1. Introduction
Volleyball is the most attractive sport in the world. Approximately two hundred million players in 105 countries are active in this sport [1]. Volleyball can be a really rivalrous game and requires high motive skills. The major bodily characteristics of flexibility, explosive power, static balance, muscular strength, speed and muscular endurance are fundamental. A player on the volleyball court always needs to change direction, speed, and body position. S/he needs effectual deceleration or acceleration in space in an instant time to perform it well [2]. As for all functioning constituents, a volleyball player must be in control of his physique and transmit energy from the ground before hitting the ball [3]. In each volleyball technique, the core muscles play a vital role in movement efficiency, keeping body posture, flexibility and endurance power during repeated execution, and supporting the stabilization and strength of arms and legs [4]. The firmness of the core region as an intermediary with the effectual transferring of the produced force in the lower extremity to the upper extremity into and out of the trunk contributes to finer athletic performance [5]. Hodges et al. (2004) provided for the first time a complete definition of core stability. They defined core stability or lumbar-pelvic stability as a dynamic process of controlling stability in functional conditions so that it allows controlled movement of the trunk in different situations [6]. Core muscles provide the stability for power generation in the lower limbs and effective control of body movements. Deficiency or imbalance in core muscles can increase fatigue and the risk of injury and also decrease endurance in athletes [7]. The core region muscles are of an effect on the activation of the muscles of the organs; so, any weakness in these muscles leads to a delay in the activation of the organs and the occurrence of various injuries [8]. The muscles weakness in the core region is associated directly with the occurrence of more injuries in the lower extremities, particularly in sports that need leaping, fast running and jumping. Increasing the core region firmness makes the neuromuscular response to increases in order to reduce back discomfort and prevent lower limb injuries [9]. As researches have shown, the reduction of strength in the core body region muscles increases body fluctuations, and consequently, it may cause a disturbance in the body’s balance [10]. As Jeffreys et al. report, core stability and balance are necessary components for proper performance in athletes. The sports require this because most athletic movements are performed in all three planes of motion; they require the athlete to have optimal strength in the thigh and trunk muscles, which ensures effective core stability. According to reports, most athletes need proper balance, strength, power, endurance, and a symmetrical body, but having a stable body is an essential part of all these features. As researches show, the lack of core strength and stability causes unfavorable efficiency of athletic techniques and makes athletes prone to injury [11]. Improving the speed, agility, and strength of the lower and upper body muscles for volleyball players is vital. Besides the specified requirements physiologically considering volleyball, the core muscle groups coordination is notable if utilized effectually [12]. Core exercises are very important for stability and strength growth [13]. Exercises of the body’s core region are an inseparable part of preparation and strength training. This means that stronger core stability probably supplies a foundation for create more force in the lower and upper extremities [14]. Core stability exercises are essential for volleyball and can ensure the balance and strength of the core region [15]. A simple analogy: Even with the best tires, smoothest gearbox, and simplest body, a car is only as good as its engine, and its power plant. Similarly, an athlete without stable and flexible core stability produces less than maximum efficiency and falls well short of his athletic ability [16]. CSE are able to enhance the ability of the nervous system in order to organize muscle coordination for increasing exercise performance. By neglecting core stability exercises, athletes cannot control and use whole-body muscle strength well, which may increase the risk of sports injuries [17]. According to the theoretical foundations and studies on the core stability exercises and their effects in various areas, in preventing injuries, improving the level of sports performance, and improving the movement capabilities of athletes, especially volleyball players, they can be useful in their athletic future. It seems necessary to summarize existing research until now. Therefore, the present study reviews systematically the researches on the impact of CSE on the volleyball players movement capabilities.

