Acute effect of a static- and dynamic-based stretching warm-up on standing long jump performance in primary schoolchildren

The aim of the present study was to compare the acute effect of a static- vs dynamic-based stretching warm-up on standing long jump (SLJ) performance in primary schoolchildren. The sample was composed of 76 schoolchildren, 43 girls and 33 boys, aged 9–10 years old from three fourth-grade classes of Primary Education. The three groups were cluster-randomly assigned to the control (CG), static (SG) or dynamic (DG) groups. All the schoolchildren performed a standardized warm-up consisting of mobility exercises (five minutes), jogging (five minutes) and the SLJ test. Afterwards the CG schoolchildren received jump theory (eight minutes), the SG performed static stretching (eight minutes) and the DG performed dynamic-bounces stretching (eight minutes). Afterwards, all of them performed the SLJ test again. The results of the one-way ANOVA (F2,73 = 34.184; p < 0.001; η2p = 0.484), followed by the pairwise comparisons with the Bonferroni adjustment, showed that the DG students (MΔ = 11.07, SE = 1.42) made a significant statistical improvement in their SJL levels compared with the CG (MΔ = -3.00, SE = 1.89; p < 0.001, d = 1.51) and SG students (MΔ = -1.85, SE = 0.67; p < 0.001, d = 1.38). However, statistically significant differences between the CG and SG students were not found (p > 0.05, d = 0.12). The dynamic-bouncing stretch as a final part of a warm-up improves explosive strength performance in primary schoolchildren, and seems to be a good option before carrying out explosive strength activities of the lower body.


Measurements
All the measures were performed during the regularly scheduled PE classes. Prior to the intervention, information about participants' gender, age and extra-curricular sport participation was collected.
Then, anthropometric measures were also taken during the same class. A week before of the SLJ measurement, all the participants carried out a familiarization session. During the familiarization session, the execution of the SLJ was explained in detail and students were able to practice it several times. Then, students carried out the SLJ test before (pre-intervention) and after (post-intervention) the warm-up. A description of the protocol of each measurement is described below.
Extra-curricular sport. Students' extra-curricular sport participation was collected through a short survey. Participants were asked if they were enrolled in any organized sport during their free time, which kind of sport(s), if they were federated, and how many days and average time per session they used to participate. Afterwards, based on previous studies [24,22], data were dichotomized as schoolchildren that either regularly participated or did not in extra-curricular sport at least twice per week with an average session time equal or higher than 60 minutes.
Anthropometric. Participants' body mass and height were measured and then the body mass index (BMI) was calculated as body mass/ height squared (kg/m 2 ). During the measurement of body mass and height participants were in shorts, T-shirts and barefoot. For the body mass measure, once the scale read zero, the participant stood in the centre of the scale without support and with their weight distributed evenly on both feet. For the body height measurement, the student stood with their feet and heels together, buttocks and upper part of their back touching the scale, and with their head placed in the Frankfort plane. The average of two measurements for both body mass and height was retained [25].
Standing long jump test. Students' lower body explosive muscular strength was measured with the standardized protocol of the SLJ test. A detailed description of the procedure has already been published [9]. In short, the participants stood behind the starting line with their feet together and jumped forward as far as they could. The distance was measured from the start line to the point closest to the landing line (the back of the heel). The test was repeated twice, and the best score was recorded (in cm). The SLJ test was carried out by the same tester, instrument, and under similar environmental stimulation, and an increase in peak torque [17]. In this sense, as doubts about the value of static stretching grow, dynamic exercise offers a more beneficial warm-up procedure [2].
Thus, there is currently a debate on whether stretching should be included in the warm-up prior to doing sport and how a stretching should be designed and executed [18]. Some authors compared warm-up protocols in children and found that warming up with static stretching decreased vertical and long jumps compared to performing dynamic exercises and jumps in the warm-up [19].
Similarly, other controlled studies in children found that the warm-up with static stretching decreased vertical jumps, while the one containing dynamic exercises increased vertical jump performance [20,21]. To our knowledge, there are no previous studies comparing the static-passive versus dynamic-bouncing stretching (soft bounces) within a warm-up, in the SLJ performance of schoolage children. Consequently, the purpose of the present study was to examine the acute effect of static and dynamic stretching (soft bounces) as a final part of a warm-up on the performance of the SLJ test among primary schoolchildren in a PE setting.

