Nice article, i'm sorry you have developed RSI at a young age.

I was trained when I was a teenager near 40 years ago. I don't keep those training materials, but I will try to summarize it. I think most of the materials
are now in the memories of the 60s/70s years old. But there are some online resources [0], a video is not like an teacher in the 80s, but it can help.

My training was directed by a teacher (female, old, no jokes, focused, loved her work). Now I known there is a big difference having a teacher, she can correct you. It's like heavy lifting, everybody can try to pull a bar. But there are small mistakes that can be corrected, and will avoid common injuries.

The first day in the training, was about "position". The teacher corrected me for the first week (a lot of times, teenagers are stubborn). The first day was about:

* sitting position.

* forearm, wrist, hand position. The forearm, the wrist and the hand shall be aligned. Somehow floating, but not floating, resting with no pressure even typing. It's difficult to describe but this video [0] of the 50s can help.

* Not placing the forearm aligned with the hand is one of the most common mistakes in people. Some people align the thumbnails of both hands, that's a mistake that put pressure on the wrist and forearm muscles. The ergonomic keyboards are designed to correct this, and to return the hand/forearm to the right alignment. There are some videos from the 50 [2], mentioning this.

* Shoulder shall be relaxed, one effect of not having proper forearm/hand alignment is also compression in the shoulder.

* Training for IBM Selectric and training for a mechanical typewriters was mostly similar. But everybody begins with the mechanical ones. You need to "hammer" the mechanical ones. They needed more pressure and force using the fingers (not the wrist). When changing to the Selectric, the words/minute increased automatically. They keys do not jam due to fast typing, and the Selectric has memory and automatic carriage returns. But position, and memory muscles remained.

* Stretch the hands at the end. (I was forced to do it and can't go out of school immediately after finish assignment). Now, 40 years after, I know it helps after long typing sessions, but as a teenager it was another boring rule to break.

* Caps-locks: I see a lot of comment in this thread about cap-locks. Each of us shall use the flow that best suits their work. But for me, I have memory muscle of using the Caps when there are more than two continuous uppercase. It's intuitive, i don't think about it. When it's not there, my memory muscle rebels. My hands feel it deeply wrong due to two months of training 40 years ago. If remapping caps fits you, go for it.

* Shift balance. Only one finger from each hand at the same time. This avoid forced positions in a single hand. (applies also to ctrl and special characters).

* Space-key balance. space pressed with the opposite thumb to the previous used hand.

Once you were properly in position, the finger training begins (repetition of key patterns). But for the few first days, the teacher corrected us the position continuously (interrupting the key repetition). After a few days: the focus on position, changed to focus on typing mistakes. Nobody went out of the room until they completed the exercises without errors. For me that means expending two hours instead of one like my school mates that were there for one only one hour.

I don't keep materials, but looking for similar books, this "Elementary Typewriting I"[3] from 1960 gives some hints. I'm pretty sure, back in the days, there were a lot of methods.

Check the us navy film [0]. The position was also their first lesson. Those days, clerks spends 8 hours of continuous typing, and an injury means full stop.

[0] US Navy training material, first day. Emphasis on position. https://www.youtube.com/watch?v=H1e4BC41wRU

[1] 1930 contests. Show the position of clerks using mechanical typewriters. https://www.youtube.com/watch?v=M-x9JX8ypJ0

[2] common typing mistakes. Use your fingers, not the wrist. https://www.youtube.com/watch?v=9mQszbUhE30

[3] US Army book on typewriting https://play.google.com/store/books/details?id=K_f3apVfwOYC&...