2. Materials and Methods
Search strategy: This study was a PRISMA systematic review [18]. It reviewed comprehensively the impact of CSE on volleyball players. It carried out searching in Google Scholar, PubMed, Science Direct, PEDro, EMBASE, Web of Science, and Scopus databases and in Persian Google Scholar, Megapaper databases, IRANDOC, Magiran, SID, Medilib, IranMedex. The keywords in two groups including: Group 1: ”core stability exercise” OR “core stability training” OR “core stability endurance” OR “trunk stability” OR “central stability exercises” OR “core intervention” OR “core stability control” OR “core strengthening exercise” OR “trunk resistance exercise” and group 2: ”Volleyball” OR ”player“ OR “athletes” from 2000 to February 2023. Sometimes, all terms were combined with no research restrictions in the search. The study performed a manual search and thorough review of article references to find articles that were not found through the electronic search. This search used both English and Persian languages. Following gathering of the outcomes of research, the researcher studied firstly the title and secondly the articles abstract. Where the articles were consistent with the criteria of exclusion and inclusion, their outcomes were utilized in the current review or were abandoned otherwise. The structure of the PICOS framework can be seen in Table 1.


Inclusion criteria and selection of studies: In the first stage, the title and abstract of the studies were screened by Hamid Zolghadr and Adibeh Baharmast Hossein Abadi focusing on the impact of CSE on the volleyball players movement capabilities, it was published in Farsi and English the second stage of all the text of the articles by Hamid Zolghadr and Adibeh Baharmast Hossein Abadi It was investigated according to the publication criteria and the specificity of the target community. Another senior researcher Parisa Sedaghati checked the last list of selected articles to ensure the alignment of all articles with the study aims. Inclusion criteria included volleyball players, core stability interventions, original research articles, experimental, and semi-experimental studies, randomized clinical trials, English and Farsi articles, and studies as published full text. Exclusion criteria comprised articles published as a summary in congresses and seminars, single case studies, pilot studies, and reviews. Based on the stated criteria, we extracted 146 articles, of which only 18 articles were under review.
Quality of the reviewed researches: The research evaluated the methods quality of the relevant studies through the Pedro scale [19]. Current scale encompasses 11 modules. They measure the following criteria: 1) Determination of eligibility criteria for participants, 2) Participants random allotment, 3) Allotment of concealment, 4) Similarity among groups in relevant variables in the pretest stage, 5) Deprive of understanding of the participator, 6) Deprive of understanding of the researcher who implemented the exercise programmed, 7) Deprive of understanding of the evaluators to measure the dependent variables, 8) The ratio of participator, who have not less than one key outcome in the measured dependent variable, 9) Complaisance of participators with the intervention, 10) Comparison among groups statistically, and 11) Measures of point and variability for not less than one dependent variable. Satisfaction of a criterion had a score of 1, and making a summary of the outcomes got out of items 2 to 11 provided an overall score [20]. Studies whose total score is less than 5 and over 6 were of “low” and “high” quality, respectively [19, 20]. Table 2 reports the outcomes of the evaluation of quality of the reviewed researches.



3. Results
Figure 1 shows selecting studies. We got 146 articles. After removing duplicate titles, we selected 127 abstracts for reviewing. Following analysis of the abstracts of the papers, 87 papers got removed and 40 papers got chosen for studying the whole text. Following reviewing all of the papers, the current study removed 22 articles and included 18 articles. Table 3 summarizes the findings of the articles.







Training period
The duration of core stability exercises varied between 6 and 12 weeks. In only one study, core stability exercises were performed for 12 weeks [27]. In two studies, the duration of their exercises was 10 weeks [23, 28]. The remaining 15 studies administered the exercises between 6 and 8 weeks. Also, 456 subjects (experimental group: 274 people and control group: 182 people) were present in these studies. The largest number of subjects (60 people) and the number of training weeks were related to the study by Sazvar et al. [23].

Evaluation groups and interventions
Two studies had three groups of subjects [24, 32]. And in other studies, two groups participated. All training groups received the intervention of core stability exercises. CSE were executed on two types of stable and unstable surfaces.

Dependent variables under the impact of CSE
Throughout 18 studies included in this review paper, the impact of CSE on variables such as agility, explosive power, dynamic and static balance, aerobic and anaerobic power, speed, endurance of muscle, proprioception, range of motion, landing mechanism, rate of control Breathing, lower limb strength, flexibility, jump-landing pattern, service speed and accuracy, spike skill, and kinematic variables were investigated.

4. Discussion
This review investigated the impact of CSE on the movement capabilities of volleyball players. According to the inclusion and exclusion criteria, the present review included 18 articles. Therefore, it discussed the results of 18 studies in different groups.

