1. Introduction
In recent years, many countries have made great efforts to win medals in various sports competitions, including the Olympic Games and World Championships [1]. Swimming is one of the sports where athletes have won many medals in the Olympic Games. In order to improve front crawl swimming records, it is necessary to perform effective techniques. One of the effective factors in improving the performance of swimmers is the stroke index (stroke length and stroke number). Strong core stability transfers the produced forces of the lower limbs to the upper limbs and improves athletic performance [2]. On the other hand, weak core stability impairs energy transfer and reduces athletic performance [3]. 
Therefore, it seems appropriate to use regular core stability exercises to reduce injury, improve the record and performance in swimmers. Due to the importance of the subject and the little research done in this field, especially in relation to swimmers, the present study aimed to examine the effect of 6-week core stability exercise on the stroke index and the front crawl swimming record in adolescent elite swimmers.
2. Participants and Methods
This is a quasi-experimental applied study with pretest/posttest design. Study samples were 24 swimmers with a mean age of 12.60±1.60 years, mean height of 165.05±12.54 cm and mean weight of 48.90±10 kg. They were selected using a convenience sampling technique and based on inclusion criteria. No history of tobacco use, no supplementation before participating in the training program and being a member of the national team or the selected team of the province were some of the criteria for entering the study. 
Having cardiovascular disease, diabetes, high blood pressure and a history of previous spinal and lower extremity injuries were the exclusion criteria. First, 30 samples completed a health and physical questionnaire (surveying mental and emotional states, the amount of physical activity per day, and problems during physical activity), and after analyzing the questionnaire, 24 were selected to participate in the study. Samples voluntarily participated in the study and signed a consent form. They were then randomly divided into exercise (n=12) and control (n=12) groups.
The core stability training program was performed for 6 weeks, 3 sessions per week, each for 45-50 minutes. It included 15-min warm-up, aerobic exercise for 10-25 minutes, and 10-min cooling down. Warming up session included stretching movement and general warming of the joints. The intensity of the training gradually increased from the first to the 5th week and decreased in the last week due to the participation of subjects in the functional tests. The training program was performed on both core stability muscles including small (muscles that attach to the lumbar vertebrae and affect intercostal movements) and large muscles (those attach to the pelvis and hip). 
On the first day and after a special warm-up, swimmers were asked to swim 100 meters breaststroke in a 25-meter-long pool with all their might to estimate the distance of the stroke and the frequency of the stroke. On the first day and after a special warm-up, swimmers were asked to perform 100 meter front crawl in a 25-meter-long pool with all their power to estimate stroke rate and distance per stroke. Each subject was tested three times and the best record and the best stroke number were recorded. The test taker counted and recorded the number of strokes by moving along the pool. At the end of the swim, the best time was considered as the swimmer’s record. 

At the end of each session, the cooling down was performed by stretching and relaxation movements. During this time, the subjects in the control group performed their routine exercises. At the end of intervention and after one day of rest, the tests were performed in both groups. The collected data were analyzed in SPSS v.16 software. After confirming the normality of data distribution using the Shapiro-Wilk test and the homogeneity of the variances by Levene’s test, student t-test was sued for to intergroup comparison and ANCOVA for intergroup comparison of mean changes. The significance level was set at P<0.05.
3. Results
Based on the results presented in Table 1, the t-test results showed that the 6-week core stability training program led to a significant improvement in the subjects’ 100-meter front crawl record (P=0.02), but there was no significant change in their stroke index (P=0.4). In the control group, no significant changes were observed in subjects’ front crawl record and stroke index. According to the results in Table 1, the ANCOVA results showed no significant difference in the mean scores of stroke index (P=0.4) and 100-meter front crawl record (P=0.39) between and within the two study groups.
4. Conclusion
Core stability training can improve swimmers’ front crawl records; so, it is recommended that these exercises be included in the swimmers’ dryland training. Moreover, due to the lack and contradiction of the results regarding the effect of core stability exercises on the performance of swimmers, more studies are recommended in this area.
Ethical Considerations
Compliance with ethical guidelines
All ethical principles were considered in this study. The subjects were free to leave the study at any time, and were assured of the confidentiality of their information.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Authors' contributions
All authors contributed equally in preparing this article.
Conflicts of interest
The authors declared no conflict of interest.