Child and adolescent sex differences in sit-and-reach flexibility

The sit-and-reach assesses hamstrings extensibility, and to a lesser extent, lumbar extensibility. The sit-and-reach is arguably the most frequently performed test of flexibility in human history. Thus, the sit-and-reach provides, with high statistical power, a unique opportunity to explore sexual dimorphism of a proposed fitness attribute. In adults, greater female than male sit-and-reach flexibility is well established. However, the evolution of this sex difference throughout childhood and adolescence is less clear, and relevant meta-analyses are now 30-40 years old. The purpose of the current study was to use meta-analysis to update the question of sex differences in sit-and-reach flexibility in children and adolescents. The analysis included 407 effects from 95 studies. Studies were published between 1983–2023. The total sample was 944,292 (484,323 boys, 459,969 girls). At all ages assessed (3-16 years old), girls had greater sit-and-reach flexibility than boys. Between 4–10 years old, the effect size was fairly consistent and moderate in size (g = -30 to -0.50). This 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 is similar between countries and has remained relatively stable since the 1980s. Overall, the results confirm that sexual dimorphism in flexibility occurs early in human development, is impacted by puberty, and is broadly consistent across time and place.