Background: It has been demonstrated that core strength training is an effective means to enhance trunk muscle strength (TMS) and athletic performance in youth. However, the role of instability with core strength training is unresolved in youth. This study specifically will investigate the effects of core stability (CST) compared to core instability strength training (CIST) on physical fitness in adolescents.

Methods: Twenty-seven (14 girls, 13 boys) healthy adolescents (age 14±1 years) will be assigned to a CST (n=13) or a CIST (n=14) group. Both training programs will last 6 weeks (2 sessions/week) and included frontal, dorsal, and lateral core exercises. During CIST, these exercises will be conducted on unstable surfaces (e.g., TOGU© DYNAIR CUSSIONS, THERA-BAND© STABILITY TRAINER).

Expected Results: Based on selected results reported in the literature, we hypothesize that participants performing CIST as compared to CST will show larger improvements in physical fitness tests (i.e., strength, speed, flexibility, coordination, balance) following training. Of note, training induced gains in strength, speed, flexibility, coordination, and balance are of vital importance for sports performance, everyday activities, and injury prevention.

Adaptations following core strength training performed on stable (CST) as compared to unstable surface conditions (CIST) will be assessed in a pre- versus post-test design. The training period will last 6 weeks to induce training induced changes in measures of strength, speed, flexibility, coordination, and balance. These health (i.e., strength, flexibility) and skill-related (i.e., balance, coordination, speed) components of physical fitness will be assessed using physical fitness tests (i.e., Bourban TMS test, standing long jump test, 20-m sprint test, stand-and-reach test, jumping sideways test, Emery balance test, Y balance test).

Twenty-seven healthy adolescent boys and girls will participate in this study after the experimental procedures were explained. An a priori power analysis with an assumed Type I error of 0.05 and a Type II error rate of 0.20 (80% statistical power) was calculated for measures of trunk muscle strength and revealed that 13 participants per group would be 105 sufficient to observe medium Test x Group interaction effects. Study participants will be recruited from local sports clubs.

Both core strength training programs will be supervised and conducted by 2 experienced physiotherapists. The two programs will be organized as circuit training with each instructor supervising 6-7 participants. Both training programs will comprise 2 training sessions per week with a total of 12 training sessions for each intervention group. Each training session will last 30 min, starting with a brief warm-up program mainly consisting of core strength exercises at moderate intensities and ending with a cool-down program (i.e., dynamic stretching). During the main part of training, both groups mainly conduct the "big 3" exercises as described by McGill. These include the curl-up, side bridge, and quadruped position. In other words, every single training session will consist of frontal, dorsal, and lateral core exercises. The only difference between the 2 intervention groups is that the CIST protocol comprised core exercises that will be conducted on unstable elements (e.g., TOGU© DYNAIR PRO, SENSO, TOGU© REDONDO BALLS, TOGU© POWERBALLS, THERA-BAND© STABILITY TRAINER, THERA-BAND© EXERCISE BALL), whereas the CST program will contain the same exercises on stable surface only.

Prior to testing, all participants will perform a standardized 5-minutes warm-up which consists of bipedal and monopedal balance, submaximal plyometric, and skipping exercises. Thereafter, physical fitness tests (i.e., Emery balance test, Y balance test, stand-and-reach test, 20-m sprint test, jumping sideways test, standing long jump test, Bourban TMS test) will be assessed.

A multivariate analysis of variance (MANOVA) will be used to detect differences between study groups in all baseline variables. The effects of core strength training on variables of physical fitness will be analysed in separate 2 (Group: CST, CIST) x 2 (Test: pre, post) ANOVA with repeated measures on test. When Test x Group interactions reached the level of significance, group-specific post hoc tests (i.e., paired t-tests) will be conducted to identify the comparisons that were statistically significant. Additionally, the classification of effect sizes (f) will be determined by calculating partial eta squared.

Inclusion Criteria:

• healthy; physically active

Exclusion Criteria:

• musculoskeletal, neurological or orthopaedic disorders that might affect their ability to perform physical fitness tests and core strength training