All methods were implemented according to relevant guidelines and regulations.
Participants
We distributed a recruitment form to recruit elementary school boys who can volunteer from a soccer club near the facility to which the author belongs. Prior to measurements, the purpose of this study was explained in writing and orally to the participants and their guardians, and written consent was obtained. This study was conducted after receiving approval (approval number: 2020-242) from the “Ethics Review Committee for Research Involving Human Subjects” of Waseda University. The exclusion criteria were as follows: those who did not agree with the purpose of the study, those who had pain in the lower limbs, those who could not participate in all three measurements, those with missing data, those who were unable to wear the arch support for an average of at least 1 h per day during the intervention period, and those with no decline in the MLA were excluded (Fig. 1). The sample size was estimated by G*Power 3.1.9.2 software. The minimum sample size was calculated to be 21, considering the previous studies [11, 14], and based on an effect size of 0.50, an α-level of 0.05, and a power of 0.80. Finally, considering possible dropouts due to COVID-19and long-term intervention, 31 participants were recruited.
Outcome measurement
Classification of foot morphology
The foot posture index-6 (FPI) was used to classify foot morphology [1]. The FPI is an evaluation tool developed considering its simplicity and convenience. FPI scores of + 1 and + 2 feet in the MLA shape item were defined as flat feet with low MLA and were included in the intervention. A physical therapist with 7 years of clinical experience who has a history of clinical examinations and graduate studies research conducts FPI measurements alone after confirming in advance that sufficient reproducibility can be obtained.
Foot morphology evaluation
An automatic three-dimensional foot measuring machine (Real Foot, Dream GP Inc, Osaka, Japan) was used to evaluate foot morphology (Fig. 2). Measurements were taken in static standing (on both feet) and sitting positions after marking the scaphoid bone length. The measurements included foot length (from the back of the calcaneus to the tip of the longest toe), foot width (from the first metatarsal to the fifth metatarsal head), foot circumferences (circumference at the first metatarsal head—fifth metatarsal head), and navicular height (from the floor surface to the lowest end of the navicular rough surface). The difference in navicular height between the upright and seated positions, the navicular drop, was expressed as an index of arch morphology and also as arch height ratio (Standing navicular height divided by foot length times 100). All measurements were performed at the same place and at approximately the same time. Measurements were performed by a single, well-practiced and appropriate technique person.
Muscle cross-sectional area evaluation of intrinsic and extrinsic foot muscles
The intrinsic and extrinsic foot muscles were measured using an ultrasound imaging system (SonoSite Edge II, FUJIFILM SonoSite, Inc, USA) in the B-mode. A probe (Linear Probe HFL38xp, FUJIFILM SonoSite, Inc, USA) with a frequency of 6–13 MHz was used for the measurements. This is approximately the same frequency used in previous studies [14,15,16]. In addition, at this frequency, it was used after confirming in advance that the muscle of the measurement site could be imaged in a sufficient state for analysis with several hundred legs. The intrinsic muscles of the foot flexor hallucis brevis (FHB) muscle, abductor hallucis (ABH) muscle, and flexor digitorum brevis (FDB) muscle were placed in a dorsal recumbent position with the knee in slight flexion and the ankle in slight plantar flexion. The extrinsic ankle muscles—the flexor digitorum longus (FDL) muscle, flexor hallucis longus (FHL) muscle, peroneal (PER) muscle, and tibialis posterior (TP)—were placed in an end-sitting position with the ankle joint in the mid-position, the knee joint in 90° flexion, and the hip joint in 90° flexion. The participants were instructed to relax their feet without applying pressure to the lower leg, and then the measurements were performed. The FHB and FDB muscles were measured by applying a probe in the short-axis direction to the proximal portion of the first metatarsal head [15], ABH and FDB to the medial aspect [14] and plantar surface [14] of the foot between the navicular tuberosity and medial tubercle of the calcaneus, respectively (Fig. 3a). The FDL, FHL, PER, and TP were imaged by applying a short-axis probe to the proximal 50% [16] of the medial end of the tibial plateau and the inferior end of the medial end of the medial tibial plateau, the 60% [17] of the proximal end of the fibular head and the inferior end of the external capsule, the 50% [16] of the proximal end of the fibular head and the inferior end of the external capsule, and the 30% [18] of the proximal end of the lateral knee joint cleft and the inferior end of the external capsule, respectively (Fig. 3b). A physiotherapist with 7 years of clinical experience who has experienced clinical examinations and graduate studies confirmed the high reliability of the measured values in preliminary experiments, and performed all measurements and data analysis by himself.
