a good left arm warm up that I use and that works is..(it's also good practice for the right arm..)

-------4-3-2-1-----------
-----3---------2---------
---2------------3--------
-1---------------4-3-2-1-

Then try the same pattern but on different strings or higher/lower on the fretboard.. also.. try sweeping (not really sweeping, but just practicing downpicking/uppicking at high speeds to evenually be able to sweep)the "1-2-3-4" parts and alternate pick the "4-3-2-1"s..

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