Daily energy expenditure through the human life course

What makes you say fidgeting is /a/ movement? Fidgeting is likely a combination of many positive and negative things; I can imagine: more anxious people are subconsciously wanting an escape from a situation and fidgeting more, more energised and impatient people are itching to hurry things up, more musically or rhythmically interested people are grooving with internal beats and earworms, uncomfortable people are pushed to move by their chair/clothes/posture/tools, it could be a form of stimming behaviour such as enjoying the feeling of the movement of clothes against skin or the finger movements taking some concentration and helping focus.

And on the negative side, non-fidgety people might be more depressed or anxious mentally, more physically drained or fatigued or lethargic physically, may have had upbringings where movement drew negative attention from adults telling them to sit still, may have social upbringings where fidgeting was seen as 'acting out', may have been part a band or group where being still was trained into them, may find their own movements distracting or annoying...

As a student, I used a bus to get to school. As an adult, I walk by foot, more than an hour a day. Also, I didn't exercise in my youth, and I do now. Yet, it is now that I am fat.

    Strava’s calorie estimation is awful.

Re-reading your post made me think: I bet this is intentional design -- overestimate number of calories burned. Then, people will tell their friends about this amazing device from Strava that burns an unreasonable number of calories...

I just spent the last six months dieting down to 6% bodyfat and I haven't once gone under 2000 calories. Other than women, I know zero bodybuilders who can sustain anywhere near 1200 a day. Hell, Neggy Shelton's death was just exposed in the Washington Post from extreme dieting and she was doing about 900 calories a day, plus 2.5 hours of steady state cardio, and she was a 124 lb woman.

Those Minnesota starvation study guys you're referencing had to use an extreme deficit to get down to concentration camp weights like that over the course of a year. It doesn't say in there, but that article with Michael Rea was written in 2006, and finding other stuff on him, he seemed to have been doing calorie restriction since at least 1999. A small calorie deficit can add up when you keep at it for a very long time. Even if his TDEE was 2200 or so, that would drop 30 pounds or so in a year eating 1900.

For whatever it's worth, I've been that size, and it was not on purpose. I grew very quickly in middle school and was extremely active. By 8th grade, I was 6'2" 120 lbs and it took many years before I got much bigger than that, and I used to buy entire boxes of donuts, Little Debbie snacks, and family-size bags of potato chips on the way home from school and eat them in addition to the pizza and cheeseburgers they served at the school. And when I got a car and started driving all my friends home, we'd usually go by McDonald's every day, and in addition to just the actual food, I'd get a McFlurry, pretty much every single day.

But unlike everyone else who seems to be wondering how that was possible and what happened when they were skinny in youth, it's not a mystery to me at all. I sometimes played basketball for 18 hours in a day. When I was driving my friends home, that was usually after cross-country practice, running for up to three hours after school. I walked or rode my bike everywhere. I almost never sat down. I rarely even slept. I could eat basically anything because it may as well have been the Michael Phelps Olympics diet. I was insanely active and using a ton of energy every day.

And yeah, when life normalized in my mid-20s, I too gained a bit for a short while, then I adjusted and ate less, and now I'm very lean again, but also bigger because I started lifting and learned to eat on purpose, in an intentional, measured way calibrated to actually meet my energy needs, not just ad hoc having whatever I crave at any given minute and not thinking about it. I understand why people don't want to do this. Even though I'm not restricting like him, at least not permanently, my diet habits are basically like Michael Rea's. I weigh everything, prepare all my own food, and eat pretty much exactly the same thing every day. It probably sounds like a slog and people want to just free graze and wonder why we can't live like humans must have lived for most of the past 300,000 years, when diets and hunger levels seemed to calibrate to energy needs automatically, nobody thought about it, yet almost nobody was fat except a tiny number of super rich idle nobility. But that isn't the environment we live in any more. People are extremely sedentary and randomly selected food from anywhere is utter trash loaded with extra calories in every possible form for no good reason. Unless you have the activity level of a middle schooler from the 90s or an Olympian, you need to actually try.

It's actually nowhere near as simple as that, and it depends on how you define the "same activity".

Moving your body from one location to another at a particular speed results in a fairly static amount of work done (in the basic physics sense). So if the same you hops on a bicycle and cycles up the same hill in the same conditions with the only difference being that one of you has trained hard for the last 5 years and the other hasn't (but somehow your body mass has stayed the same) then you'll burn exactly the same amount of energy. Untrained you will find it much harder, but the energy burn will be roughly the same.

There are some things that can be different as you go from "untrained" to "trained", for example lighter people burn less energy moving themselves around than heavier people. Trained people tend to do activities harder/faster so the "same activity" could actually be a much harder activity despite it not feeling that way. Although if the activity involves travelling a set distance some of the extra effort involved in doing it faster is offset by the fact that it takes less time, so the difference between the two is not as large as you'd think.