Impact of CSE on the volleyball players’ balance
On the impact of CSE on the of volleyball players balance, there were eight studies [21-23, 28, 31, 32, 34, 37]. These studies investigated the impact of balance on dynamic and static balance. As the results showed, CSE can enhance both dynamic and static of volleyball players’ balance. All the studies showed the positive effect of CSE on the volleyball players’ balance. Sports like volleyball, in which players perform jumping and landing movements during the game, make it necessary to have proper balance to maintain the landing position followed by a successful jump. As the studies reported in this review showed, having a strong core region can improve balance in volleyball players. Therefore, balance is the most important movement skill in volleyball players, and its improvement can improve the level of athletic performance and prevent injuries. According to Kibler’s findings, the activation of the core region muscles in the movement pattern of the end organs improves posture control; the body uses the activation of the core stabilizing muscles to generate rotational force around the body and create the movement of the limbs [38]. Stabilizing muscles of the hip and thigh undertake to hold the accurate alignment of the lower extremity during dynamic movements [39].

Impact of CSE on endurance and strength of muscles of volleyball players
About the impact of CSE on the muscular endurance and strength of volleyball players, there were eleven studies [21, 23-25, 27-33]. These studies investigated the impact of CSE on the muscular endurance and strength of the trunk muscles, the strength, and power of the of the upper and lower extremities muscles. As the results showed, CSE can enhance the strength and endurance of the muscles of the trunk, and the power and strength of the upper and lower extremities. Thus, all the studies showed the positive the impact of CSE on the improvement of the relevant variables. Only the study of Fatahi et al. [27] revealed that CSE improved the muscular endurance of the trunk and upper limbs, but did not improve the strength of the lower limbs. Seemingly, the tests for evaluating the strength of the lower limbs were not suitable for this study and they could have used better and more accurate tests, such as Isokinetic tests. Having the endurance and strength of the muscles of the trunk, consequently having the muscle power and strength of the lower limbs, is an essential factor in volleyball players. Because of the long duration of the volleyball, having proper muscle endurance and strength can maintain a player’s athletic performance. Therefore, the weakness and reduced endurance of the posterior, anterior, and lateral stabilizing muscles of the trunk causes reduced strength and efficiency of the muscles around the thigh. The thigh muscles play an important role in transferring power from the lower limb to the upper spine during vertical or standing activities [39]. The weakness of the core stabilizing muscles can disrupt the accurate alignment of the lower extremity in dynamic movements and disturb the movement pattern in the lower limb [39].

Impact of CSE on volleyball players agility, anaerobic power and speed 
About the impact of CSE on speed, anaerobic power and agility of volleyball players, there were four studies [21-23, 32]. These studies investigated the impact of core stability training on speed, anaerobic power and agility of volleyball players. The results showed that stability exercises improve volleyball players speed, anaerobic power and agility of agility and high speed, and having the right anaerobic capacity in the execution of techniques and consecutive rallies in volleyball, is a fundamental factor. Since the speed of throwing a ball is very high in volleyball, having agility and speed in implementing defensive and offensive techniques is very important for a volleyball player. Therefore, as the results of the present review show, core stability exercises can improve these vital factors in volleyball players.

Impact of CSE on kinetic and kinematic variables of volleyball players
About impact of CSE on kinetic and kinematic variables of volleyball players, there were five studies [25, 28, 30, 33, 35]. These studies investigated the impact of CSE on the jump-landing pattern, landing mechanism, trunk and trunk kinematics, kinetic and kinematic variables, as well as biomechanical analyzes. The results showed that core stability exercises correct and improve the kinetic and kinematic variables of the trunk and lower limbs of volleyball players. Because many jumps and landings occur in volleyball, it is essential to consider the mechanism and patterns of jump and ford of volleyball players. Therefore, the results of the current review emphasize the impact of CSE on improving the jumping and landing patterns of volleyball players.

Impact of CSE on volleyball players athletic performance 
About impact of CSE on volleyball players athletic performance, there were four studies [4, 28, 31, 33]. These studies investigated impact of CSE on the speed and accuracy of service skills, athletic performance, and spike skills. As the results showed, CSE improve the athletic performance and skills of volleyball players. Only in the study of Tsai et al. [33], core stability exercises did not improve the athletic performance of volleyball players. Seemingly, the low number of subjects and the use of general tests to evaluate the athletic performance of volleyball players causes insignificance in this study. By improving balance, strength, endurance, agility, and other relevant factors in volleyball players, core stability exercises can improve their athletic functioning. The outcomes of the current review confirm this finding as well.

