THE EFFECTS OF TWO DIFFERENT HIIT RESTING PROTOCOLS ON CHILDREN’S SWIMMING EFFICIENCY AND PERFORMANCE

On children, HIIT program is being used by coaches as a way of rapid improvement. The values of the intervals vary from the target of the period. The study’s purpose was to find out if HIIT in different interval protocols improves children’s swimming performance. In the study participated 18 rookie swimmers (9 boys and 9 girls), aged 11.6 ±1.5. The HIIT protocol in both groups included the set of 2 × 4 × 17.5 m (freestyle and freestyle kick, respectively). The first group (HIIT 1) had a 10 sec interval while the second (HIIT 2) – a 1 min between the HIIT reps. Swimmers performed in 35 m freestyle (35F) and 2 min free kick (2’ FK) trials at maximum effort. The performance (T), the number of strokes (SN) and the stroke length (SL) were measured at 35F, the covered distance at 2’ FK and the heart rate (HR), the rated perception of exertion (RPE) at 35F and 2’ FK. Regarding the results, all parameters improved significantly (p = 0.01). The use of a 4-week swimming program with training 4 times per week, in which two were HIIT, improved children’s performance and efficiency in swimming. Intervals of 10 sec or 1 min did not alter the improvement.


Introduction
Many of training methods contribute to the athletes' increase of physiological adaptions and performance. Common training methods are High Volume Training (HVT) and High-Intensity Training (HIT). These two methods seem to be helpful for an athlete's performance (Sperlich, Haegele, Achtzehn, De Marees, Mester, 2009a;Sperlich et al., 2009b). These training methods are used as the main part of a training session.
A training method which proves the most interesting in studying is High-Intensity Interval Training (HIIT). HIIT training is one of the most beneficial methods to improve almost all physiological and performance responses in a wide range of training. Moreover, they found significant performance improvements that reached from 6-20%.

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The Effects of Two Different HIIT Resting Protocols on Children's Swimming Efficiency and Performance The results in both studies suggested that 120 sec was the most beneficial rest time between maximal repetitions of exercise (Balsom et al., 2005).
So, the purpose of this study was to determine if a 4-week swimming program, 4 times per week, with HIIT, two times per week, with two different passive interval protocols, it could improve children's swimming performance. Children, due to their physiological demands, needs less time to recover after an exercise in addition to adults (Tabata et al., 1997). Thus, it is great of interest in this study to examine if there will be any difference between the two resting protocols.

Participants
All tests were conducted according to the ethical guidelines of the Research Committee of Aristotle University of Thessaloniki. In the study participated 18 children (9 boys and 9 girls) aged 11.6 ±1.5. The participants' swimming level was moderate. Height (H), weight (W) and body mass index (BMI) were measured (Table 1), while the participants were questioned if they were well on their health. Moreover, they were examined by a cardiologist before the study. All children's parents were informed about the procedures of this experiment and the possible improvements that this type of exercise could provide to their children's health. At the 16 exercise sessions, the attendance was 12 ±3 for HIIT 1 and 13 ±2 for HIIT 2.

Study design
The study measured pre-post differences in a 4-week swimming program with training 4 times per week in which two programs were HIIT, in a 17.5 m pool. Premium school children took part in the study. The participants were informed about the protocols they would follow and the way they would have to answer about the rate of perceived exertion (RPE). Before training, the children's attendance was monitored and the training volume was calculated to find out if the training's frequency and volume were enough for performance adaptions.

Procedures
The 18 boys and girls were divided into two equal groups depending on their swimming level. The first group was named HIIT 1 and included 5 girls and 4 boys, and the second group was named HIIT 2 and included 4 girls and 5 boys. The HIIT protocol for both groups included 4 × 17.5 m freestyle and 4 × 17.5 m freestyle kick with a kickboard in maximal intensity. The difference between HIIT 1 and 2 was the interval. The passive interval in the sets was Konstantinos Papadimitriou, Stratos Savvoulidis 10 seconds for the first group and 1 minute for the second group. Between the freestyle swim and freestyle kick protocols, there was a 5-minute passive interval for both groups.

Measures
To find out if there were any adaptions from the two HIIT programs the children were tested in two swimming protocols. The first protocol took place at the beginning (0 weeks) of the experiment and the other one at the end (4 weeks). These two protocols included 35 m freestyle and 2 min freestyle kick (Papadimitriou, Loupos, Tsalis, Manou, 2017) in maximal intensity, respectively. In the 35 m freestyle, the time (T), the number of strokes (SN) and the stroke length (SL) were measured. On the 2 min freestyle kick the covered distance was measured (D). Additionally, the heart rate (35F HR-2'FK HR) and the rate of perceived exertion (35F RPE-2'FK RPE) were measured for both tests. The HR parameters were estimated in 10 seconds (Nussinovitch et al., 2011) with the use of POLAR Electro.

Statistical analysis
For the statistical analysis the Shapiro-Wilk test of normality was used. Furthermore, to assess the effects of two HIIT resting protocols, before and after the intervention, in the parameters studied, there was used a two-way ANOVA with repeated measures (group × time). Moreover, a Pearson's -r correlation was conducted between the parameters that were studied. The analysis was performed using the SPSS Version 25.0 for Windows (SPSS Inc., Chicago, IL, USA). The level of statistical significance was set at α = 0.05.

