Childhood Sex Differences in Strength are Muscle-Specific
New meta-analysis of over 30 studies shows boys' strength advantage is greatest in upper-limb muscles
Dear Subscribers and Followers,
Over the past couple of weeks ago, I have been working on the last of three large-scale analyses on sex differences in physical performance in children and adolescents. I am happy to announce that this final analysis is now complete, and a pre-print of the paper describing the study’s methods and findings is now freely available at SportRxiv. The paper is titled, “Sex differences in upper- and lower-limb muscle strength in children and adolescents: a meta-analysis.”
Why this new research?
In August, I completed a meta-analysis on sex differences in grip strength in children and adolescents. Focusing only on grip strength for that paper was appropriate because a substantial amount of data exists on grip strength compared to all other strength tests, and this substantial data allows for testing of multiple hypotheses. To have included other strength tests in that paper would have made it excessively long and less readable.
The findings from that study were that boys, on average, exert greater grip forces than girls from birth onward (see my interview at Men are Good for a full summary of the study). Nevertheless, results observed in muscles involved in gripping should not be assumed to reflect sex differences in other muscles of the body, particularly leg muscles. In fact, in adults, I have previously shown that the size of the sex difference in muscle strength is greater in arm muscles than leg muscles.
Thus, the aim of my new analysis was to determine whether the muscle-specific sex differences in strength that are observed in adults are also present in children and adolescents.
What were the findings?
In an upcoming essay, I will elaborate on the results from this new analysis. For now, here is a list of some of the study’s key findings:
• Boys are physically stronger than girls throughout development, including before puberty, and the size of this sex difference is more pronounced in muscles of the upper-body than of the lower-body.
• Between 5-10 years old, girls have 85% of boys’ strength for upper-body muscles and 94% of boys’ strength for lower-body muscles.
• Between 14-17 years old, girls have 65% of boys’ strength for upper-body muscles and 76% of boys’ strength for lower-body muscles.
What is next?
This new paper is currently undergoing peer review. In the meantime, you can download a copy of the pre-print from SportRxiv. SportRxiv is not an academic journal. SportRxiv is a server that hosts initial drafts of papers that are currently undergoing academic peer review (i.e., pre-prints). Because peer review takes several months, a purpose of pre-prints is to communicate new research as quickly as possible. Eventually, the academic journal will return feedback from peer reviewers. The paper is then revised and eventually published in a journal. Typically, the version of the paper that is published in the journal does not differ significantly from the pre-print. Nevertheless, the pre-print is not the final version of the paper.
Response on X (Twitter)
Over the past 48 hours, this new pre-print has received a fair bit of attention on X. I was happy to see many people engage with the paper.
Some people (including those who promoted the findings to serve their cause) have accused me of “duh science” - meaning, the results are so obvious that we did not need a study on the topic. I will not bother to address the many ways in which this criticism is misguided, but I did want to quickly assure readers that many of the results from my recent meta-analyses on childhood sex differences in fitness were not already established within the scientific community (explained here in interview). I know this because I have read every paper that has included relevant data and some studies do not find statistically significant differences in muscle strength between boys and girls prior to puberty, particularly for leg muscles. Thus, the only way to figure out what the totality of research says about a question is to conduct a meta-analysis of all relevant studies. So, that is what I did.
Your continued support is greatly appreciated.
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If you have appreciated my three separate meta-analyses on childhood sex differences in grip strength, lower-body strength, and flexibility, please consider supporting The Nuzzo Letter with a one-time or recurring donation. Your support is greatly appreciated. It helps me to continue to work on independent research projects and fight for my evidence-based discourse. To donate, click the DonorBox logo. In two simple steps, you can donate using ApplePay, PayPal, or another service.
The Go Fund Me for the research on childhood sex differences in muscle strength will remain open for anyone who prefers to provide further support for the project that way. Your donations are greatly appreciated, as the process of addressing reviewer comments takes many hours of work. Thank you!
