Flexibility plays an important role in both daily life and performance in various sports. This study evaluated the intrasession and the intersession reliability of flexibility measurements, examining the effects of sport, age, and sex. The sample included 80 wrestling athletes (40 boys: 20 children/20 adolescents; 40 girls: 20 children/20 adolescents), 80 taekwondo athletes (40 boys: 20 children/20 adolescents; 40 girls: 20 children/20 adolescents), and 80 non-athletes (40 boys: 20 children/20 adolescents; 40 girls: 20 children/20 adolescents). The participants performed two assessment sessions, which included two tests (back scratch/sit and reach). The results showed high intrasession and intersession reliability for boys and girls among wrestling and taekwondo athletes (children: ICC = 0.988-0.998, SEM% = 2.31-7.44; adolescents: ICC = 0.993-0.999, SEM% = 1.13-5.19). Additionally, the results demonstrated good/high intrasession and intersession reliability for boys and girls among non-athletes (children: ICC = 0.992-0.997, SEM% = 3.40-9.98; adolescents: ICC = 0.996-0.998, SEM% = 2.81-8.94). The SEM% values were slightly higher in non-athletes vs. athletes (wrestling, taekwondo), as well as in children vs. adolescents, indicating that athletes and adolescents present better reliability than non-athletes and children, respectively. No differences in reliability were observed between boys and girls. In conclusion, the sit and reach and the back scratch are reliable tests in assessing flexibility during the developmental ages. It seems that age and engagement in sports affect the reliability of measurements.
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