Not necessarily what it appears to be. I went through high school in the 60s and there were plenty of fatbodies in gym and plenty of skinny guys (I was one) as well.
A our HS they had the normal calisthenics and rotated through different activities such as rope climbs, weights, gymnastics, wrestling, track, swimming, etc. My strong point was swimming and lifesaving.
The gym teachers assumed that if one person could do a kip on the high bar then all students should be able to do that. Obviously this wasn’t so but that is how we were graded. I was satisfied with my Cs in gym and put my efforts elsewhere.
When I dropped out of college to enlist in the Marines, I ran into one of my old gym teachers who sneered and said that I would never make it; I was too weak.
I made it, aced the final PT and went on to complete my enlistment. When I went through, we started out with 60 recruits, some fit, some fat, some skinny. Not everyone made it but the 35 of us who did had the perseverance to stick with the physical demands and succeed.
When I went back after infantry training I made a point of finding that asshole and proving to him that he didn’t know what he was talking about.
I’m retired from the military and see the kids around town as well as the airmen on base. There seems to be the same basic mix of body styles and abilities as there was when I was in HS 50 years on.
“From 1963 to 1970, 4.2 percent of 6 to 11 year olds and 4.6 percent of 12 to 19 year olds were obese. In 1988, 11.3 percent of 6 to 11 year olds and 10.5 percent of 12 to 19 year olds were obese. In 2001, just over 16 percent of 6 to 11 year olds were obese. The last survey, from 2007 to 2008, 19.6 and 18.1 percent of 6 to 11 and 12 to 19 year old kids were obese. Between 1971, the first year results were available for 2 to 5 year olds, and 2008, young kids went from an obesity rate of 5 percent to 10.4 percent.”
https://www.livestrong.com/article/364363-the-history-of-obesity-in-children/
“a kip on the high bar” Shessssh, easy for me and I was 6’4” in my senior year, ‘61