Introduction
The arches of the foot, including the medial arch, play a critical role in performing numerous functions of the foot, such as absorbing and distributing forces and maintaining body stability [1]. In foot pronation, the foot’s medial arch decreases, causing the navicular bone on the inner side of the foot to move downward or inward, which can also cause other anatomical abnormalities [2]. The decrease in the height of the longitudinal arch is due to the condition of the bones and ligaments surrounding it and it plays an important role in maintaining balance and performing motor skills. Foot pronation is a risk factor for lower limb and trunk injuries during activities such as walking and running [3]. The navicular drop test is used to quantify foot pronation. It has been reported that people with pronated feet have greater external angles during walking compared to normal people [4]. Therefore, pronated foot is one of the risk factors of injuries in runners, such as medial tibial stress syndrome and runner’s knee [5]. The present study aims to investigate the effect of long-term sand training on the frequency of lower limb muscle activity in men with flat feet during walking.

Methods
This is a randomized clinical trial. Participants included 60 men with foot pronation who were randomly assigned to two groups of control (n=30) and training (n=30). The sample size was determined using G*Power software. To record the electrical activity of muscles, an 8-channel electromyography device with a sampling rate of 500 Hz, a bandwidth of 12-50 Hz, a band-pass filter of 10-500 Hz, and surface bipolar electrodes [20] were used every day from 9 AM to 3 PM at the Health and Wellness Center of the University of Mohaghegh Ardabili. A 50 Hz notch filter was used to remove the noise caused by the power line frequency in the system. 
The training group performed walking, running, and jumping exercises on the sands of Shourabil Lake in Ardabil city every day from 5:00 to 7:00 PM for 8 weeks, three sessions per week, each for 40-50 minutes. The training program included 5 minutes of warming up and 5 minutes of cooling down. The exercises on the sand began with walking, followed by running and finally jumping [21]. To prevent the interference of acute physiological responses to exercise, the training group participated in a post-test assessment 6 days after the last training session.
All statistical analyses were performed in SPSS software, version 23. The Shapiro-Wilk test was used to examine the normality of the data distribution. Repeated measures analysis of variance was used for data analysis, followed by the post hoc Bonferroni test. Independent t-test was used to compare the demographic characteristics of the two groups during the pre-test. 

Results
The results of the independent t-test showed no significant difference between the groups in demographic characteristics. 

During the loading response phase of gait
The effect of time was statistically significant on the frequency of tibialis anterior (P=0.043) and biceps femoris (P=0.034) activity. Their activity was higher in the post-test phase compared to the pre-test phase; 
The effect of the group factor was statistically significant on the frequency of the medial gastrocnemius medialis (P=0.032) and vastus lateralis (P=0.000) activity;
The interaction effect of time and group was significant only on the frequency of tibialis anterior muscle activity (P=0.009). Its activity significantly increased in the training group (but not in the control group) during the post-test phase compared to the pre-test phase.

During the mid-stance phase of gait
The effect of time was significant on the frequency of the rectus femoris (P=0.019), which was lower in the post-test phase compared to the pre-test phase. 
The effect of the group factor was significant on the frequency of the tibialis anterior (P=0.003), vastus medialis (P=0.000), and semitendinosus (P=0.046). Their median frequencies were higher in the training group compared to the control group.
The interaction effect of time and group was significant only on the frequency of the gluteus medius muscle (P=0.026). Its activity increased in the control group and decreased in the training group during the post-test compared to the pre-test phase.

During the push-off phase of gait
The effect of time was statistically significant on the frequency of the vastus lateralis muscle (P=0.000), which was lower in the post-test phase compared to the pre-test phase. 
The effect of the group factor was statistically significant on the frequency of the tibialis anterior muscle (P=0.001). Its median frequency was higher in the training group compared to the control group.
The interaction effect of time and group was statistically significant on the frequency of the gluteus medius muscle (P=0.022). The frequency of this muscle in the control group did not change in the post-test compared to the pre-test phase, but decreased in the training group.

Conclusion
In mean with foot pronation, Long-term training on sand can increase the electrical activity of tibialis anterior and biceps femoris in the loading response phase, reduce the activity of the rectus femoris in the mid-stance phase, and reduce the activity of vastus lateralis in the push-off phase of gait.

Ethical Considerations
Compliance with ethical guidelines
This study was registered by the Iranian Registry of Clinical Trials (IRCT), Tehran, Iran (Code: IRCT20191211045704N1). This study was approved by the Research Ethics Committee of Ardabil University of Medical Sciences (Code: IR.ARUMS.REC.1398.484).

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
Conceptualization, study design, data collection, analysis, interpretation and writing the original draft: All authors; Project administration, supervision, review and editing: AmirAli Jafarnezhadgero.

Conflicts of interest
The authors declared no conflict of interest.


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