Impact of CSE on other movement capabilities of volleyball players
About impact of CSE on other movement capabilities of volleyball players such as proprioception, range of motion, flexibility, and breathing patterns, there were three studies [24, 26, 27]. These studies investigated the impact of CSE on proprioception, range of motion, flexibility and volleyball players breathing rate. The results of these studies show that core stability exercises can improve relevant variables in volleyball players. Some core stability exercises are necessary to control the lumbopelvic chain, which causes breathing control in performing the movements accurately in these exercises and improves the breathing rate and the function of the diaphragm muscle.
Of course, caution need to be taken in order to interpret the conclusions of the current study, considering the restrictions. Such as the difference in the number of samples under study, measurement methods, and components of different core stability protocols. However, these outcomes can assist sports teachers, coaches, therapists and physiotherapists to enhance the movement capabilities of volleyball players. Comprehensive researches are necessary. Limitations in the studies: 1. No research investigated the durability of the impact of the exercises after the completion of the follow-up exercise period. Consequently, in the future researches, after the follow-up exercises, examining the effect of their durability is necessary for several weeks. 2. Most of the studies did not clearly explain the method of blinding and preventing bias in the research. 3. They did not clearly state the training environment and supervising these exercises. 4. Finally, all the studies reported in the current review were at a low level in terms of quality according to Pedro’s scale. Therefore, future studies should conduct high-quality research with accurate reports so that we can see the effectiveness of the studies, the improvement of the balance and endurance of female athletes.

5. Conclusion
The outcomes of current review show that CSE can improve movement capabilities, performance, and sports skills as well as prevent injuries from volleyball players. Because of the diversity of its movements, we can easily level the core stability exercises according to the level of the players for achieving their training goals. 

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research. This article is a systematic review with no human or animal sample.

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

Authors' contributions
The authors equally contributed to preparing this article.

Conflict of interest
The authors declared no conflict of interest. 

Acknowledgments
The authors hereby thank the Educational and Research Officials of Guilan University and the Faculty of Sports Sciences.


