Study design and participants
A prospective transversal study was designed with the aim of analysing the outdoor footwear fit of a sample of schoolchildren in length, width and height. Through a purposive sampling and using as the selection criterion the geographical proximity to the education centre where the study was carried out, three centres from the city of Sevilla (Spain) were selected. The randomized sample included one Early Childhood Education centre and two Early Childhood and Primary Education centres. For the samples election, a probabilistic sampling was used from a study population composed of 726 schoolchildren. With the consent of their parents or guardians, schoolchildren between 3 and 12 years of age were selected, who did not have any malformations or previous history of foot surgery. The children who wore boots or high-top footwear on the day of examination were excluded from the sample. The final study sample consisted of 505 schoolchildren. The study was carried out from February to May 2015, within the framework of a school health program, and it was approved by the Research Ethics Committee of the Universidad of Sevilla (Sevilla, Spain).
Measurements
A measuring device was designed to determine the maximum foot length (FL), foot width (FW) and height at the level of the first metatarsal head (FH), and transfer the measurements to the inside of the shoe, in order to verify its fit. To carry out the measurements, the longest foot was selected, which was determined by measuring both feet separately from the heel to the longest toe using a retractable measuring tape while the participant was standing. When the measurement obtained was identical for both feet, the foot to be studied was randomly selected by flipping a coin in the air (heads = left foot, tails = right foot). To make the measurements traceable, the participants stood barefoot, with both feet at the same height and their knees extended, on an acetate sheet placed on a methacrylate base, which had a calibrated template imprinted in it (Andalusian Centre of Metrology; model CAM-V-00014-GRID-POD-03). This base had two protruding surfaces that made up a 90° angle at one of its corners, where the heel and the medial or lateral edge of the study foot were placed. Using a digital height gauge with a scribe marker (Andalusian Centre of Metrology; model CAM-PDVC-150-GRAMILPOD-01), it was possible to transfer the relative position of the reference points taken from the foot to the acetate sheet placed on the Grid (one point for the FL and two for the WL) (Fig. 1). The length was obtained directly by transparency through the scale of the Grid and the width was obtained from the X and Y 115 coordinates of each point using Access 11.0 (Microsoft Office 2010) software.
Unlike the measuring devices used in other studies [11, 13], our measurement took into account the maximum length of the foot according to the digital formula of the schoolchildren. To achieve this, we considered three different digital formulas, Egyptian foot formula (first longest toe), Greek foot formula (second longest toe) or square foot formula (first and second toe of the same length). To perform the measurement, the interior capacity of the footwear was taken into account taking as reference the maximum foot length. The maximum length of the foot (FL) was defined as the distance from the posterior side of the heel to the end of the longest toe [14]. The maximum metatarsal width (FW) was considering as the distance from the most protuberant medial and lateral points corresponding to the head of the first and fifth metatarsal bones [9, 13]. The maximum height (HL) was defined as the distance from the floor to the highest area of the first metatarsal head [15]. Measurements were precise to 0.01 mm. All the measurements were taken at schools by two operators.
The inter-observer reliability of the researchers and the measurements were calculated with an interval of 1 week. Inter-observer intraclass correlation coefficient (ICC) with level of confidence of 95% (Cronbach’s α) was 0.99 for FL, 0.98 for FW, 0.99 for footwear length (FWL), and 0.99 for footwear width (FWW). Coefficient of variation (CV) was 13.15% for FL, 12.59% for FW, 13.08% for FWL, and 9.74% for FWW. Inter-observer relative technical error of measurement (TEM %) was 0.20, 0.07, 0.21, and 0.06 respectively.
Once these measurements were obtained, they were transferred to the inside of the footwear using a acetate transparent insole that had approximately the same size as that of the foot of the schoolchild, where the length, width and height reference points obtained were placed using a hook-type Velcro surface tape. In order to calculate the length, width and height of the inside of the footwear, telescopic gauges with protractors were used (Andalusian Centre of Metrology; interior gauge, model CAM-0-150 mm-POD-02). Using a loop-type Velcro surface tape, the gauge was fitted to the insole at the points marked with hook-type Velcro surface tape and it was then inserted into the shoe (Figs. 2 and 3). The gauge had a brake that could be released once inside the footwear, which allow edit to expand longitudinally, transversally and upwards until it met the inner edges of the shoe; at this point, the break was activated again and, once the gauge was extracted from the shoe, it was possible to accurately determine the length, width and height of the footwear using a retractable measuring tape. Foot measurements were obtained from the longest foot, and were compared with the inner length, width and height of the footwear.
The expansion of the measuring device lengthwise was determined by the resistance generated by the material of the heel counter and of the reinforcement of the toe tip. For the width and height measurements, the position of the touch probes when they reach the measuring points without deforming the material was taken into account. An adequate toe allowance (TA) of 5–15 mm in length and 10 mm in width was estimated.
Statistical analysis
The statistical analysis was conducted using the statistics software IBM SPSS v22 (SPSS, Inc., Chicago, IL, USA). To determine the three-dimensional fit of the footwear, the difference between the shoe and foot measurements of length, width and height was calculated. This way, a positive result showed that the footwear was longer or wider than the foot and, on the contrary, a negative result indicated that the footwear was shorter or narrower. If the result was zero, it meant that there were no differences between the dimensions of the footwear and those of the foot. Based on the TA considered, it was established for the present study that the footwear was well-fitted when the distance between the toecap and the longest toe was between 5 and 15 mm. In the same way, the footwear was considered narrow when the space for the expansion of the foot in width was less than 10 mm. To compare the length and width fit, the Wilcoxon signed-rank test for related samples was used. To compare the length, width and height fit as a function of gender, the Mann-Whitney U-test was used. For the inferential analysis, a 95% confidence interval was considered; thereby, the experimental p-value was compared with a 5% significance level.