Short-term effects of on-field combined core strength and small-sided games training on physical performance in young soccer players

This study aimed to compare the effects of 6-weeks combined core strength and small-sided games training (SSGcore) vs. small-sided games (SSG) training on the physical performance of young soccer players. Thirty-eight amateur soccer players (age: 16.50 ± 0.51 years) were randomly assigned to either a SSGcore (n = 20) or a SSG group (n = 18). The SSGcore group performed upper and lower body core strength exercises combined with SSG including 2-, 3- and 4-a-sided soccer games third a week. The SSG group performed only the SSG periodization. Baseline and after the 6-week training period the Yo-Yo Intermittent Recovery Test level 1 (YYIRTL-1), 5–20-m sprint test, countermovement jump (CMJ), squat jump (SJ), triple-hop distance (THD), zigzag agility with ball (ZAWB) and without ball (ZAWOB), three corner run test (TCRT) and Y-balance test. The SSGcore group demonstrated meaningful improvements in 20 m sprint time (SSGcore: -9.1%, d = 1.42; SSG: -4.4%, d = 0.76), CMJ (SSGcore: 11.4%, d = 2.67; SSG: -7.7%, d = 1.43), SJ (SSGcore: 12.0%, d = 2.14; SSG: 5.7%, d = 1.28), THD (SSGcore: 5.0%, d = 1.39; SSG: 2.7%, d = 0.52) and TCRT (SSGcore: -3.7%, d = 0.69; SSG: -1.9%, d = 0.38). Furthermore, the SSGcore group demonstrated meaningfully higher improvement responses in both leg balance score (d = ranging from 2.11 to 2.75) compared with SSG group. These results suggest that the inclusion of core strength training to a SSG periodization is greatly effective to improve speed and strength-based conditioning in young soccer players.


INTRODUCTION
training strategy to increase soccer-specific strength and endurance performances in young soccer players. Consequently, this training strategy may be a part of strength and conditioning program as a timeefficient alternative training method for improving physical conditioning during the preseason period.
While some previous studies examined the long-term (8-12 weeks) on-field training effects of core strength training programs [17,20], other recent studies investigated the effects of strength training on physical performance responses in young soccer players [18,21].
To our knowledge, no study has performed regarding the short-term effects of combined core strength and small-sided games training on physical performance responses in young soccer players during the off-season period. Therefore, this study aimed to compare the of rest between bouts. The study results demonstrated that the players' physical fitness, anaerobic test performances and technical skills are higher in the SSG training group.
It is well known that soccer players perform numerous strengthbased activities during the matches. Consequently, core muscle strength is not only critical skill-related component of physical fitness such as coordination, balance and speed, but also important indicator of health-related components of young players [15]. Numerous studies have demonstrated that the considerable positive training effect of core strength in young soccer players improved sport-specific performances, including balance [15], endurance [16] and agility [17]. Furthermore, concurrent training method, combining strength and endurance [18] or strength and SSG [19], has also more effective Effects of core strength training on physical fitness short-term effects of on-field combined core strength and smallsided games training (SSG core ) vs. small-sided games (SSG) training on aerobic, anaerobic and balance performance responses in young soccer players. We hypothesized that the SSG core training will lead to greater improvement in speed and strength-based performance responses such as sprinting, jumping and balance.

Experimental Approach to the Problem
A parallel group controlled design was used in order to compare short-term effects of on-field SSG core vs. SSG training on aerobic, anaerobic and balance performance responses in young soccer players during the off-season period. The study design consisted of 1-week of baseline test, 6-weeks of training intervention (3 times per week) and 1-week of post intervention tests. All players completed maximal oxygen consumption (V O 2max ), 5-20 m sprint test, countermovement jump (CMJ), squat jump (SJ), triple-hop distance (THD), zigzag agility with ball (ZAWB) and without ball (ZAWOB), three corner run test (TCRT) and Y-balance test measurements with same order before and after the 6-weeks intervention period. In this study, we especially preferred these performance determinant tests that have been shown high relationship with match performance of young soccer players [4,22]. The players were already familiar with all the tests and SSG trainings. Each training session started with a 15-min stan-

Subjects
Forty-four young male soccer players agreed to participate at the beginning of this study ( Figure 1). During the study, a total of 6 players (2 players from the SSG core , 4 players from the SSG) were excluded due to dropouts (e.g., non-attendance at training, injuries, failing to attend test sessions 62.58 ± 6.08 kg, training age: 4.11 ± 0.58 years). These players were members of a regional amateur U-17 league teams and they were also familiar with training workload of ≥ 3 training units per week (~100 minutes) and have been involved in training and competitive soccer matches at least 3 years. All players were performing 3 training sessions which includes low to moderate aerobic training, plyometric and small-sided games without core strength training at the time of the study. Players were not familiar with upper and lower core strength training. They and their parents were notified of the research requirements, procedures, risks and benefits before being given the informed consent forms. Written these forms were then obtained from all the players and their parents. The present study was approved by the Research Ethics Committee of the local university and was conducted in accordance with the Declaration of Helsinki.

