The aim of the study is to evaluate the effect of an eight-week specific Core Stability Training (CST) on young rhythmic gymnasts. The literature suggests CST may help improve sport performance, however, more studies implementing functional CST and accurate methodology are needed to validate this statement. Little is known about Core Stability (CS) and Rhythmic Gymnastics (RG). The study intends to find out whether CST contributes to the Rhythmic gymnasts core stability, balances and jumps execution enhancement, very important RG performance determinants. It is hypothesized the rhythmic gymnasts will increase their CS, balances and jumps performance once the specific CST is conducted.

Gymnasts were recruited from the Gymnastics Club "Gimnàstica Muntanyenc". All participants were rhythmic gymnasts who had at least two-year experience competing in RG at regional federated and school competition level and trained three times per week. Individuals who didn't meet these criteria or were injured at the moment were excluded. The participants were invited to take part in the study voluntarily, and along with their parents were informed verbally and in writing as to the characteristics of the program. Parents or legal guardians signed informed consent forms, and a document that explained the objectives and planned activities of the provided program. The study was approved by the FPCEE Blanquerna Research Ethics Committee. All protocols applied in this research (including the management of the personal data of the participants) complied with the requirements specified in the Declaration of Helsinki of 1975 and its subsequent revisions.

The specific CST program is aimed for Rhythmic gymnasts between 8 and 15 years of age who train in the competitive group and their goal is to improve their RG level. All the study participants were asked to sign an informed consent document that was drafted in keeping with all applicable research ethics protocols, including a guarantee of the confidentiality of personal data and an assurance that participants can choose to abandon the study at any time.

Control group (CG), experimental group (EG) and pre-test, post-test methodology was selected. EG performed the specific CST while CG carried out the traditional RG training.

The CST intervention had been carried out over the course of one academic year (2018-19). The first 3 months allowed investigators to recruit the participants and design the CST adapted to the Rhythmic gymnasts and pursuing the highest specificity for the CS exercises. This design took into account prior experiences of a CS physiotherapist expert and a RG national coach. The load of the sessions was controlled according to a Rate of Perceived Exertion (RPE) scale, a valid method of quantifying exercise training during a wide variety of types of exercise. A training protocol trial will be applied where the subjects will be asked to point out which is their perceived exercise intensity, thirty minutes after the training ends. The repetitions will be adjusted to the 7-8 RPE scale values (very hard) allowing the correct execution of the exercises to the participants and yet permitting adaptations produced by the training. The conclusions drawn from this initial experience formed the scientific basis for the definitive version of the CST.

During the fourth month a pre-test was carried out including an anthropometric study of the gymnasts, three CS tests, seven balance tests and six jump tests. The participants warmed up before each test except for the anthropometric study and the order of the subjects and the order of the applied tests were randomly determined using a true random number generator to avoid order effects. A group blinded specialized physiotherapist conducted the anthropometric study and the CS tests, and a group blinded RG expert led the rest of the tests.

CS tests: Two pressure biofeedback unit (PBU) tests were applied to evaluate the lumbopelvic motor control (LPMC) and the pelvic tilt tests were conducted to evaluate CS endurance.

Balance tests: One leg stand test (OLST) was performed on a force platform to evaluate the gymnasts' balance. Right and left and open and closed eyes conditions were used to perform the four Romberg variants balance tests. Three specific Rhythmic balances were chosen to execute over the force platform and were also evaluated by three RG judge experts.

Jump Tests: Counter movement jump (CMJ) and single leg countermovement jump (SLCMJ) were performed on a force platform to assess the gymnasts' jump. Three specific RG jumps outside the force platform were evaluated by three RG judge experts.

Anthropometric study: A complete anthropometric study was carried out by a professional physiotherapist and will allow the investigators to verify the results are not affected by group initial differences.

From February to April the eight-week CST sessions were offered in the gymnasts' regular training space suitable for the intervention exercises and guaranteeing the safety of the participants. The investigators provided all the extra equipment required for the study. The EG participants performed the CST 3 times per week at the same time of their scheduled training. The duration of the specific training was approximately 30 minutes for 8 weeks with a total of 24 sessions. EG gymnasts were observed while performing the exercises to ensure the training was performed correctly.

Thirty minutes after every CST session all gymnasts (EG and CG) were asked to pick the score of the RPE scale that best described their sensation. This score, multiplied by the minutes of the session (RPE session), helped us modulate the training periodization plan. The average RPE session values were calculated to ensure no load training differences existed among groups.

This CST program involves exercises that challenge the balance, postural control and explosive strength of the gymnasts, executed with specific RG elements and postures.

The specific CST training contains three blocks; a circuit, plyometric exercises and CS exercises. Three different balances and leaps very common in RG (novice to intermediate competitive level) were selected to convert the core and the explosive strength exercises into a very specific and functional training. The balances selected were the passé balance, the side balance with help and the arabesque, the three leaps were the scissors, the stag from assemble and the split leap. The mentioned RG elements were selected due to their lower limb position planes variety and different techniques since it is advised that a range of exercises should be performed to challenge the core musculature in all three planes and ranges of movement to develop total CS. Moreover, all exercises were executed equitably for the right and left side. Unstable superficies as the Bossu, unstable discs and soft balls were included to perform core and balance exercises. The gymnasts performed 3 exercises of the circuit on an unstable surface. It is reported, when standing on an unstable support, the stabilizing muscles make anticipatory adjustments subsequently minimizing postural destabilization. The protocol training and the exercises were conducted by Club Muntanyenc professional RG coaches.

A post-test was performed after the eight-week specific CST following the same procedure as the pre-test.

The Statistical Analysis will be performed using an Excel spreadsheet and then exported to the computer program SPSS statistics 22.0, to then perform the various statistical tests. Initially, a normality test will be performed to check the distribution of the data and to decide whether parametric or non-parametric tests are applied. The G-POWER program will be used to calculate the statistical power of the sample with a value of p <0.05 and an estimated effect size of 0.5.

Inclusion Criteria:

• Competition Gymnasts from Club Muntanyenc Sant Cugat training minimum 3 times per week

Exclusion Criteria:

• Injured Gymnasts

The assessment of the test is conducted by masked assessors (judges and physiotherapists)