I had a similar experience, I had to drop my entire repertoire and relearn my right hand starting with the simplest stroke, took two years. The reason I did it was, well, things had changed in the fifteen years before. And I wasn't connecting with my music or my audiences, so I was desperate. My teacher knew what to do, so I followed his advices. A lot of what you say rings a bell, so to speak, so let me try to add some information that helped me.
First, let me say, I read something about "surgery" and that implies that you had physical therapy. It is important to say that not everyone needs physical therapy. Aaron Shearer's book, written after his tendonitis, was his physical therapy and lots of people believed that that was the way to play. But they did not need physical therapy. My students sit across from me, sometimes for years, and they don't think about it, it's just the way to play, effortlessly and with an articulated sound. For you, I can see you reached the point where you are desperate (maybe like me) and excited about your progress (like me). That's great, thank you for sharing your knowledge, hope and experience. Be careful, though, not to think that everyone needs physical therapy. And, unless you've worked with a neurologist, I would be careful about throwing the word "dystonia" around, it is very serious.
That being said, Charles Duncan, in 1982, first introduced "awareness of the release of tension" as the basis of classical guitar technique. My teacher, in the intro to the first edition of his book, mentioned it, but subsequent editions (that changed according to the needs of the students who were coming to him, which constantly evolves) went straight to the technique, so the reference may be hard to find. He publishes the text from his books, The Art & Technique of Guitar
by Richard Provost, free online, and you are welcome to read them, and there are people on the forum who can help you understand. If you talk to the top players, like Barruecco and Tennant, they will tell you they play like this, read his books, and constantly ask him if he has any new ideas.
Scroll down the page to Downloads, Scale & Arpeggio ->
Let's start with what you posted. Tremolo is actually in book three, considered advanced. There are two sounds in the tremolo, the place sound (on a) and the sequencing sound (on m-i). From what I can hear, you are observing both of these. The unevenness will go away, don't worry about it. Also, sitting in the backseat of your car may be affecting how your hand falls on the guitar, I hope you don't play there all the time
In the tremolo, the fingers play in a natural sequence. The fingers come out and a goes the string. a plays, drawing m to the string. m plays, joining a in the hand, drawing i to the string, and i plays, joining m and a in the hand, and p goes to the string. Then it starts again, p plays and a, m, i come out together and a goes to the string. I didn't make it up, they just work this way, if you let them. That's why it's called "sequencing", with the minimum number of steps.
In my playing, I try not to "do" anything. Instead, I find it in the music. You seem to have discovered the "release" of tension, I would suggest that the next step is to decide where to put it. There are four parts to the stroke: 1) release (the end is the beginning), 2) go to the string, 3) push across (don't pull) and 4) snap/'follow through'. And by 'follow through', I don't mean to imply that you keep swinging, just let the finger stop by itself.
The confusion comes from trying to do two things at once: as once finger plays, the other releases. The exchange occurs between i & m and there is a constraint in the hand that makes them want to work together. Trying to force the exchange has been known to lead to injury, I had a student who had RSI from keyboarding and he just about crippled himself because he was forcing the exchange. On the other hand, a good reason to learn the release and exchange is to avoid injury, but he insisted he knew what he was doing. That is why it is called "awareness" of the release of tension, he was not aware. Oh, well.
So you get the release, the "noodle". Okay, the next part of the stroke, go to the string, is pretty natural. The finger swings from the main knuckle. The entire finger action is from the top knuckle. Keep it as simple as possible. On the next part, the finger will push but the tip joint resists, until the tip joint releases and snaps. This is literally like snapping your finger. Push, release the tip joint, snap.
So, where in this does the exchange occur? As I said before, I like to find it in the music. Now were are going to talk about something that hardly ever comes up in the guitar, articulation. Other instruments have volumes of articulation studies but I can't name one for guitar. Articulation is controlling the duration, the ending point of the note. When you play your beautifully sequenced tremolo arpeggio, each note is shortened by the stroke that follows. But the tremolo doesn't have an exchange, unless you count p to a, but you can't really hear it. Exchange is a scale sound, sometimes occurs in complex arpeggios, mainly between i & m.
I tend to put the release on the "go to the string" step, the articulation or muting of the previous note. If you are crossing strings, the sound is not apparent but on one string it is obvious. It is a beautiful day out and at this point I'm going to take a break. Here's is an exercise I give my students, play the rests by releasing (one finger) and going to the string (other finger) at the same time. As you have discovered, it is the release finger that needs all your attention to get this to work. More later, thanks for the topic.
Even exchange of i & m, simple articulation
Simple does not mean easy, someone said.
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Kevin Collins, Amherst, Mass, USA All rights reserved.