The day after a workout, I sometimes develop twitching in a random muscle; my triceps, or today my "upper" abs shake randomly.
What causes this ?
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Like most benign conditions, it's not extensively studied. Research (fortunately) tends to go where's the greatest need and unraveling the mysteries of a harmless eye twitch doesn't get the attention those with twitching eyes probably would like.
Anecdotally there are several suspected causes of fasciulations, but no smoking gun to demonstrate that some particular repeatable and testable pattern is present in a certain population (or all populations). The best I've been able to find is a 2010 study which loosely links exertion:
However, strenuous physical exercise caused a temporary increase in fasciculations, but only in lower leg muscles. Fasciculations above the knee should raise suspicion and may warrant further investigation.
In the mean time, you might find some help in reading up on people's particular anecdotal and personal experiences.