I got the "Speed mechanics" book on monday, and I"ve been spending some time working on it. I'm pretty bad at tremelo picking, so I spent an hour or so working on it. I would practice this tremelo picking lick until the point where I physically couldn't anymore, take a break, then do it again. Today, even though I've warmed up, I can't pick very well at all. Do you think my wrist just needs a rest because of yesterday?
I understand Stetina's logic in how he presents picking: start on the simpler case
of picking on a single string before moving to multiple strings. So you get that
and tremelo picking in the earlier chapter and cross string picking later.

It makes logical sense, but I think he's wrong. The real fundamental you want to
develop first is cross string picking because that will teach you in a more general
sense what your picking mechanism (arm, wrist, hand) will need to do in a more
demanding situation. So skip tremelo and go right to cross string exercises (his
"7 basic mechanics of cross string picking" don't make sense to me. why 7? but

As you're discovering, in your attempt to tremelo pick on 1 string, you're probably
locking tension in in the attempt to keep your pick on a single string. Ultimately
that's a disaster. It will appear to allow you some progress tremelo picking, but it's
useless for your general picking.
yeah I"ve wondered about that myself. As far as I can tell, I don't have any tension, it's just I do it for a few minutes straight to build muscle in my wrist.
I think most really good guitarist get good at picking by just having proper form and practicing an assload. I know Malmsteen and Frusciante both just got their picking skills by practicing, they never really practiced the 1-2-3-4 speed stuff that's always recommended.