Behind every perfectly tailored “Bridgerton” costume and ad for Lululemon leggings lies the work of fit models, the unsung heroes who help designers create clothes that actually work on real bodies.

For bodybuilders and fitness enthusiasts, changing body composition often reflects hard work and discipline—but knowing body composition is useful for anyone, and for more than just bragging rights.

Dana Santas, known as the “Mobility Maker,” is a certified strength and conditioning specialist and mind-body coach in professional sports, and is the author of the book “Practical Solutions for Back ...