Methods
Participants
The study protocol was first approved by the Ethical
Committee of the University of Granada. After the school
approvals were obtained, schoolchildren and their legal
guardians were fully informed about all the features of the
study [i.e., a thorough description of the methods, potential
risks, expected benefits, etc.; based on Thomas’s et al.
(2015) guidelines] and were required to sign an informed
consent document. A sample of 150 schoolchildren, 70
boys and 80 girls, aged 7-10 years old from six different
third/ fourth-grade PE classes of a public primary school
center participated in the present study. 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 trial design was used (Mayorga-Vega
et al., 2013; Merino-Marban et al., 2015). The
six natural classes balanced by grade were assigned randomly
to form one of the following study groups: experimental
group 1 (EG1), experimental group 2 (EG2) or the
control group (CG).
All the participants were free of orthopedic disorders
such as episodes of hamstring and/ or lumbar injuries,
fractures, surgery or pain in the spine or hamstring
and/ or lumbar muscles over the past six months. The
inclusion criteria were: (a) correctly performing all the
flexibility evaluations, and (b) having an attendance rate
of 90% or higher for PE classes during the intervention
period. Finally, although all the 150 invited schoolchildren
agreed to participate, only 140 participants met the
inclusion criteria. Figure 1 shows the flow chart that corresponds
with the participants included in the present
study. For general characteristics of the included participants,
see the Results section.