²) using bioelectrical impedance analysis to produce a two-component model of fat mass (FM) and fat-free mass (FFM). The dynamic arch index also was determined from electronic footprints captured during gait using a capacitive pressure distribution platform with a resolution of 4 sensors/cm². Results: While significant correlations were noted between FFM and the area of both the hindfoot (r = .75, p < .05) and forefoot (r = .72, p < .05), the midfoot area was correlated only with FM (r = .54, p < .05). Similarly, the arch index was significantly correlated with the FM percentage (r = .67, p < .05). Conclusions: The findings of this pilot study suggest that body composition influences arch index values in overweight and obese subjects. Consequently, body composition may be a confounding factor in interpreting footprint based estimates of arch height and, as such, these estimates would best be used with supplementary measures of body composition.