Biological sex and flexibility in kids
New meta-analysis does not stretch the truth about early sexual dimorphism
Dear Subscribers and Followers,
A couple of weeks ago, I announced that I was on the verge of completing another large-scale analysis on sex differences in physical performance in children and adolescents. The new analysis is now complete, and I am happy to report that a pre-print of my latest paper, “Child and Adolescent Sex Differences in Sit-and-Reach Flexibility: A Meta-Analysis,” is now publicly available at SportRxiv.
What is this new research and what did it show?
My last paper, which I overviewed in a recent interview at Men are Good, examined sex differences in grip strength in kids. This new meta-analysis examined sex differences in sit-and-reach flexibility in kids.
The sit-and-reach test was developed in 1952. It assesses hamstrings flexibility, and to a lesser extent, low back flexibility. It is part of many physical fitness test batteries around the world. Arguably, it is the most commonly performed test of flexibility in human history. When thousands of results from sit-and-reach tests performed by boys and girls over the past several decades are aggregated into one meta-analysis, they provide a high-powered way to further explore early sexual dimorphism in a proposed fitness attribute (flexibility1 is the only component of fitness in which adult females score higher than adult males).
In an upcoming essay, I will elaborate on the findings from this research as well as the research on grip strength. In the meantime, here are some quotes from the paper that summary the study’s key findings:
• " At all ages assessed (3-16 years old), girls had greater sit-and-reach flexibility than boys. "
• "This sex difference [in sit-and-reach flexibility] widened at age 11…peaked at age 12…then returned to pre-pubertal levels by age 16."
• “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.”
What is next?
This new paper on flexibility is now undergoing peer review at an academic journal. I will share the final version of the paper when it is published in the journal.
For now, please consider downloading and reading the pre-print from SportRxiv. SportRxiv is not an academic journal. SportRxiv is a server that hosts pre-prints, which are initial drafts of papers that are currently undergoing academic peer review. Because peer review takes several months, a purpose of pre-prints is to communicate new research findings as quickly as possible. Eventually, the academic journal will send back the feedback from peer reviewers. The paper is then revised accordingly and eventually published in the journal. Typically, the version of the paper that is published in the journal does not differ significantly from the pre-print version of the paper. Nevertheless, the pre-print is not the final version of the paper.
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Related content at The Nuzzo Letter
Previously, I have argued, based on various lines of evidence, that flexibility be de-emphasized or retired as a component of fitness. That is a story to be retold another time!