1.    Introduction
Walking is one of the fundamental human movements, and various factors can influence an individual's gait pattern. One such factor is foot pronation, a common biomechanical condition that often results in functional inefficiency at the ankle joint due to structural changes. These alterations may increase the risk of injury in the affected region (3). The prevalence of foot pronation in adults has been reported to range between 2% and 23% (4). Research has shown that individuals with pronated feet demonstrate altered plantar pressure distribution during walking, including reduced vertical ground reaction forces (GRFs), decreased peak pressure, and increased maximum force in the forefoot region (4). Pronation deformity is characterized by a lowering of the medial longitudinal arch during weight-bearing, often accompanied by misalignments in the ankle structure. This abnormality causes medial displacement of the talus and navicular bones, contributing to the development and progression of the deformity. Moreover, excessive foot pronation has been associated with increased mechanical stress on the ankle, knee joints, and pelvic girdle (5).
Recent studies suggest that modifying gait patterns through targeted training or environmental interventions can aid in the correction of flatfoot deformities (6). Additionally, differences in the vertical component of GRFs between individuals with and without foot pronation have been well documented (7). Identifying effective interventions to reduce or manage this condition may play a crucial role in restoring normal force distribution patterns. One promising approach is the use of orthopedic insoles. Anti-pronation insoles are designed to support the medial arch and realign foot posture during walking. By understanding their impact on the frequency spectrum of GRFs, researchers and manufacturers may develop more effective and individualized insole designs. Given the reported benefits of such insoles, the present study aimed to compare the frequency spectrum of ground reaction forces during walking with and without the use of anti-pronation insoles in individuals with pronated feet (1, 2).
2.    Methods
This semi-experimental, laboratory-based study included 20 male participants with pronated feet (mean age: 21.12 ± 1.47 years; mean weight: 71.52 ± 5.18 kg; mean height: 178.15 ± 4.18 cm). The required sample size was calculated using G*Power 3.1, indicating a minimum of 18 subjects to achieve a statistical power of 0.80, an alpha level of 0.05, and an effect size of 0.8. Participants were selected based on the presence of foot pronation and the absence of any prior lower limb injuries. The dominant foot was identified as the foot preferred for kicking a ball. Informed consent was obtained from all participants, and the study was conducted in accordance with the Declaration of Helsinki.
Gait analysis was conducted using a Footscan platform (0.5 × 0.5 × 0.02 m, 4363 sensors) placed at the midpoint of a 20-meter walkway. Vertical ground reaction force (GRF) data were recorded at a sampling rate of 300 Hz using RSScan software, under two conditions: with and without an anti-pronation insole made of EVA material, manufactured in Iran. Participants completed three familiarization trials followed by three successful trials under each condition. Only steps with full foot contact within the central area of the platform were considered for analysis. GRF data were filtered using a fourth-order Butterworth filter (cutoff frequency: 20 Hz) and analyzed in the frequency domain using harmonic analysis in MATLAB 2016. Extracted parameters included 99.5% power frequency, median frequency, frequency bandwidth, and the number of essential harmonics (ne). Harmonic decomposition provided insight into the signal’s frequency content and fluctuations in the force-time curve. Data normality was verified using the Shapiro–Wilk test. A paired t-test was conducted to compare conditions, with statistical significance set at p < 0.05. Effect sizes (Cohen’s d) were calculated for each comparison.
3.    Results
The results of the vertical ground reaction force frequency spectrum are summarized in Table 1. The analysis revealed that the frequency at 99.5% power in the vertical component of the ground reaction forces was significantly higher during walking with insoles compared to walking without them (p = 0.047, d = 0.85). However, no significant differences were found in the other frequency-domain variables (p > 0.05).

4.    Conclusion
The aim of the present study was to compare the frequency spectrum of ground reaction forces (GRFs) during walking with and without insoles in individuals with foot pronation. The results indicated that the frequency at 99.5% power in the vertical component of GRFs was significantly higher during walking with insoles compared to without them in individuals with pronated feet. Moreover, the frequency at 99.5% power was also higher in the big toe and toes 2 to 5 when using insoles. Similarly, higher frequencies were observed in the vertical GRF component of the first to fourth metatarsal regions. This finding may be interpreted by considering that a reduction in the frequency content of the vertical GRF signal reflects increased fluctuations during walking (9). Previous studies have shown that one of the contributing factors to such fluctuations is instability caused by the collapse of the navicular bone (10). The observed increase in frequency when using orthotic insoles suggests that they effectively modify the distribution of vertical forces during foot-ground contact. This may be attributed to enhanced foot stability and improved control over pronation movements, thereby influencing the dynamic loading patterns (11). Overall, the use of orthotic insoles in individuals with pronated feet appears to alter gait biomechanics by improving the distribution of vertical forces and promoting more effective engagement of different foot structures. These findings have practical implications for the design and clinical application of anti-pronation insoles to improve walking performance and reduce injury risks associated with foot deformities.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
Funding
This research did not receive any financial support from government, private, or non-profit organizations.
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declare that there are no conflicts of interest associated with this article.