This meta-analysis was conducted to determine the effects of CST on 50-meter freestyle performance in male and female swimmers. The study was properly registered in the International Prospective Register of Ongoing Systematic Reviews (PROSPERO) under the identification number CRD42024579857. The research was based on the collection and analysis of randomized controlled trials and controlled clinical trials, following the guidelines of the Cochrane Handbook and the statements established by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [11,12].

A systematic search was conducted during August 2024, using databases such as PubMed, Scopus, Web of Science and Bireme. The search was optimized using MeSH, DeCS and free terms. Details of the search strategy and history for each of the databases are available in the supplementary material.

Two reviewers made the selection of articles through a preliminary reading of titles and abstracts to determine their relevance. Subsequently, the reviewers independently analyzed the full texts of the pre-selected articles to decide on their inclusion in the review. In case of discrepancies, a third reviewer intervened to resolve the disagreement. The collaborative application Rayyan was used for this process [13]. Data were extracted from each study including title, first author, year of publication, objective, population, methodology, training protocols, and measures or mean differences of the results in the 50-meter freestyle, along with their respective standard deviations. In case of missing data, authors were contacted through the correspondence author's email or through the scientific social network ResearchGate to obtain the missing information.

Quality of the included studies was assessed using the clinical trial checklist provided by the Scottish Intercollegiate Guidelines Network (SIGN) [14] and the Consolidated Standards of Reporting Trials (CONSORT) approach [15]. These tools provide predefined criteria for grading quality based on the presentation and reporting of studies. The assessment was performed by two investigators and, in case of disagreement, a third reviewer was involved.

Each study was individually assessed for risk of bias using the Risk of Bias 2 (ROB 2) tool of the Cochrane Handbook of Systematic Reviews [16]. The assessment was performed independently by two reviewers, with the intervention of a third reviewer to resolve discrepancies. Although it was considered to perform an analysis of the risk of publication bias using a funnel plot and Egger's test with the t statistic in STATA, it was decided not to carry it out due to the lack of at least 10 studies in each sex, which is the minimum number required to ensure the feasibility of these approaches [17-19].

The extracted data were analyzed following the guidelines of the Cochrane Handbook of Systematic Reviews [11]. For the global estimation of effect, means (M) and mean differences (MD) were used, as the measurement units and evaluation methods were consistent across studies. Therefore, it was not necessary to employ the standardized mean difference. Additionally, when needed, the pooled standard deviation (pooled SD) formula was applied to calculate the standard deviation of the mean difference effect. In cases where studies did not provide the standard deviation of the outcomes of interest, it was calculated from confidence intervals or standard errors, according to the guidelines of the Cochrane Handbook [11]. RevMan web software was used to assess statistical heterogeneity, applying statistical methods such as 𝐼² and 𝜒², along with their degrees of freedom and 𝑝 values, in the fixed-effects model, assuming that all individual studies share a common true effect [20].

A total of 2,323 articles were identified through the database search. After eliminating duplicates, 1,000 manuscripts remained. Subsequently, they were submitted to a rigorous selection process, first by reviewing titles and abstracts, and then through full-text analysis. Finally, 7 studies were chosen for inclusion in the meta-analysis [21-27] (see Figure 1).

Figure 1

The seven studies analyzed in this review were published between 2017 and 2023, and together included a sample of 142 swimmers, aged between 11 and 20 years. Of these, 71 swimmers were part of the intervention group and 71 of the control group, distributed in 48 females and 94 males. The studies used various dosages and training strategies: five implemented 8-week programs [22,24-27], while two opted for 6-week programs [21,23]. Most investigations established a training frequency of 3 sessions per week [21-23,25,26], although two studies increased the frequency to 4 and 5 sessions per week, respectively [24,27]. Table 1 presents a detailed summary of the most relevant aspects of each included study.