Assessment of developmental age
Since the level of foot growth differs depending on the biological maturity [13], the peak height velocity age (PHVA), an index of biological maturity, was calculated using the BTT method and examined. The BTT method estimates physical maturity by assessing predicted height structure using the Bock, Thyssen and du Toit (BTT) mathematical structural growth model in AUXAL software [19]. PHVA, the age at which height increases the most, was estimated from the history of each participant's height data using a dedicated software (AUXAL3.1, Scientific Software International Inc, USA). Developmental age is the difference between age and PHVA at the time of measurement and is an indicator of maturity [20].
Procedure
To assess the developmental effects, the intervention study was conducted as an 18-weeks crossover study (Fig. 1). The study protocol included a pre-intervention session prior to the start of the experiment, in which an overview of the experiment, the wearing method of the arch supporter, and precautions were explained. Obtained consent before conducting the experiment. The participants were randomly divided into two groups: the first-half supporter group received a supporter intervention period of 9 weeks in the first half and an observation period of 9 weeks in the second half. The second-half supporter group received an observation period of 9 weeks in the first half and a supporter intervention period of 9 weeks in the second half. The intervention and observational phases were switched at 9 weeks after the intervention. Since a previous report showed that the intervention was effective after 8 weeks in adults [11], the intervention period was 9 weeks. For the allocation, one examiner prepared a list table in the order in which the participants arrived at the measurement site. Another examiner blinded the measurement results and randomly allocated numbers 1 and 2 from the top of the list table, dividing them into two groups. The first group was designated as the first half supporter group and the second group as the second half supporter group. Measurements were taken at three time points: pre-intervention, midterm, and post-intervention. For the results, the difference between the second measurement and the first measurement was defined as the period I change, and the difference between the third measurement and the second measurement was defined as the period II change.
For the arch support, an arch supporter (Solvo-Tate Arch Supporter, Sanjin Sangyo Co Ltd, Saitama, Japan) with an arch pad made of a viscoelastic polymer material attached to a stretchy knit was used (Fig. 4). The small (S) or large (L) size of the supporter was selected according to the participant’s foot length. The height of the arch pad was 8 mm for size S and 10 mm for size L, and the cloth thickness was 1 mm). The method of wearing the supporter was explained using the manufacturer’s instructions, according to which the pads attached to the supporter were aligned with the medial arch of the foot and worn barefoot with socks worn over the top. The participants were instructed to wear shoes for as long as possible, except during strenuous exercise, sleeping, and bathing. They were also instructed to record the time for which the arch supporters were worn. An in-person site visit was conducted between the 4th and 5th weeks of intervention where the examiner evaluated the wearing method of the arch supporter and the wearing time using questionnaires. Additional instructions were provided as needed to increase compliance with the arch supporter use.
Statistical analysis
We compared the means of the two groups’ physical characteristics by using either an unpaired t-test or the Mann–Whitney U test. To examine the effect of the intervention, the sum of the means of the change in the first half of the supporter group in period I and the change in the second half of the supporter group in period II and the change in the first half of the supporter group in period II and the change in the second half of the supporter group in period I were compared using a corresponding t-test for items for which normality was found and a t-test for items for which normality was not found. The Wilcoxon signed-rank sum test was used for items that were not identified. For items that were indicated to be significant by the corresponding t-test or Wilcoxon signed-rank sum test, an additional analysis was performed using analysis of covariance (ANCOVA) with the developmental age as a covariate to take into account the effects of growth. The significance level was set at less than < 0.05, and the effect size was calculated using Cohen's d (small: 0.20, medium: 0.50, and large: 0.80). A statistical software (SPSS Statistics 27, IBM, USA) was used for the statistical analysis.