This is how the the US Navy tried to get recruits to stand tall during World War II, with anatomical studies of slouching. How do you know if you’re slouching? Well, here’s how, point by point.
This from the 1943 Physical Fitness Manual of the U.S. Navy. There are lots of fascinating illustrations throughout, showing how to do handsprings and the like, and more photos of diving and pullups and rope climbing without your feet. But the sloppy trainee could become an alert trainee by doing this with his body.
I found the manual here, on archive.org.