Table 1

Author	Year	Type of study	Intervention group (CST + SWT)	Control group (SWT)	Core stability training program
					# weeks	Frequency	Training surface	Type of exercises	# exercises
Gül [26]	2020	Randomized clinical trial	n: 7 male swimmers (11.57 ± 1.2 years)	n: 7 male swimmers (11.43 ± 1,2 years)	8	3 times x week	Stable	Does not report	6
Kurt [24]	2023	Randomized clinical trial	n: 11 male swimmers (12-15 years)	n: 11 male swimmers (12-15 years)	8	4 times x week	Stable	Dynamics	10
Eskiyecek [22]	2020	Randomized clinical trial	n: 12 male swimmers (11.25 ± 0.75 years)	n: 12 male swimmers (10.42 ± 0.51 years)	8	3 times x week	Stable	Dynamics	10
Karpiński [21]	2020	Randomized clinical trial	n: 8 male swimmers (20.2 ± 1.17 years)	n: 8 male swimmers (20.0 ± 1.9 years)	6	3 times x week	Stable and e unstable	Dynamics	4
Khiyami [23]	2022	Quasi-experimental clinical trial	n: 9 male swimmers (13 ± 2 years)	n: 9 male swimmers (13.11 ± 2,6 years)	6	3 times x week	Stable	Dynamics and statics	14
Gencer [27]	2018	Quasi-experimental clinical trial	n: 12 female swimmers (10.58 ± 1.31 years)	n: 12 female swimmers (10.75 ± 1.29 years)	8	5 times x week	Stable	Dynamics and statics	7
Zarei [25]	2017	Randomized clinical trial	n 12: female swimmers 12 (14 ±1.08 years)	n: 12 female swimmers (14 ± 1.27 years)	8	3 times X week	Stable	Dynamics	16

Note. Abbreviations = CST: Core stability training; SWT: Swimming training.

In the assessment of reporting quality using the CONSORT checklist, it was observed that items 2a, 2b, 5, 6a, 6b, 7b, 11b, 12a, 12b, 13a, 13b, 15, 16, 17a, 20, 21 and 22 were fulfilled in all the reviewed studies. Nevertheless, items 1a, 4a, 4b, 8a, 8b, 9, 10, 11a, 23 and 24 were not always adequately reported. The main deficiencies were concentrated in items 8a, 8b, 9, 10 and 11b, which are related to the sample size calculation, the randomization mechanism and the blinding of both participants and evaluators.

Regarding the specific quality of the studies, assessed using the clinical trial checklist proposed by SIGN, six studies were classified as acceptable quality [21-26] and one as low quality [27]. No study complied with item 1.4, referring to blinding of the subjects, investigators, and assessors. Still, it is recognized that given that the intervention in question is physical exercise, implementing the blinding of participants and of the investigators responsible for the training program represents a significant challenge.

After applying the ROB 2 tool to assess the risk of bias in the included studies, all of the studies were found to have a low risk of bias in the domains “Deviations from intended interventions” and “Missing outcome data” (see Figure 2). However, concerns and a high risk of bias were identified in the domains “Randomization process” and “Selection of reported outcomes”. Figure 3 presents the detailed individual risk of bias assessment for each of the 7 included studies.

Figure 2

Figure 3 Note. Forest plot comparing the effect of core stability training combined with swimming training versus swimming training on specific performance in the 50-meter freestyle in male swimmers. Abbreviations= CST: Core stability training. SWT: Swimming training.

This study represents the first systematic review with meta-analysis investigating the effects of CST on the specific performance of swimmers, positioning itself as a pioneering work in this field. Among the main strengths of the study, the methodical approach used in the search and selection of the studies, as well as in the data extraction and evaluation of the risk of bias, quality of the reports and quality of the included studies stand out. No studies were excluded by language or year of publication, which ensured the inclusion of all available evidence. In addition, the fact that at least two reviewers were involved in the initial phases of the search, selection, extraction and evaluation of the studies helped to reduce bias and improve the quality of the process.

Nevertheless, there are some important limitations, particularly related to the risk of bias in the considered studies. One of the studies showed a high risk of bias [27], while the others presented some concerns [21-26], mainly due to the lack of trial registration and lack of clarity in the report on the concealment of the randomization sequence, which generates some uncertainties. On the other hand, it is important to note that six out of the seven included studies analyzed a population of athletes aged 10 to 13 years [21-27]. This age range corresponds to a critical stage of biological and hormonal maturation, which can significantly impact physical performance [35,36], potentially leading to an over- or underestimation of the intervention's true effect. Additionally, the studied population consisted exclusively of semi-professional level swimmers. This suggests that the findings may not be generalizable to other groups of swimmers, such as amateurs, who might benefit more substantially from this type of training, or elite-level swimmers, whose highly developed physical characteristics could result in a more modest response.

Another relevant limitation is the underrepresentation of female swimmers in the sample; only two studies included female participants [25,27]. This imbalance could result in an overestimation of the effect observed in this group. The limited data on female athletes weakens the robustness of conclusions regarding their response to the intervention, highlighting the need for greater inclusion of female samples in future studies. Given these limitations, we recommend that future researchers conduct additional primary studies involving both amateur and professional swimmers, as well as address the significant gap in data on female populations. This approach will allow for more accurate conclusions and a more balanced representation of responses to the intervention across diverse populations. Finally, although meta-analysis allows combining studies and increasing statistical power by including a larger number of subjects, which would not be possible in individual studies, the total population in this meta-analysis remains relatively small. Still, without this synthesis, the number of participants would be even more limited.