GENERAL INFORMATION 
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1. Dataset title: Datos BSSR en Primaria.

2. Authors: Rafael Merino-Marban, Daniel Mayorga-Vega e Iván López-Fernández.

3. Author contact information: Rafael Merino-Marban, email: rmerino@uma.es


METHODOLOGICAL INFORMATION 
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Material and methods
The purpose of this study was to research the effect of a stretching program on hamstring flexibility in schoolchildren from Primary and Secondary Education. 
Methods. For practical reasons and the nature of the present study (i.e. intervention focused on natural groups in a school setting) a cluster-randomized controlled design was used. Balanced by school and grades, natural classes were assigned randomly to form one of the following study groups: warm-up group (WUG), cool-down group (CDG), mix group (MXG) and non-stretching group (NSG).
A total of 222 children from Primary Education (117 girls and 105 boys; average age: 8.82 ±1.63 years; weight: 36.89 ±10.69 kg; height: 137.18 ±11.13 cm; BMI: 19.26 ±3.54) 
A total of 289 from Primary Education, average age: 9.5 years; weight: 37.1 ±10.8 Kg; height: 137.2 ±11.1 (3 experimental groups and 1 control group). The 3 experimental groups performed hamstring stretches for 4 minutes during each Physical Education class over a two-month period, 2 sessions per week (8 weeks, for a total of 16 PE sessions), with a stretch duration of 20 seconds, utilizing the static passive technique. The EG1 performed the stretching exercises during warm-up (4 minutes), the EG2 performed the stretching exercises during cool-down (4 minutes) and finally the EG3 performed the stretching exercises during warm-up (2 minutes) and cool-down (2 minutes) period. The control group followed the standard class program of PE classes. Hamstring flexibility was measured using the PSLR, PKEA and BS-SR test in a random order before and after the program. Finally, after two weeks without performing stretching exercises, a new measurement was taken with the aim of observing the levels of retention. 

Procedures
A stretching intervention program was applied to WUG, the CDG and the MXG during the PE classes. These groups participants performed a stretching intervention program twice a week on nonconsecutive days for 8 weeks. The stretching program was conducted and supervised by the same PE teacher for the groups from the same school. The WUG students performed four minutes of stretching at the end of the warm-up, the CDG students performed four minutes at the end of the cool-down and the MXG students performed two minutes at the end of the warm-up + two minutes at the end of the cool-down. The NSG students followed the same standard PE classes without performing any stretching exercise.

Statistical analysis
Descriptive statistics (means and standard deviation/standard error or percentage) were computed for general characteristics of the participants and the dependent variable. Firstly, all statistical test assumptions were checked by common procedures (e.g., histograms and Q-Q plots for normality) and met. Afterward, the comparison of the effect of the hamstring stretching programs on schoolchildren’s BSSR scores was examined. Since the implementation of the missing data requires strong assumptions that are hard to justify, “complete case” analyses including only those whose outcomes were known were used. Since the unit of randomization and intervention was the class, a Multilevel Linear Model (MLM) with participants nested within classes as random effects, and with the between-group factor group (WUG, CDG, MXG, NSG) as fixed effects on the change in BSSR scores (i.e., post-intervention – pre-intervention) was selected (i.e., one-way nested ANOVA). Since none of the potential confounding variable explored (i.e., gender, grade, body mass, height, body mass index, and pre-intervention BSSR scores) showed statistically significance, covariables were not included. The maximum likelihood estimation method was used. Subsequently, the post-hoc between-group pairwise comparisons with the Bonferroni adjustment were carried out. 
Effect sizes were estimated using the Cohen’s d (continuous variables) and Cramerʼs V (categorical variables) for the pairwise comparisons. All statistical analyses were performed using SPSS version 25.0 for Windows (IBM® SPSS® Statistics). The statistical significance level was set at p < 0.05.



DATA-SPECIFIC INFORMATION:
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1. Name File: Datos BSSR en Primaria.

VARIABLE		DESCRIPTION

Código	    		Participant identity
GRUPO                   Grade and group belonging
PROGRAMA                Participant group: 1= Warm-Up Group, 2= Cool-Down Group, MXM= mixed Group, 4= No Stretching Group
CURSO                   Participant grade: 1= first cycle of primary, 2= second cycle of primary, 3= third cycle of primary.

GENERO			Participant sex: 1 = male, 2 = Female
EDAD                    Participant age (years)
SEMESTRE                Participant birthday: 1= first semester, 2= second semester of the year.
PESO                    Participant weight (kg)
TALLA                   Participant height (cm)
BSSR_D_1                First Back Saber Sit-and-Reach Test in Right Leg (cm)
BSSR_Iz_1               First Back Saber Sit-and-Reach Test in Left Leg (cm)