Participants
The study protocol was first approved by the Ethical Committee of

Statistical analysis
Descriptive statistics (mean ± standard deviation/error or percentage) for general characteristics of the participants and dependent variables were calculated. Statistical test assumptions were checked by common procedures (e.g., histograms and normal Q-Q plots for normality). Chi-squared analyses were carried out to test the ratio differences of gender and extra-curricular sport practitioners be- Standing long jump performance. Figure 3 shows the acute effect of a static-and dynamic-based stretching warm-up on SLJ perfor-tween the three groups. A one-way analysis of variance (ANOVA) was conducted to examine potential differences between the three groups in terms of body mass, body height, body mass index, and SLJ baseline levels. Then, the acute effect of the static-and dynamic-based stretching warm-ups on SLJ test performance was examined using a one-way ANOVA, including group as an independent variable (CG, SG and DG) and SLJ change scores (i.e., post-intervention -pre-intervention scores) as a dependent variable.
Subsequently, for the post hoc pairwise comparisons, the Bonferroni adjustment was used. From all the potential confounding variables explored (i.e., gender, body mass, body height, body mass index, and extra-curricular sport), any covariable was used. Effect sizes were estimated using the partial eta squared (η 2 p ) and Cohen's d for the overall and pairwise comparisons, respectively. All statistical analyses were performed using the SPSS Version 21.0 for Windows (IBM® SPSS® Statistics). The statistical significance level was set at p < 0.05.

RESULTS
General characteristics. Table 1 shows the general characteristics of the participants and differences between the three groups. The chi-square analysis showed that the three groups had a balanced representation of gender and extra-curricular sports practitioners (p > 0.05). Additionally, the one-way ANOVA results did not show statistically significant differences in terms of body height and SLJ baseline scores between the three groups (p > 0.05). However, the

DISCUSSION
The purpose of the present study was to examine the acute effect of  [19]. Similarly, other controlled studies in children found that the warm-up ending with static stretching decreased vertical jumps, while the one containing dynamic exercises increased vertical jump performance [20,21]. These findings suggest that warm-up protocols that include only static stretching may have unintended adverse consequences on anaerobic performance in children [2]. On the other hand, dynamic exercises can reverse the negative effect caused by performing static stretches [26].
All these studies compared the effect of static stretching with dynamic exercises, not with dynamic stretching. Although some authors suggest that these dynamic exercises could be taken as dynamic stretches since the muscles are stretched to a new range of motion and then forced to contract to perform the desired action [2].
But in any case, it could be said that dynamic exercises are dynamic stretches using a bouncing technique.
Some authors stated, it is important to understand that a dynamic stretch does not involve a bouncing-type movement, which is characteristic of a ballistic stretch, but rather a controlled elongation of a specific muscle group [2]. We completely disagree with the opinion of these authors. A bouncing-type movement is simply one of the possibilities within a range of dynamic stretching techniques [27].
On the other hand, among young people the practice of dynamic techniques has been rejected because of a greater potential risk of muscle injury [28]. Nevertheless, as several authors suggest [27,29], dynamic stretching exercises may not be harmful when practiced gently.
Perhaps the greatest jumping performance achieved due to dynamic stretching when compared with static stretch as part of a warm up is due to an increase in heart rate, core temperature, greater nervous system stimulation, and an increase in peak torque associated with the dynamic stretch [17]. An 8-minute warm-up improves sprint performance and decreases the perception of fatigue when compared to 15 and 25 minute durations [30]. In the present study, the duration of the exercising in the warm-up was 18 minutes for the 2 groups doing stretching and 10 minutes for the group not stretching (because for 8 minutes the students were receiving theoretical information). However, the 18-minute warm-up with dynamic stretching performed best compared to the other two groups. It would be interesting to compare different warm-up durations with the same stretching technique. However, some limitations should also be considered. First of all, due to human and time restrictions, a larger sample could not be examined. This fact could limit the generalizability of the obtained outcomes to the particular setting studied.
A more conclusive study could be a crossover study so that each warm-up condition (without stretching/static/dynamic respectively) could be compared, in one big sample. However, for practical reasons and the nature of the present study (i.e., intervention focused on natural groups in a school setting) a cluster crossover design would be the only feasible option. However, since there were 3 study conditions (i.e., Static, Dynamic, and Control), which represents an important novel in this study (i.e., comparing the static and dynamic stretching in the same study), 9 groups (classes) were required and there was no access to that number (only 3). Therefore, in the present study, a randomized control trial cluster design was chosen instead.
Future research interventions should compare the effectiveness of different stretching techniques within warm-up protocols on