Procedures
Testing Procedure. Body weight and body fat percentage were measured using the bioelectrical impedance measurement (BC-418, Tanita, Tokyo, Japan) before breakfast. Each player performed vertical jump height in terms of the CMJ and SJ tests with hands kept on the hips to minimize the contribution of the upper limbs. Jump performances were assessed using a portable force plate (Newtest, Finland).
A standard tape measure was used for measuring the maximum distance in the TDH, which is a valid test for evaluation of lower limb strength and power, is to reach maximum distance with 3 consecutive hops without losing balance and touching the ground with the any hands or other leg [23]. Each player performed a 20-m sprint test (with 5-m and 10-m splits), ZAWB, ZAWOB and TCR. A zigzag agility test consisting of 4-to 5-m sections was set out at 100° angles [24].
The selection of this test was based on rapid acceleration, deceleration, and balance control required for a short running time, which represented the result of the test [25]. The TCR test was used to evaluate the speed and anaerobic endurance of the players. After the starting command, the players ran to flag post (1) 80-m away from the start.
Then, they turned to the left-hand side and ran to flag post (2) 20-m away from flag post (1). Subsequently, the players ran back to flag post (3) 82.4-m away from the start. Finally, the players turned to the right-hand side, ran to the finish line (4), and crossed it [26]. The times for these tests were measured and recorded using photocells (Newtest, Finland). Before the Y balance test, each players' length of the both legs were assessed with a standard tape measure from the anterior superior iliac spine to the most distal aspect of the medial malleolus during supine lying position. The test, consisting three different movement directions such as anterior, posteromedial and posterolateral) was used to evaluate the dynamic balance with single leg stance whilst reaching in those direction. After six trials on each foot for familiarization, each player had three successful reaches separated by 15 seconds [27]. After the test, a composite score (CS) was calculated using the following formula [28]: To evaluate of aerobic capacity, The YYIRT-1, which is a reliable and acoustically progressive test [29] was performed on a natural grass pitch and according to procedures were described by Bangsbo

Statistical Analyses
Data were expressed as mean ± standard deviation (SD). Betweengroup differences were analyzed using a 2-factor repeated-measures analysis of variance with 1 "between" factor (group: SSG core vs. SSG) and 1 "within" factor (time: baseline vs. post). Effect sizes (Cohen's et al. [30]. After the test, the estimated V O 2max was calculated using the following formula Training Interventions. The training procedure is summarized in

RESULTS
Baseline and the short-term effects of on-field combined core strength training on anthropometric and performance responses of the participants are summarized in Table 2. No significant within-group differences were found in anthropometric measurements such as body mass and body fat percentage following training in both groups ation in the order of exercises [33] and one have used a cohort approach [19]. In our study, a parallel design tested a combination of SSGs and core training against only SSGs, with a clear benefit to those who experienced the combination. Specifically, in the current study aerobic performance was largely improved by both groups. The improvement is in line with findings revealed by meta-analysis [34,35].
In fact, SSGs assumes an high-intensity interval training modality and it is one of the suggested approaches for improving capacity to repeatedly perform high-intensity efforts with changes of direction [36,37].
The capacity to stress both aerobic and anaerobic systems [36], while promote some muscular tension and load can justify the benefits in field-based aerobic tests as YYIRT. In this field, none of the groups took advantage, since both meaningfully improved the aerobic capacity with no significant changes between groups.
In neuromuscular-based tests as CMJ and SJ, players experiencing the combination of SSG and core training revealed significant benefits comparing to those only performed SSGs. The SSG core group had average improvements of 11 and 12% respectively for CMJ and SJ, while the group with only SSG improved 8 and 6%, respectively.
The core performance it seems to play a beneficial effect on vertical jump performance since studies have reporting large correlations between both outcomes [38] or revealing the benefits of adding core training to regular field-based sessions [39]. Improvements in 20-m linear sprinting were also significant in SSG core comparing to SSG. This is also interesting, since a previous study establishing relationships between core performance and linear sprinting revealed large magnitudes (dependency relationship) between both [38]. Finally, balance capacity was also significantly improved by SSG core comparing to SSG, thus suggesting that core training can be highly interest- ing to promote balance as demonstrated in previous studies [40].

CONCLUSIONS
This study is not absent of limitations. The sample size is not large enough to generalize the findings and no control group was used to compare the effects. Additionally, quantifying the volume of strength training and field-based training should be considered in future studies and used as a covariable for the pre-post analysis. compared with SSG group (Figure 2).

DISCUSSION
The purpose of this study was to analyse the effects of two SSGsbased programs (SSG core and SSG) on fitness levels after a 6-week period of intervention. Within group analysis revealed that aerobic performance, linear sprinting and change of direction performance measured at zig-zag tests were significantly and largely improved by both SSG-based groups. Between groups analysis revealed greater improvements of CMJ, SJ, linear sprint at 20-m, three corner run test and both leg balance in SSG core comparing to SSG group.
The combining of SSGs with other training methods have been tested. Some articles revealed beneficial effects of combining SSGs with high-intensity running-based interval training [32,33] in aerobic capacity. A more recent study combining SSGs and strength training (in weight room) also showed beneficial effects in neuromuscular fitness [19]. However, from these three studies, only one have used a parallel design [32], while one tested the same protocol with a vari-