I'm having the same manifestation in my left arme and it's quite coinsidantial, since it is comming forme my braquialus also. And I have also been going lots of static holds and controling the weight on the excentric portion of the lift. Could be fatigue or over stressing the muscle.
I'ves been taking 300mg of citrate Magnesium every night for the last couple of year's . So, I can rule out the Magnesium from being the cause.
Is been twitching for the last 24 houres and does not seme to be slowing down.