Despite the many investigations on development of the foot arch, little is known about the direct measurement of foot arch height and cut-off values in children and adolescents. This study is a large-scale investigation to assess natural history of children and adolescents foot arch height using the direct measurement of the MLA, to establish normative and cut-off values for the AHI, and to analyze associations between age or anthropometric characteristics and foot arch across childhood and adolescence. This investigation of foot arch posture includes healthy children and adolescents aged from 6 to 19 years.
This study confirms that the mean NH and AHI increases with age in a linear pattern, from 2.62 cm and 13.9 at age 6 years to 4.19 cm and 16.4 at age 19 years, respectively. Importantly, the NH range of variation was broad: 1.4 to 3.6 cm at age 6 years, and NH 2.7 to 6.4 cm at age 19 years. Flatfoot or low arched foot was generally found to decrease with age. In this study, the prevalence of flat foot was 11.9%; the prevalence decreased from 27.6% in 6-year-old children to 5.9% in 19-year-old adolescents. This study also found very little gender bias for NH measures and AHI values. These statistical differences demonstrated that males have a mean NH and AHI, 3.64 cm and 15.32 versus 3.40 cm and 15.0 in females, respectively.
In the current study, normative and cut-off values for normal and abnormal difference in AHI among different age (years) groups based on the 68% and 95% prediction intervals were presented. The mean foot arch in the children and adolescents, as found in this study was AHI 15.16 (2.61). Clinically this implies that around 68% of study population have AHI measure between ± 1 SD from the mean, range 12.5 to 17.7 (AHI normal category) (Table 3). Further, around 95% have AHI value between ± 2 SD from the mean, range 9.9 to 20.3 (AHI low arched, normal and high arched categories) (Table 3). Clinical alert is indicated for foot arch > ± 2 SD, representing 5% of expected abnormality (Table 3).
To our knowledge, no studies have reported cut-off values for AHI (NH/FL) in children and adolescents. However, several studies [7, 16, 18] have reported mean values for AHI in different age groups, but our findings differ slightly from the results in these studies. Waseda et al.  in a large-scale investigation of the foot arch have reported a mean AHI of 14.9 in 6 to 18 years old children and adolescents. Our higher AHI (15.1) may be related to the age range of students in this study (6 to 19 years old) in comparison with the study by Waseda et al. . In the study by Williams and McClay  they reported mean AHI of 16.3 for a sample of younger and older adults (mean age 27). That study  was a reliability study investigating the reliability and validity of several measurements of the MLA in adults 19 to 43 years old and different weight bearing, and was therefore not designed to investigate normative values. Morita et al.  reported a mean AHI of 14.6 and 14.2 in 9- and 11-year-old children, respectively. The mean values of 14.7 and 14.6 reported in the current study in children aged 9 and 11 year, respectively; were close to the proposed values reported by Morita et al. .
More importantly, the findings in our study showed that the mean NH in males gradually increased to the age of 11 and then accelerated from the age of 12 to 14 years and tended to increase after the age of 14. In females the mean NH gradually increased to the age of 10 and then accelerated from the age of 11 to 12 years and reached a plateau at 16 years of age. The NH tended to increase after the age of 14 in both genders but there was small degree of changes and not significant. This outcome may be associated with the process of development of the foot arch, which is in agreement with previous studies by Waseda et al.  and Rai et al.  that have reported an increase in the longitudinal arch with age, using NH measurement method.
This study found minimal differences between male and female foot arch posture in the NH and AHI values. This is again corresponds well with other recent studies that have reported little gender bias, despite using other measurement techniques such as footprints, radiological and anthropometrical measures or observational techniques [12, 20–22].
In the current study, we found that age and foot length are indicative associated factor with foot arch posture. This finding is in accordance with previous studies [9, 21, 24, 25], which show that age and foot length had a significant association with foot arch posture. In agreement with earlier studies [6, 12, 24], we found that BMI does not seem to be an important predictor of children foot arch posture. While other studies found BMI to be related to foot arch posture in children [26, 27]. The results in the literature regarding the association between BMI and foot arch posture in children are still inconclusive.
Some helpful insights can be derived from this study. A low arch AHI category, and even severely low arch, can be expected in children aged less than 8 years. Hence, in the first decade of life, the presentation of a child with low arched foot (flexible flatfoot), which is painless, can usually be confirmed as normal for age . The severely low arched and severely high arched foot postures, must be considered as ‘abnormal’, with approximately 2.9% and 1.8%, respectively seen in this sample of children and adolescents (Table 4). Thus, in the physical examination of foot arch posture, clinicians must more closely consider the severely low and high arched foot (as a clinical/neurological alert) than the asymptomatic low arched foot .
The findings of this study have important implications for clinicians, parents and future research. Such normative reference data help appreciation of the range of ‘normal range’ for foot posture . Further, the children flatfoot that is becoming flatter as a child becomes older should alert clinicians, and direct differential diagnoses, for factors such as joint hypermobility, connective tissue disorders, altered neurological tone or muscular conditions . The data also provides mean and standard deviation values to act as comparators for future studies in a range of potentially pathological groups.
The present study has some limitations. First, the sample of the study population was students between 6 and 19 years old and results could not be generalized to other age groups. Second, it may be difficult to palpate the tuberosity because of the navicular bone’s round shape and the undesirable influence of the local soft tissue . Finally our study is cross-sectional and can only provide some insights, a prospective longitudinal study is needed to clearly show foot arch change over time