Girls exhibit greater sit-and-reach flexibility than boys across all ages from 3 to 16Girls show greater sit-and-reach flexibility than boys through age 16

INTRODUCTION: The sit-and-reach, which primarily assesses hamstrings extensibility, is arguably the most frequently performed flexibility test in history. Thus, the sit-and-reach can be used to explore, with high statistical power, sexual dimorphism of a proposed fitness attribute. Here, the aim was to examine sex differences in sit-and-reach flexibility throughout development. METHOD: A meta-analysis was performed on 408 effects from 96 studies that were conducted in 38 countries between 1983 and 2023. The total sample was 944,420 (484,380 boys, 460,040 girls). RESULTS: At all ages (3-16 years old), girls had greater sit-and-reach flexibility than boys. Between 4 and 10 years old, the effect size was fairly consistent and moderate in size (g = -0.30 to -0.50). The sex difference widened at age 11 (g = -0.59, 95% confidence intervals (CIs) [-0.75, -0.44]), peaked at age 12 (g = -0.78, 95% CIs [-0.96, -0.61]), then returned to pre-pubertal levels by age 16 (g = -0.42, 95% CIs [-0.61, -0.52]). Secondary analyses revealed that the sex difference in sit-and-reach flexibility was similar between countries and has remained relatively stable since the 1980s. CONCLUSION: Sexual dimorphism in sit-and-reach flexibility occurs early in development, is impacted by puberty, and is broadly consistent across time and place.