It depends completely on the technique that you are trying to gain speed on.
If you want fast, palm muted power chords (like metal guitar), then it's in the wrist.
If you want fast open chords (like Pinball Wizard), then it's more in your wrist / forearm (I guess you might consider forearm to be bicep).
Fast single notes (like Dick Dale), is either wrist or a twisting of your forearm.
Fast picking for lead lines could be a million different things, including alternate picking, finger technique, sweep picking, etc.
The one thing that took me a long time to realize is that at the end of the day, you'll be faster if you are relaxed.
It may not feel like that to begin with, but tensing up never really helps.
music.stackexchange.com/users/71/yossarian
i would say it's all in the flick of the wrist
the goal is that you don't really want to be straining your muscles much if at all.
it's really about twitching the wrist quickly without flexing or constricting muscle much, to not tire your arm.
you don't need strength for this.
just speed.
music.stackexchange.com/users/899/topwik
Just like my other favorite past-time: It's all in the wrist.
With that being said, Al DiMeola has forearms like a ham, but if you watch him play
it's amazing the economy of movement
that's concentrated in his wrists.
Steve Morse, at his peak in the late 70's seemed to use his entire arm, which always looked like it shouldn't produce the insanely accurate, musical flurries of notes - especially during his "country" solos's.
Herb Ellis, one of jazz's fastest guitarists, always seemed to get most of his speed/strength out of his thumb and forefinger - which, seems to be the case for a majority of accurate speed players: a wrist movement that eventually becomes such a second nature muscle memory that it winds up being a fluid motion in the thumb and forefinger area.
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There is a famous part of Eruption that quotes Etude
No 2 by Kreutzer ( reference here ).
For that style of picking, it is important to lock the wrist, and use the forearm in a tight, fast pattern (yet staying relaxed ;-)
Here is a link to the quote, which shows Eddie's technique
music.stackexchange.com/users/203/Michael Easter
There are likely as many routes to fast picking as there are pickers... For bluegrass-type fast runs that are mostly done in a pretty strict alternate-picking style, it seems to come from the elbow with the wrist relatively rigid.
Doc Watson refers to "picking from the elbow" when digging in on such passages.
Note that I'm talking about steel-string acoustics here, and usually set up with rather stout medium-gauge strings.
Playing an electric with "slinkies" can be quite different.
music.stackexchange.com/users/549/M. Werner
The pick type makes a great difference to me - if I'm chugging I'll use a fairly thick pick, but for speed I'll go for a 0.4 or 0.45 mm pick, and as new as possible...if it is worn you have to move it further to get the point of the pick past the string to return back across it.
music.stackexchange.com/users/104/Doktor Mayhem
A fast picking speed comes from mainly the hand both working to move the pick quickly.
However the bicep does a little work, keeping the hand and arm steady to provide accurate picking.
music.stackexchange.com/users/8/Ali
Where specifically on the arm/hand should rapid picking speed come from?
Where does picking speed come from? Forearm, hand, or bicep?
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music.stackexchange.com/users/159/bobobobo
There is little consensus on which part of the body is responsible for fast picking. Contenders include wrist, wrist/forearm and elbow. The majority seem to favor the elbow. This depends on the individual style of the player. However, many believe that it is all in the wrist. Fast picking may also come from the hand and the bicep plays a steadying role. It also depends on the style of music. Fast, muted chord speed comes from the wrist. Fast-strummed open chords come from the wrist and forearm. Fast single notes come from wrist or twisting of the forearm.
Fast, muted chord speed comes from the wrist, whereas fast-strummed open chords come from the wrist and forearm, and fast single notes come from wrist or twisting of the forearm It's all in the wrist. It depends on the individual style of the player. Fast picking mainly comes from the hand but the bicep keeps things steady.