References

1. Beneka A, Malliou P, Gioftsidou A, Tsigganos G, Zetou H, Godolias G. Injury incidence rate, severity and diagnosis in male volleyball players. Sport Sciences for Health. 2009; 5:93-9. [DOI:10.1007/s11332-009-0083-9]
2. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. The effects of core proprioception on knee injury: A prospective biomechanical-epidemiological study. The American Journal of Sports Medicine. 2007; 35(3):368-73. [DOI:10.1177/0363546506297909] [PMID]
3. Smith CE, Nyland J, Caudill P, Brosky J, Caborn DN. Dynamic trunk stabilization: A conceptual back injury prevention program for volleyball athletes. Journal of Orthopaedic & Sports Physical Therapy. 2008; 38(11):703-20. [DOI:10.2519/jospt.2008.2814] [PMID]
4. Mohamed HN,  Rezk M. The effect of core stability training on the accuracy of back row attack for volleyball beginners.  International Journal of Technical Research and Application. 2015; 22:94-9. [Link]
5. Nesser TW, Lee WL. The relationship between core strength and performance in division I female soccer players. Journal of Exercise Physiology Online. 2009; 12(2):21-8. [Link]
6. Hodges P. Lumbopelvic stability: A functional model of the biomechanics and motor control of lumbopelvic stability.  In: Richardson C, Hodges PW, Hides  J. Therapeutic exercise for lumbopelvic stabilization. London: Churchill Livingstone; 2004. [DOI:10.1016/B978-0-443-07293-2.50005-4]
7. Rivera CE. Core and lumbopelvic stabilization in runners. Physical Medicine and Rehabilitation Clinics. 2016; 27(1):319-37. [DOI:10.1016/j.pmr.2015.09.003] [PMID]
8. Clary S, Barnes C, Bemben D, Knehans A, Bemben M.  Effects of ballates, step aerobics, and walking on balance in women aged 50-75 years. Journal of Sports Science & Medicine. 2006; 5(3):390-9. [PMID]
9. Mannion AF, Dvorak J, Taimela S, Müntener M. [Increase in strength after active therapy in chronic low back pain (CLBP) patients: Muscular adaptations and clinical relevance (Persian)]. Der Schmerz. 2001; 15(6):468-73. [DOI:10.1007/s004820100034] [PMID]
10. National Center for Injury Prevention and Control.  Preventing falls: How to develop community-based fall prevention programs for older adults. Atlanta: National Center for Injury Prevention and Control; 2008. [Link]
11. Jeffreys I, Powys C. Developing a progressive core stability program. Strength & Conditioning Journal. 2002; 24(5):65-66. [DOI:10.1519/00126548-200210000-00017]
12. Gabbett T, Georgieff B. Physiological and anthropometric characteristics of Australian junior national, state, and novice volleyball players. The Journal of Strength & Conditioning Research. 2007; 21(3):902-8. [DOI:10.1519/00124278-200708000-00042] [PMID]
13. McGill S. Core training: Evidence translating to better performance and injury prevention. Strength & Conditioning Journal. 2010; 32(3):33-46. [DOI:10.1519/SSC.0b013e3181df4521]
14. Behm DG, Anderson K, Curnew RS. Muscle force and activation under stable and unstable conditions. The Journal of Strength & Conditioning Research. 2002; 16(3):416-22.[DOI:10.1519/00124278-200208000-00012]
15. Şatıroğlu S, Arslan E, Atak M. Voleybolda core antrenman uygulamaları. Paper presented at: 5th Training Science Congress. 3 July 2013; Ankara, Turkey. [Link]  
16. Luo S, Soh KG, Soh KL, Sun H, Nasiruddin NJM, Du C,  et al. Effect of core training on skill performance among athletes: A systematic review. Frontiers in Physiology. 2022; 13:915259. [DOI:10.3389/fphys.2022.915259] [PMID] [PMCID]
17. Faries MD, Greenwood M. Core training: Stabilizing the confusion. Strength & Conditioning Journal. 2007; 29(2):10-25. [Link]
18. Asar S, Jalalpour S, Ayoubi F, Rahmani M, Rezaeian M. [PRISMA; preferred reporting items for systematic reviews and meta-analyses (Persian)]. Journal of Rafsanjan University of Medical Sciences. 2016; 15(1):68-80.
19. Maher CG, Sherrington C, Herbert RD, Moseley AM, Elkins M. Reliability of the PEDro scale for rating quality of randomized controlled trials. Physical Therapy. 2003; 83(8):713-21. [DOI:10.1093/ptj/83.8.713] [PMID]
20. Moseley AM, Herbert RD, Sherrington C, Maher CG. Evidence for physiotherapy practice: A survey of the physiotherapy evidence database (PEDro). Australian Journal of Physiotherapy. 2002; 48(1):43-9. [DOI:10.1016/S0004-9514(14)60281-6] [PMID]
21. Çakir M, Ergin E. The effect of core training on agility, explosive strength and balance in young female volleyball players. Journal of Sport Sciences Research. 2022; 7(2):525-35. [DOI:10.25307/jssr.1171779]
22. Bora H, Dağlıoğlu O. Effect of core strength training program on anaerobic power, speed and static balance in volleyball players. European Journal of Physical Education and Sport Science. 2022; 8(5):72-80. [DOI:10.46827/ejpe.v8i5.4355]
