I cannot create a summary of the article you requested because the search results provided do not include an article titled "Physical activity for public health in the 21st century" from the journal Nature Medicine published on March 9, 2026. The search results include other articles related to physical activity and health, but not the specific article you have listed. In order to create an accurate and factual summary in accordance with your requirements, I would need access to the full text of this particular article.