Volleyball is a sport dominated by strength and power, in which several physical fitness factors are involved [4]. Explosive jumps in this sport require high lower limb strength, and strong spikes require the strength and endurance of upper limb muscles. And maintaining balance when performing these skills is also provided by the strength and endurance of the trunk muscles [5]. As a result, coaches and players are always trying to maintain and improve these factors at a desirable level by examining and controlling these features. Physical fitness characteristics, endurance, flexibility, and strength play a key role in optimizing performance and preventing injury in volleyball. The quarantine period and immobilization due to coronavirus are seriously threatening these factors [6]. The study aimed to evaluate the effect of a period of combined training and central stability with the observance of health protocols on selected variables of physical fitness of adolescent and young volleyball players during the coronavirus epidemic.
2. Materials and Methods
Fifteen adolescent and young male volleyball players participated in this study by available random sampling method. After the first quarantine period, the volleyball players returned to the volleyball hall to resume training. At the beginning of their arrival and before exercise, the selected key factors of physical fitness, including lower limb strength, upper limb, trunk endurance, and flexibility, were measured as a pre-test by a volleyball coach specializing in assessment.
After the pre-test phase, a 12-week combined exercise program and central stability emphasized physical fitness exercises followed by all health protocols announced by the World Health Organization (WHO). This step was performed by the players and warming up and cooling down to prevent injury under the supervision of their coach. Then, following the second quarantine period and at the end of the twelfth week, the tests were repeated by the volleyball coach who had recorded the pre-test tests to evaluate the post-test.
The SPSS software v. 21 was used for statistical analysis. Mean±SD was used to analyze descriptive statistics, and the Shapiro-Wilk test was used to test the normality of data distribution. Then, to evaluate and compare the differences between the means of pre-test and post-test variables among volleyball players, paired t-test was used. All stages of statistical tests were performed at a significant level of 0.05 (P≤0.05).
3. Results
The results of the Shapiro-Wilk test showed the distribution of data was normal in all variables. The descriptive statistics results showed that the post-test stage’s mean size in all variables was higher than the average of the pre-test stage (Figure 1). 



The results of inferential statistics related to the paired t-test of the variables of the present study showed there is a statistically significant difference between the two stages of pre-test and post-test in the variables of upper limb and trunk endurance as well as flexibility (chest press, back arm press, sit-ups and sit and reach tests). However, no significant difference was observed in the lower limb power variables (three-step jump tests and Sargent jump) at two levels (P≤0.05).
4. Conclusion
This study aimed to investigate the effect of a combination training course and central stability along with health protocols on selected variables of physical fitness of adolescent and young volleyball players during the coronavirus epidemic. The present study results showed there was a statistically significant difference in the variables of the upper limb and abdominal muscle endurance and muscle flexibility. Still, this difference was not observed in the lower limb strength variable in the pre-test and post-test levels (P≤0.05).
According to available studies, no research has been found on the effect of a specific exercise program by following health protocols of a particular group of athletes such as volleyball players. The only study in this field was the research of Tertipi et al. (2020) [12], whose main emphasis was not only on the physical changes of volleyball players during pandemics immobility, instead, but it has also been on limiting volleyball players’ access to gyms as well as its economic consequences in volleyball.
A review of the results of this study shows an increase in the rate of jumping in the study group. Probably the lack of plyometric exercises as one of the most critical training programs in the development of lower limb strength in volleyball players, also, the impossibility of performing spike and defense skills on the net in conventional volleyball exercises due to being held outdoors and on the grass is one of the reasons for the lack of significant differences between the pre-test and post-test results in this study.
Improving the range of motion of the joints and flexibility, especially for the muscles of the back of the body, is effective in preventing injuries and has a direct relationship with the level of performance of volleyball players [6]. Implementing a central stability training program by applying pressure and stress on the muscles of this section and stimulating their neuromuscular characteristics may be one of the most important reasons for observing significant differences in the flexibility parameter in the two levels of pre-test and post-test among young and adolescent volleyball players.
Endurance of the upper and abdominal muscles is a component of physical fitness vital to the successful and consistent performance of skills such as defense, attack, and service in volleyball [16]. A review of the results of the exercises planned in this study shows a significant increase in upper and abdominal muscle endurance in young and adolescent volleyball players. 
It is possible that the load and volume of central body stability training during the training period and the proportion of time required for the muscles to adapt to the training pressure justify this statistical increase in the endurance factors of the upper extremity and trunk muscles.
Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them.
Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 
Authors' contributions
All authors equally contributed to preparing this article.
Conflicts of interest
The authors declared no conflict of interest.
Acknowledgements
The researchers express their gratitude to the players and families of the Alborz Star Volleyball Club players who helped us in this research.

 

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