Results
From the Shapiro-Wilk analysis there was found a normality in the parameters that studied (p > 0.05). In twoway ANOVA with repeated measures analysis, there was found no significant differences between the two groups (HIIT 1 vs HIIT 2) in almost all parameters studied, except for HR in 2'FK test. On the other hand, statistically significant differences were found in combined groups (HIIT 1 + 2) between 0 and 4th weeks. Figures 1, 2, 3, 4, 5, 6 show the differences between measurements and groups. Figures 7 and 8 show the correlations between the parameters between 35F, 35F SN, 35F SL and 2'FK. From the results the 35F in HIIT 1 + 2 decreased and the difference was statistically significant. The decrease was from 55.9 ±13.8 to 46.3 ±9.3 sec (p = 0.01) (Figure 1). The 35F SN decreased from 60 ±16 to 53 ±15 strokes (p = 0.01) (Figure 2).

Discussion
In the study it was found that high intensity training in different intervals contributes positively in performance and in physiological parameters. These results were common with other studies in various sports (Helgerud et al., 2001;Laursen, Jenkins, 2002;Buchheit et al., 2008;Mosey, 2009;Laursen, 2010). Probably, the anaerobic pathway used in that kind of exercise helps the athlete's physiology in gaining a stronger stimulus.
The time and the distance covered at the 35F and in 2' FK decreased and increased respectively in both of groups. Thus, HIIT independently of the interval rest between the sets helped the swimmers to improve. Probably, in both of HIIT sets, swimmers' physiology in childhood uses mainly the aerobic energy pathway. The rest of 10 sec gives an anaerobic pathway energy contribution. On the other hand, 1 min rest ensures a greater aerobic contribution. These hypotheses could be made only if the try was at its maximal intensity. The Borg's scale and HR in both of 35F and 2' KF tests in both of groups proves that swimmers swam in maximal intensity during the tests.
Why Borg's scale did not elevate in measurements after the 4-week period probably has two explanations. The swimmers' low skill level attenuated their ability to reach higher fatigue levels. Moreover, the performance a b Konstantinos Papadimitriou, Stratos Savvoulidis enhancement contributed to finding the test easier after the 4-week intervention. This hypothesis is supported by the HR parameter which increased at the second measurement in both groups and tests. Usually, after a long intervention of exercise, athletes gain the ability of lowering their heart rate in the phase of rest after set (especially elite athletes). The explanation is that cardiac sympathetic excitation outlasting causes bradycardia with enhanced sympathetic activity in athletes (Furlan et al., 1993). In the study, immediately after the tests of 35F and 2' KF, the young swimmers were found to increase their HR by 10 seconds comparing the 0 and 4 th week in both groups. The probable explanation is that the children in these ages fulfill their body demands of oxygen by increasing their HR. Young children, when involved in an aerobic exercise program, showed progressive increases in left ventricular posterior wall thickness and left ventricular mass and no changes in left-ventricular end-diastolic dimension, shortening fraction or resting heart rate (Geenen, Gilliam, Crowley, Moorehead-Steffens, Rosenthal, 1982). Thus, the swimmers' heart should pump more blood to the body to respond to the exercise demands.
Another finding in the study was that the HIIT 2 protocol, which had more resting time than HIIT 1, in 2'FK test, was found to have increased the HR response. In that case, possibly, the greater resting time in HIIT 2 caused higher anaerobic contribution and greater HR stimuli, therefore higher heart rate demands among the young swimmers. Contrary to the lower resting time, which had the HIIT 1 protocol, the energy contribution was probably more aerobic; therefore, the heart rate demands were lower. From the unique study which examined the HIIT effects in young swimmers (Sperlich et al., 2010) it was concluded that HIIT enhances VO2 peak, the rate of maximal lactate accumulation and 2,000 m freestyle performance. The common finding is that in both studies HIIT improves aerobic and anaerobic contribution. On the other hand, an unclear field, which B. Sperlich did not analyze, is the interval which was used at HIIT period.
In the study it was found that, independently, in terms of the interval between HIIT's reps (10 sec or 1 min), the young swimmers improved their performance. P.D. Balsom et al. (Balsom, Seger, Sjödin, Ekblom, 1992) and S. Seiler and K.J. Hetlelid (2005) in their studies found that the optimal rest period for maximal repetition tries is 120 sec. Thus, greater distances need higher interval periods. Moreover, the main difference between our study, P.D. Balsom's et al. (1992) and S. Seiler and K.J. Hetlelid's (2005) is that their participants' age was higher than ours. So, the interval in children does not affect the aerobic or anaerobic stimulus.
Another aspect of performance which improved from the 4-week intervention program was the stroke length (SL) and the stroke number (SN). These parameters indicate that, probably, the improvement in technique contributed to the better performance, which both groups showed. The improved efficiency in the water was probably caused by technique exercises combined with the HIIT program. Thus, this combination affected the neuromuscular coordination and facilitation as well. Moreover, in correlation analysis the contribution of SN and SL with 35F makes the hypothesis about the importance of these parameters in performance stronger.

Conclusion
The study is a novel which examines different rest protocols between HIIT sets on children. From the results it can be seen that the resting period does not differentiate the performance enhancement in young swimmers. Thus, the coaches do not need to pressure the young swimmers with exhausting sets in low intervals. On the other hand, perhaps the swimmers' skill level affects the training adaptions. Therefore, more research is needed in this field to avoid the exhausting training which many coaches prefer to use on children.