23. Sazvar A, Khodaveisi H. [The effect of ten weeks of training in water with an emphasis on central stability exercises, on balance and some selected movement functions of women volleyball players with chronic ankle sprain (Persian)]. Journal of Sport in Biomotor Sciences. 2022; 27(1):64-71. [DOI:10.22034/sbs.2022.162723]
24. Alizamani S, Ghasemi G, Lenjan Nejadian S. Effects of eight week core stability training on stable- and unstable-surface on ankle muscular strength, proprioception, and dorsiflexion in athletes with chronic ankle instability. Journal of Bodywork and Movement Therapies. 2023; 34:6-12. [DOI:10.1016/j.jbmt.2023.04.005] [PMID]
25. Mirghmali E, Minoonejad H, Seidi F. [The effects of core stability training on trunk strength and landing mechanics in female athletes with trunk defects on a stable and unstable level (Persian)]. Journal of Paramedical Sciences & Rehabilitation. 2022; 11(1):45-56. [DOI:10.22038/JPSR.2022.53041.2183]
26. Solanki HP. Effect of core stability training on breath holding time and respiratory rate among volleyball players. International Journal of Physical Education, Sports and Health 2021; 8(5):82-5. [Link]
27. Fatahi A, Yousefian Molla R, Ameli M, Khezri D. [The effect of combined and core stability training program with protective measures on selected variables of physical fitness of junior and young volleyball players during the coronavirus pandemic (Persian)]. Journal of Sport Biomechanics. 2021; 7(3):162-71. [DOI:10.32598/biomechanics.7.3.293.2]
28. Şahin E, Özdal M. Effect of core exercises on balance and vertical jump of 12-14 aged female volleyball players. European Journal of Physical Education and Sport Science. 2020; 6(4):47-55. [Link]
29. Lestari AD, Wibawa A, Dewi AA, Sugiritama IW. Providing of core stability exercise increase the core muscle strength of adult male Balinese vocational polytechnic volleyball athletes. Bali Anatomy Journal. 2020; 3(1):14-8. [Link]
30. Tafakoriollah A, Daneshmandi H, Samami N. The efect of selected core stability exercises on the jumping-landing pattern and trunk muscular endurance in adolescent volleyball players with trunk defect. Physical Treatments-Specific Physical Therapy Journal. 2020; 10(2):89-98. [DOI:10.32598/ptj.10.2.440.1]
31. Yapıcı A. Effects of 6 weeks core training on balance, strength and service performance in volleyball players. European Journal of Physical Education and Sport Science. 2019; (12):21-62. [DOI:10.5281/zenodo.3596020]
32. Devrim U, Erdem K. Evaluation of the effects of core-quick strength and core-plyometric studies on balance, agility and strength traits of volleyball players. Asian Journal of Education and Training. 2019; 5(3)482-7. [DOI:10.20448/journal.522.2019.53.482.487]
33. Tsai YJ, Chia CC, Lee PY, Lin LC, Kuo YL. Landing kinematics, sports performance, and isokinetic strength in adolescent male volleyball athletes: Influence of core training. Journal of Sport Rehabilitation. 2020; 29(1):65-72. [DOI:10.1123/jsr.2018-0015] [PMID]
34. Mirjamali E, Minoonejad H, Seidi F, Samadi H. [Comparison of the effects of core stability training on stable and unstable levels on the static and dynamic balance of female athletes with trunk dysfunction (Persian)]. The Scientific Journal of Rehabilitation Medicine. 2019; 8(1):61-70. [DOI:10.22037/JRM.2018.111331.1920]
35. Rezvankhah Golsefidi N, Sirzad E, Alizadeh M, Barati A. [Effect of 6-week core stability training on biomechanics of single- leg jump-landing in 10 to 12 years old active children (Persian)]. Studies in Sport Medicine. 2019; 10(24):83-102. [DOI:10.22089/smj.2019.6035.1322]
36. Moustafa Mabrouk Mohammad Shahin R. Effect of core strengthening training on some biomechanical and physical variables and spike among volleyball players. The International Scientific Journal of Physical Education and Sport Sciences. 2016; 3(1):88-97. [DOI:10.21608/isjpes.2016.233365]
37. Sedaghati P, Baharmast Hossein Abadi A, Zolghadr H. The effects of core stability exercise on the dynamic balance of volleyball players. International Journal of Applied Exercise Physiology. 2013; 2(2):1-10. [Link]
38. Kibler WB, Press J, Sciascia A. The role of core stability in athletic function. Sports Medicine. 2006; 36(3):189-98. [DOI:10.2165/00007256-200636030-00001] [PMID]
39. Abt JP, Smoliga JM, Brick MJ, Jolly JT, Lephart SM, Fu FH.  Relationship between cycling mechanics and core stability. The Journal of Strength & Conditioning Research. 2007; 21(4):1300-4. [DOI:10.1519/00124278-200711000-00056] [PMID]